NBS MONOGRAPH 115

The Calculation of Rotational

I Energy Levels and Rotational

Line Intensities

in Diatomic Molecules

U.S.

RTMENT

OF

MMERCE

National

Bureau
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UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans. Secretary NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

The Calculation of

Rotational Energy Levels and Rotational

Line Intensities in Diatomic Molecules

Jon T. Hougen

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

y.S 1 National Bureau of Standards Monograph 115,

Nat. Bur. Stand. (U.S.), Monogr. 115. 52 pages (June 1970) CODEN: NBSMA

Issued June 1970

For sale by the Superintendent of Documents, U.S. Government Printinfi Office Washington, D.C. 20402 (Order by SO Catalog No. C13.44:n5). Price 55 cents
mmfil BUREAU OF STANDARDS

SEP 1 2 197^

Freiace i, / i

This monograph describes procedures for making quantum mechanical calculatioiis of rota- tional energy levels and rotational line intensities in diatomic molecules. The procedures are

illustrated by sample calculations. A familiarity with the material of this monograph should enable a practicing electronic spectroscopist to carry out, though in a rather mechanical way, his own

theoretical calculations for molecules under experimental investigation.

A true understanding of the procedures described below can only be acquired by studying

the theoretical reference material cited. These references are not exhaustive; they represent sources which the author finds convenient and instructive. Unfortunately, the material in the

references is sometimes presented in a notation different' from that used here. In addition, it is sometimes slightly too general or slightly too specific to apply directly to diatomic molecules.

The material of this monograph is aimed at electronic spectroscopists who have had the

equivalent of one semester of graduate-level quantum mechanics.

Finally, I should like to point out, that the procedures described here for carrying out calculations are not new. Neither do they represent all possible correct procedures. They do represent,

however, a unified approach to the problem, which, in the opinion of the author, is easier to understand than the original literature cited, and less likely to lead to error than some of the alternative

calculation procedures.

Note added in proof: The reader is referred to the book Rotational Structure in the Spectra of

Diatomic Molecules [28] by I. Kovacs, which just appeared and which represents another author's

discussion of much of the material in this monograph.

Library of Congress Catalog Card No. 76-604235

II
Contents

Page Preface II

1. Calculation of rotational energy levels 1

1.1. Hund's coupling cases (a), (b), (c), and (d) 1

1.2. General approach to the calculations 2

1.3. Nonrotating-molecule Hamiltonian 3

1.4. Nonrotating-molecule basis set

1.5. Nonrotating-molecule matrix elements 6

1.6. Rotating-molecule Hamiltonian 7

1.7. Rotating-molecule basis set 8

1.8. Rotating-molecule matrix elements 8

1.9. Example: The Hill and Van Vleck expression for ^11 states 9

1.10. Example: The Schlapp expression for ^2 states 13

2. Symmetry properties of the rotational energy levels 15

2.1. Geometric symmetry operations 16

2.2. Permutation-inversion symmetry operations 18

2.3. The symmetry operation o"„ 19

2.4. Example: Parities of the rotational levels in a ' state 21

2.5. Example: Parities of the rotational levels in a ^2"^ state 22

2.6. The symmetry operation i 22

2.7. The symmetry operation C2 : 22

2.8. Example: Symmetry properties of the rotational levels in a 'Ha state 23

2.9. Relations between matrix elements 23

2.10. Example: Li = L2-L| 25

2.11. The time inversion operation 0 25

3. Calculation of rotational line intensities 27

3.1. Laboratory-fixed components of the electric dipole moment operator fi 28

3.2. Molecule-fixed components of ft 29

3.3. The direction cosine matrix a 31

3.4. Example: Honl-London intensity expressions for a 'D — '2"^ transition 32

3.5. Example: Rotational intensity distribution in a^S" — 'S"*" transition 34

3.6. Intensity calculations when closed-form expressions cannot be obtained. Example: Rotational intensity

distribution in a''A — transition 37

4. Perturbations 41

4.1. General remarks 41

4.2. Homogeneous and heterogeneous perturbations 42

4.3. Example: 'H — heterogeneous perturbation 42

4.4. Example: — 'H homogeneous perturbation 44

4.5. Van Vleck transformations 45

4.6. Example: A-type doubling in a 'H state 45

4.7. Centrifugal distortion corrections to rotational energy levels 46

5. References 49

III
;
The Calculation of Rotational Energy Levels and Rotational

Line Intensities in Diatomic Molecules

Jon T. Hougen

Procedures are described, in this pedagogical monograph, for making quantum mechanical

calculations of rotational energy levels and rotational line intensities in diatomic molecules. The

procedures are illustrated by sample calculations. A familiarity with the material of this report should

enable a practicing electronic spectroscropist to carry out, though in a rather mechanical way, his own theoretical calculations for molecules under experimental investigation. The material of this report is aimed at electronic spectroscopists who have had the equivalent of one semester of graduatelevel quantum mechanics.

Key words: Diatomic moleciJes; Hund's coupling cases; rotational levels; rotational line intensities; theoretical calculations.

1. Calculation of Rotational Energy Levels

1.1. Hund's Coupling Cases (a), (b), (c), and (d)

In chapter 1 we seek to calculate rotational energy level expressions for certain limiting

situations known as Hund's couphng cases [1] ' (pp. 218-237), [2] (pp. 275-302), and also for situations intermediate between the Hmiting coupling cases. Because any discussion of Hund's

coupling cases involves a consideration of various angular momenta, it is convenient at this point

to summarize the types of angular momenta considered and the notation to be used. This summary

is presented in table 1. The first column of this table specifies the type of angular momentum. We

Table 1. The angular momenta involved in Hund's coupling cases

Type of angular momentum Operator

Quantum numbers

Total Projection

Electronic orbital L L A

Electronic spin S S 1

R R

Total J=R+L+S J n = A + i

N=R+L N A

consider here electronic orbital angular momentum, electronic spin angular momentum, and the

i| angular momentum associated with the rotation of the nuclei of the diatomic molecule about the center of mass. We do not consider the nuclear spin angular momentum. Each angular momentum

has associated with it a quantum number specifying its magnitude and a quantum number specifying its projection along the internuclear axis. For the purposes of this paper, the projection

quantum numbers will be considered to be signed quantities.

It can be shown that the projection of the rotational angular momentum along the internuclear

axis is necessarily zero. Hence, we do not introduce a quantum number for this projection. As a

further consequence of this fact, the projection of the angular momentum N along the internuclear

axis is equal to the projection of L along the axis, and the projection of J is equal to the projection

of L-f S, as indicated in the table.

' Figures in brackets indicate the literature references on p. 49.

1
In this monograph we shall define the various Hund's coupUng cases in terms of two concepts:

(i) the quantum number occurring in the expression for the rotational energy levels, and (ii) the good

quantum numbers in the nonrotating-molecule problem. Table 2 describes Hund's coupling cases

(a), (b), (c), and (d) in these terms. Table 2. Brief description offour Hund's coupling cases

Coupling case

Rotational energy

level expression

Good quantum numbers in the nonrotating molecule

(see text)

Degeneracy in the nonrotating molecule

(a) A. S, 2, (a=A + 2) 2 or 1 (b) BN(N+\) A, S, 1, (Cl = A + l) 2(2S+1) or (2S+1)

(c) BJU+l) a 2 or 1 (d) BR(R+1) L, A,S,2, iQ.= A + X) (2L+1)(2S+1)

The second column of table 2 contains a rotational energy expression of the form BX{X+ 1),

where X is one of the quantum numbers from table 1. Rotational energies are given by these

simple expressions only for pure Hund's coupling cases. Rotational energies for coupling cases

intermediate between these pure coupling cases are given by much more complicated expressions.

The third column of table 2 indicates which quantum numbers are good ones in the nonrotating

molecule for the various Hund's coupHng cases. The nonrotating-molecule quantum numbers

shown in table 2 for Hund's cases (a), (b), and (d) indicate those which must be good; other nonrotating-molecule quantum numbers may, in fact, also be good ones. The nonrotating-molecule

quantum number shown in table 2 for Hund's case (c) indicates the only permissible good quantum

number.

The fourth column in table 2 indicates the degeneracy of the energy levels in the nonrotating

molecule. (This degeneracy need not be exact, but the nearly degenerate states must be separated

by energies small compared to BJ.) In Hund's cases (a) and (c) the only degeneracy in the nonrotating molecule is that associated with the two values of fi, i.e.,± |fl|. (States characterized by

fl = 0+ or H = 0" are, of course, nondegenerate.) In Hund's case (b) the 2S + 1 values of S associated

with a given value of S, and the two values of A, i.e., ± |A|, must give rise to a set of states, all of

which are degenerate in the nonrotating molecule. In Hund's case (d) the 2L+1 values of A as- sociated with a given value of L, and the 2S+ 1 values of X associated with a given value of S must

give rise to a set of states degenerate in the nonrotating molecule.

1.2. General Approach to the Calculations

The basic approach to calculations taken in this monograph derives from the fact that the

Hamiltonian for a diatomic molecule [3] (pp. 3-34) can be written in the form

H=Hev+Hr, (1-1)

where Hev is the vibrational-electronic part of the Hamiltonian, which does not involve the rotational

variables or the total angular momentum of the molecule, and where Hr is the rotational part of the

Hamiltonian, which does involve the rotational variables and the total angular momentum, and

which also involves many of the coordinates and momenta occurring in Hev Since Hev does not

involve the rotational variables or the total angular momentum of the molecule, it is convenient

to call Hev the Hamiltonian for the nonrotating molecule. Hev^Hr is then called the Hamiltonian

for the rotating molecule.

As a consequence of eq (1-1), the discussion to follow will be divided into two parts, one dealing

with the nonrotational part of the problem, the other dealing with the rotational part. In both

parts we consider the concepts of approximate Hamiltonian, limited set of basis functions, and

matrix elements of the Hamiltonian in the basis set. (As most spectroscopists know, energy levels are commonly obtained either by evaluating perturbation-theory expressions involving matrix

2
elements of the Hamiltonian [4] (pp. 151-179), or by solving the secular equation obtained from

a truncated Hamiltonian matrix [4] (pp. 191-198).)

Actually, there is some flexibility in the exact form of the Hamiltonian (1-1), depending on whether it is written in terms of laboratory-fixed or molecule-fixed spin functions. In this monograph we shall always use molecule-fixed spin functions [3] (pp. 12-14). Loosely speaking, this choice

corresponds to using Hund's case (a) or case (c) basis set functions rather than Hund's case (b) or case (d) basis set functions.

The question of which basis set functions to use is of some importance, since the same calculation in two different basis sets can appear quite different. Some authors, for example, find it desirable to use Hund's case (b) basis set functions when the rotational energy levels of the molecule

under study are very close to those of pure Hund's case (b). In my opinion, however, there is less chance of confusion and error for the novice, ii all calculations are divided into a nonrotating part

and a rotating part, so that these parts can be dealt with separately. Such a division forces one to use a basis set in which the quantum numbers of the nonrotating molecule are good, and thus

precludes the use of case (b) and case (d) basis sets. In any case, the basis set functions \ev; r) used in this report can be written as simple products

of the form

\ev; r) = \ev)\r), (1-2)

where the functions \ev) are eigenfunctions of Hgv, i-e., wave functions for the nonrotating molecule problem, and where the functions |r) are appropriate rotational wave functions for diatomic

molecules [2] (pp. 279-281).2

Units can be somewhat confusing in spectroscopic calculations. The Hamiltonian operator

represents the energy of the molecule, and as such has dimensions ML^T~^ in terms of thfe fundamental quantities Mass, Length, and Time. Diatomic spectroscopists do not normally use energy

units, however. Instead, they represent energy differences by their corresponding wave numbers,

which have the dimension L~^, and, in particular, cm~*. Conversion from energy units to cm~' is carried out by dividing energies by he, where h is Planck's constant (having dimensions ML^T~^)

and c is the velocity of fight (having dimensions LT"^). Confusion arises because the same symbol

is normally used for the "same" quantity, regardless of the units in which it is expressed. For ex- ample, the symbol E may represent E[ML^T~^] or £'[cm~*], the numerical values of the two £"s

being related by E[crn~^]= E[ML^T~^]lhc. A second source of confusion in units arises in dis- cussing angular momentum couplings of the form AhS or BP. These two quantities occur in the

Hamiltonian operator in such a way that they have the dimensions of energy, i.e., ML^T~^. Since

the matrix elements of angular momentum components are multiples of h, this requires that A

and B have dimensions M~^L~^. In formulas used by diatomic spectroscopists, however, matrix

elements of AL.-S and fij^ are normally represented by something fike AAX or BJiJ+ 1), where

A and B have the dimension cm"^, and where A, X, and 7 are dimensionless. The numerical values

of the two A's (or two 5's) are related hy A[cm~^] = A[M-^L'^]h^lhc. In this monograph we shall follow common practice and represent the energy of the state, the spin-orbit coupUng constant,

the rotational constant, etc., by a single letter each, regardless of the units being used. As a general rule, terms of the Hamiltonian operator may be assumed to be in energy units, whereas matrix

elements of the terms in this operator may be assumed to be in cm"^

1.3. Nonrotating-Molecule Hamiltonian

The electronic and vibrational Hamiltonian associated with the nonrotating molecule, as well

as its energy levels and wave functions, will not be considered in detail in this monograph.

Electronic energies and wave functions for the nonrotating molecule can be determined from

ah initio or semi-empirical treatments of the many electron problem. The former calculations require

^ The materia] cited here deals with a rotating diatomic molecule in which electron spin is ignored. It can be seen from the discussion in [3] that a closely analogous

treatment can be carried out in which spin is included.

3
extremely sophisticated computer programs and many hours of computer time; the latter calculations are not always quantitatively rehable. It will be seen below that matrix elements and energy

levels associated with the electronic part of the nonrotating-molecule problem can usually be

represented by a small number of parameters in the calculation of the rotational energy levels. Vibrational energies and wave functions for the nonrotating molecule can be determined more

easily than electronic energies and wave functions. Thus, vibrational effects can be taken into

account explicitly in many cases. We shall consider only one vibrational effect in this monograph

(see section 4.7). Because the nonrotating-molecule Hamiltonian is not considered in detail in calculations of

rotational energy levels, it is often represented simply by the symbol //ei,. Sometimes it is of interest

to consider spin-orbit interaction explicitly; then the nonrotating-molecule Hamiltonian is repre- sented by Hev + AL.-S or by ,+ 2i^(ri)lrSi, where L and S are operators representing the total electronic orbital and total electronic spin angular momenta respectively, and where h and s; are operators representing the same two momenta for the individual electrons. The spin-orbit inter- action operator /4L-S is used to compute spin splittings within a given spin multiplet. If interactions

between states belonging to different S or L values must be taken into account, then the operator

S;^(r,)lrs, must be used.

1.4. Nonrotating-Molecule Basis Set

Like the Hamiltonian Hev, the basis set functions \ev) for the nonrotating molecule are not considered in detail in calculations of rotational energy levels. Consequently, these basis set functions are often represented only formally, by symbols containing the quantum numbers used

to describe the basis set, e.g., |A5S), or |LA5S) (see table 2). At this point it is perhaps worthwhile to digress and consider the notion of good and bad

quantum numbers. A basis set is said to be characterized by a good quantum number, if each function of the basis set is an eigenfunction of some particular operator, belonging to an eigenvalue

which is some simple function of the good quantum number. For example, the quantum number fl

is said to be a good quantum number, if each function of the basis set satisfies an equation of

the form

(L,+Se)|n) = m|a). (1-3)

In this example, {Lz + Sz) is the operator, |fi) is the basis set function, D,h is the eigenvalue, and

n is the quantum number. A quantum number is not good (i.e., is bad) if equations of the form

(1-3) are not satisfied by the functions of the basis set. For example, if

I^|n)?^(constant)-|a), (1-4)

then the quantum number associated with L^, namely A, is not a good quantum number for the

basis set. We now consider the three types of nonrotating-molecule basis set functions suggested by

column 3 of table 2 in more detail [1] (pp. 212-217). It turns out that the operator L^ + Sj commutes

with Hev Consequently, wave functions for the nonrotating molecule are always characterized

by the good quantum number fl, representing the projection of the sum of the electronic orbital angular momentum and the electronic spin angular momentum along the internuclear axis. When spin-orbit interaction is very large, i.e., when the energy levels of the nonrotating molecule do not fall into recognizable multiplet groups, then the wave functions for the nonrotating molecule are characterized only by the good quantum number fl. Under these circumstances it is convenient to use basis set functions which are also characterized by this single good quantum

number. In eq (1-3), represents such a basis set function for the nonrotating molecule. The

quantum number fl can take on only integral values for molecules with an even number of elec- trons, and only half-integral values for molecules with an odd number of electrons. Both types of wave functions for the nonrotating molecule occur in degenerate pairs when fl 0, the members

4
of the pair being characterized by equal and opposite values of fi. When fi = 0, the nonrotatingmolecule state is nondegenerate. Nonrotating-molecule basis set functions characterized only by

ft give rise to Hund's case (c) states in the rotating molecule.

When spin orbit interaction is not large, i.e., when the energy levels of the nonrotating molecule

do fall into recognizable multiplet groups, then the wave functions for the nonrotating molecule

are normally characterized by three almost good quantum numbers, namely: the value A of the

projection of the total electronic orbital angular momentum along the internuclear axis (corresponding to the operator Lz), the value 2 of the projection of the total electronic spin angular

momentum along the internuclear axis (corresponding to the operator Sz), and the value S of the

total electronic spin angular momentum (corresponding to the operator S^). Under these circumstances it is convenient to use basis set functions |A52) characterized by the three good quantum

numbers A, S, and 2. Since these basis set functions are eigenfunctions of and Sz, belonging

to the eigenvalues A^ and respectively, they are also eigenfunctions of the operator (L^ + S^),

belonging to the eigenvalue Q.h . where

ft = A + X. (1-5)

Because ft is so simply related to A and 2, it is often not explicitly indicated in the symbol for the

basis set functions. (Note also, that in the true wave functions for the nonrotating molecule, the

quantum number ft is good, while A and S are only approximately good.) The recognizable multiplet groups of energy levels mentioned above consist of the 2(25+1) functions characterized by

A = ±|A|, S=fixed value, and S = S, S — 1, S — 2, . . ., — S (or consist of the (2S+1) such

functions when A = 0). As we shall see by example in section 1.9, nonrotating-molecule wave

functions characterized by A, S, and 2 give rise to Hund's case (a) rotational levels when the

energy separations among the multiplet components of the nonrotating molecule are all large

compared to BJ, and give rise to Hund's case (b) rotational levels when these energy separations are

all small compared to BJ. When spin-orbit interaction is not large, and when electrostatic interactions between the

electrons and the axial field of the diatomic molecule [1] (pp. 323-330) are not large, e.g., when

electrons are in Rydberg orbitals, then the wave functions of the nonrotating molecule are normally

characterized by four almost good quantum numbers, namely: A, S, and S as above, and the value

L of the total electronic orbital angular momentum (corresponding to the operator//^). In addition,

nearly degenerate sets of such wave functions occur, consisting of the (2L+ 1) (2S+ 1) functions

characterized by L = fixed value, S= fixed value, A = L, L — . . ., — L, and S = S, S — 1, . . ., — S. Under these circumstances it is convenient to use basis set functions |LAS2) characterized

by the four good quantum numbers L, A, S, and X. Nonrotating-molecule wave functions characterized by L, A, S, and S give rise to Hund's case (a) rotational levels when the energy separations

in the nonrotating molecule among the various components of the ^•^+^L complex are all large compared to BJ. They give rise to Hund's case (b) rotational levels when the separations between nonrotating-molecule states of different |A| are all large compared to BJ, while separations among

the multiplet components within a given 2-5+iy\ state are all small compared to BJ. They give rise

to Hund's case (d) rotational levels when the separations among the various components of the

^^^^L complex are all small compared to BJ. For any particular problem, it is necessary to choose which of the three basis sets for the nonrotating molecule wiU be used. From a calculational point of view, it is desirable to use the basis

set with the most quantum numbers, since more quantum numbers mean that more matrix elements can be evaluated expUcitly. However, if certain quantum numbers of the basis set are not good

quantum numbers at all for the actual wave functions of the nonrotating molecule, then the ad- vantage gained by being able to evaluate explicitly certain matrix elements involving these quantum

numbers is offset by the fact that the values calculated for the matrix elements are incorrect. If the good quantum numbers of the basis set are almost good for the actual wave functions, then

it is often convenient to allow for the slight errors made in the calculation of matrix elements by

5

371-908 O - 70 - 2
the introduction of a few small adjustable parameters. (See, for example, sect. 1.10, where B is replaced by 5 — V27.) In the final analysis, basis set functions for the calculation of rotational

energy levels are chosen by trial and error, to maximize agreement between the final calculations

and the experimental observations.

1.5. Nonrotating-Molecule Matrix Elements

Matrix elements [5] (pp. 90-102, 176-188) of the nonrotating-molecule Hamiltonian operator

in the basis set |n) have the following form

{CL\Hev\Ct')^ 0, if |n) and \VL') are different functions,

(1-6)

{Ci\Hev\^) = the energy of the nonrotating-molecule state |fl).

Since n is always a good quantum number in the nonrotating molecule, the exact eigenfunctions

of the operator Hev can be characterized by a value of fi. It is convenient to choose the set of exact eigenfunctions of the nonrotating-molecule Hamiltonian to be the nonrotating-molecule basis set. Such exact eigenfunctions obey the relations given in (1-6). For the purposes of calculating rota- tional energy levels and rotational line intensities, the energy of each state is represented by

an appropriate parameter, to be fit by comparison with experimental data. Matrix elements of the nonrotating-molecule Hamiltonian operator in the basis set |AS2)

can be taken to have the following form

(A52|//«, +^L-S|A'S'S') = 0, if |A5S) and |A'S'S') are different functions

(A52|//?!; + ^L-S|ASX) = the energy of the nonrotating-molecule state |A5X) (1-7) = constant + /iA2.

The two exact equalities in (1-7) are satisfied only if the functions involved in the matrix elements

are exact eigenfunctions of Hev However, the basis set functions |ASS) defined above are not exact eigenfunctions of the nonrotating-molecule Hamiltonian, because spin-orbit coupling destroys

the goodness of the three quantum numbers A, S, and S. In other words, because the spin-orbit

interaction operator Xi^(ri)li ' s; does not commute with Lz, S^, orSz, this part of the Hamiltonian

will mix together basis set functions characterized by different values of A, S, and X to form final nonrotating-molecule wave functions.

Nevertheless, when spin-orbit splittings between the various components of a given multiplet

e.g., ^Hi/j, ^Hs/J are small compared to separations between different multiplet groups (e.g., ^X, *X, ^n, ^A, etc.), then the basis set functions | ASX) are very good approximations to the

exact eigenfunctions of the nonrotating-molecule Hamiltonian. Under these circumstances it is convenient to shift our point of view a bit, and to consider the functions |ASX) to be these exact

eigenfunctions. It is then necessary, however, to remember that the quantum numbers A, S, X

are no longer perfectly good, i.e., the appropriate eigenvalue equations are only approximately

satisfied. For example,

L,|ASX)=A^|ASX) + |Si)

(1-8)

S,|A5X)=X/i|ASX) + |82),

where |8i) and I82) are small functions which vanish when the quantum numbers A and X are

perfectly good. Because of the presence of small "left-over" functions like these |8i) , the diagonal

matrix elements of the spin-orbit operator AL. • S are only approximately equal to AAX. Thus,

precise energies of the multiplet components represented by the wave functions |ASX) deviate

somewhat from the expression: constant +^AX. However, the exact energies of the nonrotating-
molecule problem can always be represented by a set of adjustable parameters in the rotational

calculations.

A shift in point of view similar to that above is also desirable for the basis set functions |LASS)

If the functions |LAS2) are taken to be the exact eigenfunctions of the nonrotating-molecule

Hamiltonian, then we can write

{LASl\Hev\L'A'S'T)^0, if \LAS1) and \L'A'S'T} are two different functions

(1-9)

{LASX\Hev\LAS1) = the energy of the nonrotating-molecule state |LASS),

where the quantum numbers L, A, S, 2 in these exact eigenfunctions of the nonrotating-molecule

Hamiltonian are all slightly bad. The energies of these exact eigenfunctions can again be repre- sented by appropriate adjustable parameters in the rotational calculations.

1.6. Rotating-Molecule Hamiltonian

The Hamiltonian operator corresponding to the rotational part of the problem in diatomic

molecules [3] (pp. 6-16) must now be examined in some detail. Hr for any molecule can be written

as a sum of three products, each product consisting of a rotational constant for the molecule and

the square of one of the three components of the rotational angular momentum of the molecule

[6] (pp. 273-284). Since there can be no rotational angular momentum of a diatomic molecule about

its internuclear axis (the z axis), the third component of the rotational angular momentum is zero, and hence absent from Hr- Hr for diatomic molecules is thus written as

^B[Ux-L^-S^V+ Uy-Ly-Sy)^], (1-10)

where the rotational angular momentum is expressed in the second line of (1-10) as the total angular momentum (J) minus the electronic orbital and spin angular momenta (L and S). For the

purposes of calculation, Hr is often written in the form

Hr = B{J^-Ji) +B{L^-Li) +BiS^-Si)

+fi(i+s_+L_s+)-fi(y+L-+y_L+)-fi(7+S-+y-S+), d-ii)

where J± = Jx±iJy, L± = Lx±iLy, and S± = Sx±iSy.

The extremely thorough student will find that the total angular momentum operator J for linear molecules is somewhat peculiar, since its molecule-fixed components do not obey angularmomentum-type commutation relations. Furthermore, the operator Hr for linear molecules does

not have precisely the simple form indicated in (1—10). Nevertheless, it can be shown that correct

results are obtained by ignoring the peculiarities associated with J in linear molecules and by

treating J like the angular momentum operator defined for nonlinear molecules in [6]. The complete Hamiltonian H describing the nonrotational and rotational parts of the problem

is of course given by (1-1). It is the matrix of the Hamiltonian (1-1) which will ultimately be diagonaUzed to obtain molecular energy levels and molecular wave functions.

Loosely speaking, one can see in eq (1-10) the origin of the various entries in column 2 of

table 2. Forgetting for a moment the absence of Rz in (1-10), and remembering that the eigenvalue

associated with the sum of the squares of the components of an angular momentum operator has

the form fi'^X{X+l), we note that: if the operators L and S in the rotational Hamiltonian (1-10) can both be ignored, then one might expect rotational energies to be given by BJ{J + 1), since 7 is the quantum number associated with J^; if the operator L in (1-10) can be ignored, but the operator

S cannot be, then one might expect rotational energies to be given hy BN(N + 1), since is the quan-
turn number associated with (J — S)-; if neither L nor S in (1-10) can be ignored, then one might

expect rotational energies to be given by BR (R + 1), since R is the quantum number associated with

(J-L-S)-^

Since the operators L and S in (1-10) affect the course of the rotational energy levels only

through the four cross terms JxLx, JyLy, JxSx, JySy, and since the selection rules for nonvanishing

matrix elements of Lx, Ly and Sx, Sy are AA and AX = ± 1, respectively, we see that the effects of

L and/or S in (1-10) can be ignored when the separation between states of the nonrotating molecule

satisfying the selection rules AA and/or AS = ±1 is large compared to BJ.

1.7. Rotating-Molecule Basis Set

It can be shown [3] (pp. 6-16), [2] (pp. 279-281) that the wave functions \CIJM) associated

with the rotational part of the problem can be characterized by one parameter and two good quantum numbers: CI, J, M. The quantum number J specifies the total (not the rotational) angular

momentum in the molecule. The quantum number M specifies the projection of the total angular

momentum along some laboratory fixed Z axis, and takes on the values J, J — 1, . . ., —J. The

parameter fi, which helps to characterize the rotational wave functions of a diatomic molecule,

is somewhat peculiar. It is convenient to consider the parameter fl in the rotational wave functions

to be the quantum number associated with the projection of the total angular momentum J along

the internuclear axis. Table 1 indicates that the projection of J along the axis is actually equal to the projection of L + S, i.e., to the quantity represented in the nonrotating molecule by the symbol

fl. (Hence the rule J ^ \Cl\.) Actually, however, fi is not a quantum number for the rotational wave

functions, since it does not correspond to an eigenvalue of some operator acting on the rotational wave functions. It arises in the rotational problem because the expression for the differential operator (1-10) [3] (p. 13) contains the operator L^ + Sz, and this latter operator, when acting on a nonrotating molecule basis set function, can be replaced by ft. Basis set functions for the complete problem consist of products of the basis set functions for the nonrotational problem and basis set functions for the rotational problem. Such functions are represented by one of the symbols:

\n; ajM) = \n)\njM)

\AS^; njM} = \ASl}\flJM) (1-12)

\LAS1; njM) = IIASS) \njM}.

The quantities fl, J, M must be integers for molecules with an even number of electrons and half- integers for molecules with an odd number of electrons.

1.8. Rotating-Molecule Matrix Elements

Most of the matrix elements of the rotational Hamiltonian (1-10) can be obtained from general

considerations of the matrix elements of an angular momentum operator in a basis set characterized by a quantum number specifying the total magnitude of the angular momentum and a quantum number specifying the projection along the z axis [5] (pp. 103-109), [7] (pp. 45-78). For ex- ample, the only nonvanishing matrix elements of the components of the spin angular momentum

operator S in such a basis set are the following

(S2|S-|SS)=^-S(S+1)

<SS|S.|SS) = ^S (1-13)

(s ^±\\s^\sx) = h [{s+x){s±x+\)yi\

8
The nonvanishing matrix elements of the components of the orbital angular momentum L can be

obtained from (1-13) by replacing S by L, and 2 by A everywhere. The nonvanishing matrix ele- ments of the total angular momentum J can be obtained from (1—13) by replacing S by J, and 2

by n everywhere, except that S± must be replaced by J+ in the third equation:

(J n±iu^iy ^) = ^[(7+^l)(y±a+l)]'/^ d-u)

This somewhat surprising difference in behavior of J from L and S is discussed by Van Vleck [8] In actual calculations, two principal kinds of complications arise. First, it is possible, as dis- cussed in section 1.5, that the angular momentum quantum numbers characterizing the basis set are not perfectly good. (Note, however, that J and fl are always good quantum numbers in the basis

sets defined above.) In that case, it is common to introduce an additional parameter in matrix

elements like (1-13) to allow for the fact that eqs (1-13) are not exact. For example, part of the

third equation of (1-13) might be written

(S S + l|S.|S2) = (l-y)^[(S-S)(S + X + l)]'/^ (1-15)

where y is a small dimensionless parameter much less than unity. This parameter can in principle

be determined from accurate electronic wave functions. In practice it is usually determined by

fitting the final calculated energy expressions to observed levels. The second complication arises because sometimes a projection quantum number is used to characterize the basis set while the total-magnitude quantum number is not, e.g., the quantum

number L is missing in the basis set |A52; ClJM). Under these circumstances, the first and third

equations of (1—13) cannot be used to obtain the matrix elements of the angular momentum con- cerned. Nevertheless, and this wiU be important below, as long as the projection quantum number

is good, the selection rules on it implied by (1-13) for the components of an angular momentum

operator are still valid [7] (pp. 45-78). Matrix elements which cannot be obtained from (1-13) are usually represented by a parameter, which either drops out of the calculation, or is determined from

a fit to experimental data. Symmetry arguments (see chapter 2) are helpful in such cases to deter- mine exactly how many different parameters can (or must) be used.

To illustrate the procedures described above we now consider two examples.

1.9. Example: The Hill and Van Vleck Expression for -0 States

The first example involves obtaining rotational energy expressions for a ^11 state of a diatomic

molecule [10], which is well separated from other electronic states, and in which the spin-orbit

splitting is small compared to such electronic separations. The rotational energy levels of such

an electronic state correspond to Hund's case (a), Hund's case (b), or some intermediate case between these two. The wave functions of the nonrotating molecule are represented by |ASS), where the quantum

numbers A, S, S are all slightly bad. The quantum number fl = A + S, however, is still good. The

basis set functions for the complete (nonrotating plus rotating) problem are represented by

\ASX; ClJM). Since we are dealing with a ^11 state, A = ± 1 and S = V2. Since S= V2, S takes on the

values ± V2. The quantum number M does not enter into the calculation of rotational energy levels

for molecules unperturbed by the presence of external electric or magnetic fields. Hence, it wiU

be ignored (even though the quantum number M will be carried along in the notation). The quantum

number J will not be assigned a numerical value, since we are interested in the energy levels as a function of J. Basis set functions of interest for this problem thus have one of the following four

forms: |1 V2 V2; ^/a JM), | 1 V2 -V2; V2 JM), |-1 V2 -V2; -3/2 JM), or |-1 V2 V2; -V2 JM).

' Note that Van Vleck's paper deals with nonlinear molecules. Because of the absence of the third Eulerian angle in linear molecules, his arguments must be

altered somewhat for diatomic molecules. No elegant discussion of this problem exists [9]. Note also, that we have ignored in eq (1~14) the quantum number Af occurring in the wave functions \ClJM), since the operators 7+, J-, and are all diagonal in M and independent of M.

9
We must now set up the matrix of the Hamihonian in the basis set given above. Let us repre- sent a typical matrix element by the expression

(A'S'X'; D.'J'M'\H\hS%; njM) = (A'S'2'; n'J'M'\Hev\ASl; OyM) + (A'S'2'; fl'J'M'\Hr\ASl; njM). (1-16)

The complete Hamiltonian for a molecule in the absence of external fields is always diagonal in the quantum numbers / and M. Thus, for the problem under consideration, the matrix element

given on the left of (1-16) vanishes unless 7' =J and M' = M.

It can easily be seen, that if we restrict ourselves to matrix elements diagonal in / and M, and

to the limited basis set appropriate for the rotational energy levels of a -11 electronic state (A = ± 1, S = V2, S = ± V2), then the secular equation which must be solved to determine rotational energy

levels is obtained from a Hamiltonian matrix having four rows and four columns. These rows and

columns are labeled by the four basis set functions given just before eq (1—16). The elements of

this 4X 4 matrix are calculated as follows. Consider first the matrix elements of Hev. The quantum numbers of the basis set have been

divided by a semicolon into two groups, reflecting the factorization of (1-12). Because of this factorization, matrix elements of Hev must be diagonal in the three rotational quantum numbers

ilJM. Values for matrix elements of Her diagonal in these three quantum numbers can be obtained

from eqs (1-7) above. If we label rows and columns of a 4 X 4 matrix by the four functions given

just before eq (1—16), then we find that the matrix of Hev has the form

E+y2A

0

0

0

0

0

0

0

0

0

0

0

E-y2A

(1-17)

The parameter E repref f nts the constant given in the third line of (1—7), and ± V2/4 represents the

spin splitting. We observe that only two distinct valines for the energy occur, namely E+y2A and

E — '^/iA. This is consistent with the general phenomenon that energy levels in the nonrotating

molecule characterized by fl # 0 are doubly degenerate, the two degenerate functions having equal

and opposite values of fl (in this case fl = ±^/2 and fi = ± V2). To be perfectly general we should

allow the energy pattern to vary somewhat from that determined on the basis of simple spin-orbit

interaction. However, in this case there is really only one relevant energy parameter in the nonrotating molecule, namely the distance between the fl = ±^/2 and the n = ±V2 states. This one

energy separation can be described completely by the single parameter A. Matrix elements of the rotational Hamiltonian (1-11) in a basis set consisting of the four functions labeling the matrix (1-17) can be obtained as foUows. The operators 5(7' —71) , 5(S- — S|)

and — B{J+S- -\- J-S+) involve components of angular momenta for which the total magnitude

quantum number and the z-axis projection quantum number characterize the basis set. Consequently, their matrix elements can be obtained immediately from (1-13)."* Each of the operators

+ B{L+S- + L-S+) and — B{J+L- + J-L+) contains as a factor one of the quantities Z,+. The operators L+ have nonvanishing matrix elements only if the selection rule AA = ± 1 is satisfied. (This

selection rule applies to L± whenever A is a good quantum number, whether or not L is a good quantum number.) Since the limited basis set appropriate for the rotational energy levels of a ^H state only contains functions characterized by A = ±l, it is not possible to construct matrix elements

satisfying the selection rules AA = ±1 within this basis set. Thus, all matrix elements of L±, and

* Actually, both S and 2 are slightly bad quantum numbers, but the results originally given by Hill and Van Vleck did not allow for this. The fact that S and X are

not perfectly good quantum numbers can be taken into account by the introduction of small parameters y analogous to the y in eq (1-15). 10
hence of the two operators + B{L+S--\- L-.S+) and — B{J+L-+ J-L+) , vanish within this basis set. The remaining operator in (1-11), i.e., B{L- — L'i), is more difficuh to deal with. From the state- ments in section 1.8, this operator has only diagonal matrix elements in the limited basis set under

consideration. It can be shown (see chapter 2) that these four diagonal matrix elements are all equal. They are often represented by the symbol B{L-^).

We now write down the matric of Hr analogous to the matrix of Her given in (1-17). When

J ^ it has the form

'BUU+i)-y,+ {Li)], -B[U-y2)U+^/2)V'\ 0 0 ~

-B[u-y2)u+^/2)y'\B[ju+i) + y4+{Li)], o o

0 0 fi[y(7+i)-v4+ (Li)], -fi[(y-V2)(y+3/2)]'/2

_ 0 0 -fi[(y-V2)(y+3/2)]l/^fi[J(y+l) + V4+(Li)^

(1-18)

The desired rotational energy levels are found by solving the secular equation obtained from the sum of the matrices foxHev and Hr. It can be seen that the sum of (1-17) and (1-18), i.e., Hev + Hr,

contains two identical diagonal blocks. Individual diagonal blocks in a Hamiltonian matrix can

always be diagonalized separately. Thus, we need only solve two secular equations of order two,

rather than one secular equation of order four. Furthermore, these two secular equations are

identical, so that the resultant energy levels occur in doubly degenerate pairs. The rotational

energies obtained (twice each) from the secular equations are £ + fi(I2) + fi[(y + V2)--l] + V2[^(^-4fi)+45-(y+V2)-]i/2

(1-19)

E + B{~Li) + B[U + y2)-- I] -y2[A {A 45-' (J + V2) ^]

(The algebraic operr'tions necessary in obtaining (1—19) are simplified if one-half the trace is subtracted from each 2x2 Matrix before diagonalizing it. This same quantity must then, of course, be added to the roots obtain^^d from the secular equation.) Since the four basis set functions which

label the rows and columns of (1-17) and (1-18) represent all the basis set functions for a given

value of J, the two (doubly degenerate) energy levels given in (1-19) represent all the energy levels belonging to that value of J. The first two (J-independent) terms in (1-19) are often ignored in discussions of ^11 rotational energy' levels. When J — V2 the basis set functions labeling the first and third rows and columns of (1-17) and

(1-18) do not exist. For this ] value the Hamiltonian matrix factors into two identical 1 X 1 matrices,

giving rise to a doubly degenerate energy level at E—ViA ¥B{L\)-\-B[JU-^^) + V4].

Limiting Hund's case (a) and case (b) behavior can be obtained by expanding the square roots

in (1-19) as power series. Consider first case (a) behavior, which occurs, from a mathematical point

of view, when \A\>BJ. If \A\^BJ, it is convenient to approximate the radical in (1-19) as follows.

[J (^ - 4fi ) + 4fi2 (7 + V2 ) - ] = 2 [ 1 - 4fiM + ( 4fi2/^ -

) (7 + V2 ) 2

]

}

= M I { 1 + V2 (- ^BjA ) + 1/2 mVA')U+ V2 )

- - Vs (- 4fiM Y' + OiBIA)^} (1-20) = |^|{l-2flM + V2(45'V^^) [(7+ 1/2)^-1]}.

11
The approximation (1-20) to the radical must now be substituted into (1-19). Since we must

both add and subtract (1-20), it is convenient at this time to replace \A\ by A. The same two

energy levels will be obtained by adding and subtracting \A \ as by adding and subtracting A

(though not necessarily in the same order). Keeping only terms through order B/A, and dropping

the first two terms in (1-19), we obtain the two energy levels + V2^ + 5[y(y+l)-V4] (l-21a)

-V2^+5[y(y+i) + V4], (i-2ib)

which, apart from an extra + V2fi arising from B{Sl) , agree with the famihar [1] (p. 220) Hund's

case (a) energy level expression B[JU+l) —^~]-^ If we retain also the terms of order B'^/A'^ in (1-20), we see that the coefficient of J{J+ 1) in eqs (l-21a) and (l-21b), i.e., the effective B value,

must be replaced by Bil + B/A) and Bil — B/A) respectively. This is also a well-known Hund's

case (a) result [1] (p. 233).

Hund's case (b) occurs, from a mathematical point of view, when BJ$>\A\. Pure Hund's case

(b) occurs when ^ = 0. If ^ = 0, the last two terms of eqs (1-19) can be written

B[{J + y2) (J + ^/2)-l] (l-22a)

fi[(y-V2)(y+V2)-l]. (l-22b)

Both of these equations are of the form 5[A^(j'V+ 1) — 1]. It is thus convenient to introduce an

integer N, which is equal to 7+ V2 for the higher energy level of given J, and equal to 7 — V2 for the lower energy level. Equations (1-22) then have the form of the familiar [1] (pp. 221-224) Hund's

case (b) expression fi[A^(A^+ 1) - A^].

The significance of the quantum number A'^, which has arisen here in a somewhat formal way, can best be understood by examining the eigenfunctions of the matrix sum (1-17) +(1—18) when

A = 0, i.e., the case (b) eigenfunctions. The two eigenfunctions obtained by diagonalizing the 2X2

submatrix in the upper lefthand corner of (1-17) +(1—18) when A — O are given on the righthand

side of (1-23) as linear combinations of the basis set functions |ASS; flJM) used to label the rows and columns of (1-17) and (1-18).

1 + 1 V2 A=i+V2 yM)-+[(y-V2)/(2y+i)]'/2| + i +i/^. + 3/^ jm^

-[U + ^/2)l{2J+l)yi^ + l 1/2 -V2; + V27M) .

1 + 1 V2 A=y-V2 yM)=+[(i + =^/2)/(27+l)]'/2| + l V2 +V2;+3/27M)

^^"^^^ + [(y-V2)/(2y+l)]i/-'| + l 1/2 -V2; + V2yM)

It can be seen that the two basis set functions in a given linear combination are characterized by

the same values of A, S, J, and M, but by different values of 1 and fl. For this reason we say that A, S, y, and M are good quantum numbers in Hund's case (b), but that X and ft are not. Although the linear combinations of functions given on the righthand side of (1—23) are not eigenfunctions of S^ and Jz, they do happen to be eigenfunctions of the operator (J — Sf, belonging

to an eigenvalue h'^N{N-\- 1) . This can be demonstrated relatively easily. Since the functions

I ASS; ftJM) are eigenfunctions of 7^ Jz, S^, the expressions (1-13) and (1-14) can be used to determine the effect of (J — S) - on these basis set functions and hence on any linear combination

^ Hund's case (a) energy levels are obtained if all terms other than the first in (1-11) are ignored. Since we are interested here in studying a transition between

Hund's case (a) and Hund's case (b), only the fourth and fifth terms in (1-11) were ignored in deriving (1-19). It is the third term in (1-11 ), i.e., ii(S'^ — SJ), which

contributes the + ViB present in (1-21) but normally absent in Hund's case (a) energy level expressions.

12
of them. Because the case (b) wave functions (1-23) belong to the eigenvalue h'^N{N+ 1) of the operator (J — S)-, we say that is a good quantum number in case (b).

It is sometimes convenient to label the eigenfunctions represented by the linear combinations on the righthand side of (1-23) by a single label of the form \ ASNJM}. These labels are found on the

lefthand side of (1-23). The absence of a semicolon indicates that the functions \ASNJM} cannot

be factored into a nonrotating-molecule part and a rotating-molecule part.

Some authors also find it convenient to use functions of the form \ASNJM) as basis set functions for diatomic molecule calculations. In this monograph we have chosen always to consider the

nonrotating-molecule part of the problem separately from the rotating-molecule part. This decision

requires us to use a basis set in which Cl and 2, or perhaps just fl, are defined, and in which A'^is not defined. Wave functions in which A'^ is defined will therefore appear to arise here somewhat

arbitrarily as linear combinations of the basis set functions, the linear combinations being obtained

by diagonalizing appropriate Hamiltonian matrices.

1.10. Example: The Schlapp Expression for ^'^ States

The calculation of rotational energy levels for a ^2 state [11] proceeds in much the same fashion

as for -n states. The basis set functions for the complete problem are represented by |ASS; flJM).

Since we are deahng with a ^2 state, A = 0 and S=l. Since S = l, S takes on the values + 1, 0, — 1.^ Note that, even though the spin projection quantum number S will not be a good quantum

number in the final ^1. wave functions, it is perfectly acceptable, and indeed, from the point of

view of the author, desirable, to use a basis set in which 2 is a good quantum number. Basis set functions for the problem thus have one of the following forms: 1 0 1 1; 1 y M). | 0 1 0; 0 7 M), or

|0 1 — 1; — 1 y M) . Fory = 0 only the second function exists, of course. The matrix elements of the Hamiltonian in this basis set can again be represented by (1-16).

Since nonvanishing matrix elements of H are again diagonal in J and M, the matrix used to deter- mine rotational energy levels for a ^2 electronic state (A = 0, S= 1, 2 = ± 1, 0) has three rows and

columns, labeled by the basis set functions given at the end of the preceeding paragraph.

Consider now the matrix elements of Hev If we employ the same reasoning used in obtaining

the energies given in (1—17), we find that all three components of the nonrotating-molecule ^2 state He at the same energy. (There is no first order spin-orbit interaction AAX in 2 electronic states because A = 0.) Nevertheless, states with different values of (I do have different energies [12], except that pairs of states related to each other by a change in sign of both A and 2 are degenerate.

From these considerations, we find that the matrix of Hev is given by

E 0 0

0 E-2K 0

0 0 E

(1-24)

The two states with n = ± 1 have been given the energy E and the state with Cl — 0 has been given

the energy E — 2k. Actually, in this problem, as in the '^11 problem, there is only one relevant

energy separation. That separation is represented here by the quantity 2A.. (Experimental values

for k in the '^2 ground state of several molecules are given in table VI of [12]). Matrix elements of the rotational Hamiltonian (1-11) can be obtained from considerations

identical to those for a ^11 state. The matrix oi Hr fory s= 1 is thus

-B[2JU+l)yi'

0

-fl[2y(y+i)]'/^

BJU+l)+2B + B(Ll)

-B[2JU + l)V'^

0

-fi[2y(y+i)]'/^

BJU+l)+B{Ll)

(1-25)

* Standard spectroscopic notation leads to a little confusion here, since the symbol S represents both an electronic stale characterized by A = 0, e.g., a state, and

also the projection of S along the intemuclear axis, e.g., x=±i,o.

13

371-908 O - 70 - 3
where we have again used arguments (see eh. 2) to conclude that the matrix elements of B{L^ — Ll)

in the basis set under consideration are all equal.

Unfortunately the sum of (1-24) and (1-25) does not immediately factor into any smaller

diagonal blocks, so we must apparently find the roots of a cubic secular equation. However, there

is one simplifying procedure which has not yet been employed. For reasons of symmetry, the size of the secular determinant for rotational energy levels can often be halved by working with the

basis set functions 2-^'^[\ASX; njM)±\—\ S — —ftJM)] rather than with the individual

functions \AS1>;CIJM) themselves. In this connection we note that

(2-i/2(a + 6) \H\2-'lHc + d)) = V2[{a|//|c) + (a\H\d) + {b\H\c) + {b\H\d)]. (1-26)

Let us consider the matrices of Hei and H, in a new basis set consisting of 2~*'^[|0 1 1; 1 J M)

+ |0 1 -1;-1 y M)], |0 1 0; Oy M),and2-i/2[|0 1 1; 1 J M) - |0 1 - 1; - 1 7 M)] . If the three rows and columns of the Hamiltonian matrix are labeled by these functions, we find that the matrix

of Hev is identical to that given in (1-24), but that the matrix of Hr has the form

BJU+l)+ B{Ll ) -2BUU+1)]

-2B[JU+l)y" BJU+l)+2B + B{Li)

0 0

0

0

BJU+l)+B{Ll)

(1-27)

The sum of the matrices (1—24) and (1-27), i.e.,Hev + Hr, now factors into a 2 X 2 diagonal block and

a 1 X 1 diagonal block.

Before actually determining the rotational energy levels, we consider one further change in the Hamiltonian matrix. As mentioned above, the matrix of Hei contains a single important adjustable parameter, corresponding to the single energy difference present in the nonrotating-molecule

problem. On the other hand, the matrix elements of Hr should contain more adjustable parameters

than the one, i.e., B, which actually occurs in (1-25). Strictly speaking, for example, there should

be three different values for B in (1-25). Because the internuclear distance wiU be slightly different

for the states with n = ± 1 than it is for the state with fl = 0, one value for B occurs in the matrix

positions (1, 1) and (3, 3), one in the position (2, 2), and one in the off-diagonal positions (1, 2), (2, 1),

(2, 3), and (3, 2). (The fact that differences in internuclear distance lead to three and only three

values for B in (1—25) can be shown from symmetry considerations, as indicated in chapter 2.) In addition to the internuclear distance effect, all matrix elements of operators involving S, which

were evaluated numerically from (1-13)' using S=l and S 1, — 1, or 0. are slightly wrong, since

S and 1 are not quite good quantum numbers. Since these matrix elements were evaluated numerically, they are somewhat difficult to find in (1-25), but the 2'''' occuring in the off-diagonal matrix

elements, for example, is not exactly 2^1^, because S and S are not exactly good quantum numbers.

Finally, {L'^) has one value for the state with fi = 0, and a slightly different value for the states with n = ±l. Unfortunately, the extent of the various deviations mentioned above can only be

quantitatively determined at the present time from experiment. For this reason it is common to introduce, in some way, additional parameters in (1-25), which are to be determined by fitting the

experimental data. Since B is the only parameter occurring in (1-25), and since each matrix element

contains B, it is possible to allow for all of the above-mentioned problems in a purely formal way by

replacing the single parameter 5 by a set of B^fis. It can be shown by symmetry arguments (see

chap. 2) that a maximum of seven fieff's could be used: three of which would occur in the positions

(1, 1) and (3, 3); three of which would occur in the position (2, 2); and one of which would occur

in the off-diagonal positions. (A single B^tf in the off-diagonal positions of (1-25) corresponds to a

single fieff in the off-diagonal positions of (1-27).)

14
We shall use in our calculations one value for B on the diagonal in (1-25) or (1-27), and one

1 value for B, written as 5 — V27, off the diagonal. Such a decision leads ultimately to Schlapp's

expression [11], and can be justified as follows. When 2k in (1-24) is very small compared to electronic energies (less than 30 cm-i, say), then the difference in equiUbrium internuclear distance

for the two multiplet components of a ^2 state is expected to be very small, and only one coefficient

for 7(7+1) is needed on the diagonal of (1-25). On the other hand, contamination of the ^2 state by other electronic states through spin-orbit interaction will cause the matrix elements of the

components of S to deviate perceptibly from those given in (1-13). On the diagonal, such deviations,

as well as the differences in B{L\), can be allowed for by adjusting the parameter X. Off the

diagonal, such deviations can be allowed for by adjusting the parameter B.

In any case, the secular equation obtained from the sum of (1-24) and (1-27) modified as de- scribed above leads to the following energies for the three states of given 7^1:

£+5(11) +57(7+1)

(1-28)

£ + 5(Li)+57(7+l) + (fi-X)± [(5-A)2+ (fi- ¥27)247(7+ 1)]'/^

For 7 = 0 the energy is E-2k + BJU +1) +2B + B{Ll) . The last hne of (1-28) can be rearranged

to give Schlapp's expressions by expanding the square root as follows.

[{B-\)^+ (5-V2r)247(7 + l)]'/2= [A2-25A + 52(27+l)2-47S7(7+l)]'/2 = [X2-2fiX + fi2(27+ 1)2] 1/2(1 -27^7 (7+ l)/[X2-2fiX + 52(27+ 1)2]} (1-29) = [X2-25A + fi2(27+l)2]l/2-y(7+V2).

(The first approximate equality in (1-29) indicates the loss of terms in 72. The second approximate

equahty indicates a power series expansion of the radical. The third approximate equality represents

the case (b) approximation BJ > \k\ for the small 7-dependent term.) Dropping the first two {Jindependent) terms in (1-28) and making the substitutions J = N, J = N—l, and J = N+l in the

three energy expressions given in (1-28) and in (1-29), we obtain Schlapp's expressions

BNiN+l)

BN{N+l)-{2N-l)B-k+[k^-2Bk + BH2N-iyyi^-yN+V2y (1-30)

BN{N+l) + i2N + 3)B-k-[k^-2Bk + BH2N+3)^'i^ + y{N+\) + y2y,

apart from the small 7-independent term + V27. (A^^ is assigned to the three levels of given J such

that the level with N= J+ 1 has the highest energy and the level with N= J — \ has the lowest.

It is interesting to note that the parameter 7, which was introduced here to allow for some

slight discrepancies in matrix elements of the spin operators, is generally introduced as a coupUng

parameter between the vectors iV and S [11]. For ^1 and ^1 states, there is only one such parameter

7, and the two view points lead to identical results. For states, however, the present approach

leads in a natural way to the introduction of two parameters 7 [13], whereas the vector couphng

approach appears incorrectly still to require only one.

2. Symmetry Properties of the Rotational Energy Levels

In chapter 2 we discuss the symmetry properties of the rotational energy levels of diatomic

molecules [1] (pp. 237-240). The rotational levels of all diatomic molecules can be classified as + or — according to their parity, i.e., according to the behavior of the complete molecular wave

15
function (apart from translation) when the laboratory-fixed Cartesian coordinates of all particles are replaced by their negatives. The rotational energy levels of homonuclear diatomic molecules can be classified in addition as 5 (symmetric) or a (antisymmetric) with respect to permutation of

identical nuclei. The symmetry designations +, —, 5, a are very important, since perturbations and

optical transitions between two rotational levels are limited by selection rules on these quantities.

It turns out that the two permutation-inversion symmetry operations mentioned in the pre- ceding paragraph, i.e., the laboratory-fixed inversion operation/, and the permutation P of identical

nuclei in a homonuclear diatomic molecule, are related [14^16] to the geometric symmetry operations found in the usual group theory tables [6] (pp. 312-340), i.e., rotation, reflection, inversion,

and rotation-reflection operations. It is convenient at this time to consider these two kinds of

symmetry operations in more detail.

2.1. Geometric Symmetry Operations

Heteronuclear diatomic molecules belong to the point group C^^v This point group [6] (p. 330)

contains as symmetry elements the identity (E), one infinite-fold rotation axis (Coo), and an in- finite number of reflection planes ((Ti ) containing the rotation axis. Homonuclear diatomic molecules belong to the point group D^^h- This point group [6] (p. 330) contains, in addition to the

symmetry elements found in C^^v, an inversion center (i), one infinite-fold rotation-reflection axis

(Soo), and an infinite number of twofold rotation axes (C2) perpendicular to the Coo axis.

It is relatively easy to visualize the effect of these geometric symmetry operations on an

electron belonging to the diatomic molecule, and to determine in this way the precise effect of

these geometric symmetry operations on the coordinates of the electron. Let

Xe = Pe sin Be COS (fe

Ye^Pe sin de sin ipe (2-1)

Ze = Pe COS de

be the Cartesian coordinates and spherical polar coordinates of an electron in an axis system fixed

in the diatomic molecule such that the internuclear axis is the 2 axis. Table 3 indicates the new

coordinates of this electron, after it has been subjected to representative symmetry operations

of the point groups Coon and Dr^h- When one of these symmetry operations acts on a function containing the electron coordinates, its effect is to replace, everywhere in the function, each coordinate

by the quantity found in table 3 at the intersection of the appropriate row and column. For example,

C%{z) i C2{y) /{. . . , (fe, . .) = C%,{z) i f{ . . .,TT — (Pe, )

. . =C%{z)-f{ . . .,W-{7T + ^e), . . .)=/{ . . .,-<Pe-e,. . .). (2-2)

Table 3. The effect of various symmetry operations on electron coordinates

Symmetry

Coordinates acted upon

operation

Xe ye Ze Pe 0e

E Xf ye Pe ft. if,. Xe cos e — y,. sin e ye COS e + Xe sin e + z,. Pe + ft. <Pe + e + Xe -y<- + z,. P, + 0e

i — X,. -ye — Ze Pe 77- -ft. n+ ipe S%(z) Xe COS e — ye sin e ye COS e + Xe sin e — Ze Pe TT-de (pe + € Cs (y) —Xe +ye — Ze Pe TT-de TT-iPe

16
These geometric symmetry operations also act on the vibrational and rotational variables of a diatomic molecule. The transformations of the vibrational variable can be obtained by considering

the effect of the various symmetry operations on the vibrational displacement vectors. Pictorially

speaking, these displacement vectors are subjected to the various symmetry operations while the

molecular framework, i.e., the equilibrium position of each atom in the molecule, is left unchanged

[6] (pp. 77-101). Table 4 indicates quantitatively the effect of representative symmetry operations

of Cxi- and D^h on arbitrary displacement vectors (di , do ) for the two nuclei of a diatomic molecule.

The vibrational variable for diatomic molecules is usually taken to be the internuclear distance r, given by | (ai + di ) — (a^ + d^) |, where a\ and 32 represent equilibrium positions for the two atoms. Since the equihbrium positions a, lie on the z axis, and since they are not changed in value by any

of the symmetry operations, it is easy to show that the transformations of table 4 leave the vibra- tional variable r unaltered.

Table 4. The effect of various symmetry operations on nuclear displacement vectors

Displacement vector component acted upon

operation

d,y d\z diy diz

E dii d\y d\z d.jr diy diz d\x cos e — d\y sin e djy COS e + d,x sin e + d,. dii cos e — diy sin e diy COS €-\-dix sin e + d2z + <fix — d,y + d,. + d,j: — diy + diz

i — dzy -d,z — d]x — d\y -du

d.j- cos 6 — d:y sin e diy COS i + dzx sin e -d,z d,s cos e — d,y sin e d\y COS (. + d\x sin e — du

CAy) + d,y -d.,z — d,x -du

The effect of geometric symmetry operations on rotational variables is not as obvious as the

effect on electronic and vibrational variables. The rotational variables of a molecule actually

represent Eulerian angles, indicating how the (right-handed) molecule-fixed axis system is rotated

with respect to some (right-handed) laboratory-fixed axis system [6] (pp. 285-286). For every

orientation of the molecule-fixed axis system there is a set of corresponding Eulerian angles. For

every set of Eulerian angles there is a corresponding orientation of the molecule-fixed axis system.

It thus seems intuitively obvious that a geometric symmetry operation corresponding to a pure

rotation should effect a change in the Eulerian angles corresponding to that pure rotation. It

is not possible, however, to represent the change from a right-handed to a left-handed axis system

by a set of Eulerian angles. Consequently, it is not intuitively obvious how sense-reversing symmetry operations (reflections, inversion, rotation-reflections) should affect the Eulerian angles.

It turns out that a consistent and useful scheme of geometric symmetry operations can be obtained

if a sense-reversing operation is defined to have the same effect on the Eulerian angles as does

the pure rotation obtained from the sense-reversing operation by multiplication by the inversion

[15, 16]. According to this prescription, i and E have the same effect on the rotational variables;

0"i (jcz) and C-ziy) have the same effect; and S^(z) and C^+'(2) have the same effect. The rotational variables of linear molecules present an additional complication [16], since

they consist of two Eulerian angles rather than three [3] (pp. 6-16). Because the third Eulerian

angle is missing, it is not possible to carry out the operation C'^(z) on the rotational variables.

Thus, this operation and 5^(z), both of which were used in classifying electronic and vibrational

levels, cannot be used in classifying rotational energy levels. The remaining symmetry operations

are E, cTi , i,.and Co. Of these, we need only investigate the effect of C-z on the rotational variables.

Note added in proof: A recently pubUshed article by Bunker and Papousek [27] presents a more

sophisticated discussion of the complications associated with symmetry operations for linear

molecules than does reference [16]. The rotational variables 6 and <p represent for a diatomic molecule the polar and azimuthal

angles of the internuclear axis in the laboratory-fixed axis system. Since C>(y) corresponds to a twofold rotation about an axis perpendicular to the internuclear axis, this symmetry operation

17
reverses the direction of the internuclear axis. Thus, Ci (y) acting on a function of the rotational

variables replaces 6 hy tt — 6 and v? by 7r + everyivhere in the function. The functions of the rota- tional variables most often considered, of course, are the rotational basis set functions \D,JM)

themselves. Explicit expressions for these functions, corresponding precisely to the phase conventions used in this monograph, can be obtained by setting « = 7r/2. fi = d, y = (p, J , /Lt' = fi and

tx = M m the quantity [(27+ 1 )/47r] ''^ • S'^Hja/Sy})^'^, where ^'^j^iia^y}) ^.-^ is given in eq (15.27)

of Wigner's book on group theory [17]. In this same connection, the 3X3 rotation matrices used

in eqs (2-3) below can be obtained by setting x = 7r/2, 9=6, and(/7 = (^in Appendix I of [6]. The effect of Ci{y) on the polar and azimuthal angles of the internuclear axis, as defined

above, is different from the effect of Ct{y) on the polar and azimuthal angles of an electron, as given in table 3. This difference arises because Cz{y) represents a rotation about an axis which

is always perpendicular to the internuclear axis, but which is not in general perpendicular to the

position vector from the origin to a given electron.

2.2. Permutation-Inversion Symmetry Operations

The relation between the geometric symmetry operations of section 2.1 and the permutationinversion symmetry operations can be demonstrated most easily by considering an equation re- lating the laboratory-fixed Cartesian coordinates {XiYiZi) of the electrons and nuclei in a diatomic

molecule to the molecule-fixed electronic coordinates {xyz), the equilibrium positions of the nuclei,

the displacement vectors d, of the nuclei, and the two rotational angles 6, (p [15, 16].

~x7 — sin — cos 6 cos ip sin 6 cos (p

cos — cos 6 sin ^p sin 6 sin ip Yv

0 sin 6 cos 6 — sin — cos 6 cos ip sin 6 cos ip

F, cos ^p — cos 6 sin <p sin 6 sin </? dly

z, 0 sin 6 cos 6 — i/Jilm: )re + diz

x> — sin (f — cos 6 cos <p sin 6 cos ip dzx

Yi cos <p — cos 6 sin tp sin 6 sin ip d>y

0 sin 6 cos 6 + (

fJil Tn->) re + d>z

The left-hand sides of eqs (2-3) contain the Cartesian coordinates of an electon and the two nuclei

in an axis system parallel to that fixed in the laboratory, but located at the center of mass of the

equilibrium configuration of the nuclei. The righthand sides of eqs (2—3) all contain a 3 X 3 rotation

matrix, the direction cosine matrix, which transforms vector components from a molecule-fixed

axis system to a laboratory-fixed axis system. This matrix is a function of the rotational variables

6 and ip, specifying the direction of the internuclear axis of the diatomic molecule in the laboratoryfixed axis system. The column vector on the far right in the first of eqs (2-3) contains the moleculefixed coordinates of an electron. The second and third column vectors on the far right contain the

positions of the two nuclei in the molecule-fixed axis system. At equilibrium (di=d2 = 0) both

nuclei lie on the z axis, with the center of mass at the origin, and with internuclear distance re; m^m-,/ (m^ + m-z) is the reduced mass of the molecule.

18
Consider now the effect of the four symmetry operations E, CTi ixz) , i, and d (y) on the coordi- nates in eqs (2-3). The transformations of the coordinates on the right side of (2-3) can be obtained

from tables 3 and 4 and from the text of section 2.1. It is fairly easy to show from (2-3) that these

operations give rise to the transformations of laboratory-fixed coordinates shown in table 5. From

table 5 we see that the geometric symmetry operation av(xz), when it is applied to the electronic,

vibrational, and rotational (i.e., to all) variables, is equivalent to the laboratory-fixed inversion operation /; and that the geometric symmetry operation C-ziy), when it is applied to the electronic,

vibrational, and rotational variables, is equivalent to the permutation P of the two (identical) nuclei

in the molecule. From table 5 we also see that the geometric symmetry operation i is equivalent to the product P • I, i.e., to the combined permutation and laboratory-fixed inversion operation.

Table 5. The effect of various symmetry operations on laboratory-fixed Cartesian coordinates of

the electrons and of the two nuclei

Symmetry

Coordinates acted upon

operation Xe Ye Ze X, y, z. X-, Y. z,

E Xe Ye Ze X, Y, z, X-, Y. z,

(Tr(xz) -Xe -Ye -Ze -X, -Y, -z, -X-, -Y-, -z.

i -Xe -Ye -Ze -X-> -Y, -Zi -X, -Y, -z,

CAy) +Xe + Ye + Ze +x. + Y-> +z. +x, + Y, +z,

It is evidently necessary to distinguish clearly between the "molecule-fixed" inversion operation i, and the "laboratory-fixed" inversion operation /, since these two operations are not equivalent. In particular, i exists only if the diatomic molecule is homonuclear, whereas / exists for aU

diatomic molecules. The precise difference between these two inversion operations can only be

understood after some study [14, 15, 16].

Rotational energy levels are said to be of even (+) parity if the corresponding complete molecular wave functions are invariant to the laboratory-fixed inversion operation /; they are of odd (—

)

parity if the wave functions transform into their negatives. Rotational energy levels of homonuclear

diatomic molecules are said to be symmetric (5) if the corresponding complete molecular wave

functions are invariant to the exchange of identical nuclei P; they are antisymmetric (a) if the wave functions transform into their negatives.

2.3. The Symmetry Operation cr,

As a further aid in understanding the geometric symmetry operation ct;,, let us consider its effect on several electronic wave functions. In particular, let us consider diatomic-molecule wave

functions derived from atomic wave functions. The one-electron atomic configuration np gives

rise to an orbital P state (L = l), which in turn gives rise to diatomic-molecule orbital X(A = 0)

and n(A = ±l) states. The wave functions for these diatomic-molecule states are

\p S) =+f{pe) cos de (2^)

|p n^)=+f{pe) 2-1/2 gin 0^ giiVe,

if we use the phase conventions of Condon and Shortley [7] (p. 52). These functions transform as

follows under the operation cri {xz) (see table 3)

cr,.|p S) =+|p 2)

(2-5)

o-,.|pn^)=-|pn^),

19
which can be written as o-„|LA)=-(-l)'-^|L - A). (2-6)

Equations (2-4) and (2—5) can be used to illustrate a fundamental point concerning the relation- ship between the choice of phase factors for a set of wave functions and their transformation

properties under symmetry operations. It can be seen that the first of the transformation equations

(2—5) is unaltered if in (2-4) is defined to be —f{pe) cos de rather than +fipe) cos dg. Thus

the transformation property represented by this first equation is independent of the choice of

phases, and is an intrinsic property of the S state under consideration. On the other hand, the — sign on the righthand side of the second of the transformation equations (2-5) can be changed to a + sign, simply by defining the functions |p n± ) in (2-4) to be +fipe) 2~''^ sin de e-'^^. The trans- formation property represented by this second equation is thus not an intrinsic property of the

n state, but depends on the choice of phases.

It turns out that all X electronic wave functions are characterized by an intrinsic transformation property under the operation av. Furthermore, this transformation property is customarily

indicated by a superscript attached to the S label; i.e., we define S+ and S~ states, such that

. ^ o-.|X^)=±|S±). (2-7)

It further turns out that aU doubly-degenerate orbital electronic states, i.e., those with | A| > 0, are not characterized by an intrinsic transformation property under the operation a^. Nevertheless,

explicit transformation equations for such functions can be written down once a set of phases for the wave functions has been chosen. To obtain correct answers in any calculation, of course, it is necessary that the phase choice implicit in one's transformation equations be consistent with the

phase choice implicit in one's expressions for the various matrix elements.

Consider now the two-election atomic configuration np n'p. This configuration also gives rise to an orbital P state, which in turn gives rise to diatomic-molecule orbital S and 11 states. The wave

functions for these diatomic-molecule states are

\np n'p PX)^ +2-'!'' fiipei) sin dei e-^e\f2(p^2) sin ee2e+'^e2

-2-'l'fr{pei) sin del >^ el f.,{p,^) sin ee2 e-^e2

I np n'p P n+ ) = +V2 /i (pel) COS del /2(Pe2) sin 6e2 e + '^e2 -V2/, (pel) sin 0el e+"^e'l/2(pp2) COS 0^2

I np n'p P n-)=-V2/i(pei) sin Bei e-'^ei Mpe2) cos 0^2 + V2/i(pel) COS eel/2(pe2)jsin 0e2 e-''^e2,

if we use the phase conventions of Condon and Shortley [7] (p. 76). These wave functions trans- form as follows under the symmetry operation (Ji{xz) (see table 3)

(Ji\np n' p PI,) =—\np n' p PX)

a-^\npn'pPU.±)^^\np n' p PU^), (2-9)

which can be written as o-.|LA)=+(-l)'^-^|L -A). (2-10)

We note in passing that the 2 state of (2-4) is a S+ state, while that of (2-8) is a 2^ state.

It turns out that electronic orbital wave functions |i^A) having phase factors consistent with

those of Condon and Shortley [7], and arising from atomic states of even parity, all transform

according to (2—10), while wave functions having such phase factors, and arising from atomic

states of odd parity, all transform according to (2-6). (The parity of an atomic state is determined by

its behavior under the inversion operation i in table 3.)

20
There are several complications which arise in deciding whether to use (2-6) or (2-10). First, wave functions of the type \LA} are most often used in discussing Rydberg states. Under these

circumstances, many of the electrons in the molecule are assigned to the "core" and are not con- sidered explicitly. Since the core plays the role of the atomic nucleus, one must consider the parity

of the atomic state which corresponds to the diatomic-molecule electronic wave function involving

only electrons outside the core. Second, L is often not a good quantum number, so that no particular

value of L suggests itself for use in (2-6) or (2-10). Under these circumstances one can often obtain

consistent and correct results for stated with | A| > 0 by arbitrarily giving L some value and arbitrarily

choosing one of the two relations (2—6) or (2-10) to represent the transformation properties. This

apparently casual choice of signs actually causes no difficulty. Of course, it does require a particular

phase choice for the wave functions, which must be consistent with the phase choice implicit in any matrix element expressions used. However, when L is not a good quantum number, matrix

elements involving the electronic orbital part of the wave function are not evaluated explicitly

(they are left as adjustable parameters). Hence, contradictions between matrix elements and trans- formation properties are not introduced (see sect. 2.8). Finally, the transformation properties of

2- states are determined by eq (2-7). Sometimes, however, it is convenient to incorporate a X

state into a transformation scheme utilizing eq (2-6) or (2-10). It is then necessary to correlate the

choice of sign and the choice of L to obtain the proper X state transformation properties.

Since the transformation properties of the electronic spin basis set functions |S2) and of

the rotational basis set functions \J CI) are more difficult to illustrate with simple examples than

are the transformation properties of the electronic basis set functions IZ-A), we shall merely

state the final results here: The functions |S2) and \JCl), when chosen to have phase factors

consistent with those of Condon and Shortley [7], transform like functions \LA} of even parity

[15, 16]. We can thus summarize the effect of the operation cry on the various basis set functions

described in chapter 1 by the following equations:

o-„|IA)=±(-l)^-^ 11 -A) (2-lla)

<T,\S1) = (-1)^-^ |S -2) (2-llb)

(Tv\Jm= (-!)'-" 17 -n), (2-llc)

where (2-llc) could be written more precisely as a-v\ClJM} = {—l)'~^\ — fLjM), and where

the + or — sign must be used in (2-lla) if the electronic state of the molecule correlates with a united atom state of even or odd parity, respectively. (In homonuclear diatomic molecules, this correlation is straightforward: g and u diatomic-molecule electronic states correlate with united

atom states of even and odd parity, respectively.) The transformation properties given in (2-11)

are, of course, consistent with the matrix elements given in (1-13) and (1-14). The transformation

properties of the complete basis set functions are seen from eqs (2-11) to be

o-„|L A5 X; ay M)=±(-l)^-A+«-^+^-" |L - A 5 -2; -O 7 M). (2-12)

It was pointed out in section 2.2 that when the symmetry operation (Ti,{xz) acts on a complete

basis set function (corresponding to both the nonrotational and the rotational part of the problem),

then its net effect is equivalent to that obtained when the laboratory-fixed coordinates of all the

particles in the molecule are replaced by their negatives. States of definite parity transform into themselves or into their negatives under this operation. It can easily be seen by application of

(2-12) that functions of the form \L ASl; ilj M) ±\L - A S -2; -117 M) have a definite parity.

We next consider two examples of the determination of the parity of rotational energy levels.

2.4. Example: Parities of the Rotational Levels in a ^S" State

The complete basis set functions | AS'S.;ClJ M) for a 'S" state can be written as

1 0- 0 0; 0 7 M). They transform as follows under av{xz):

o-,|0- 0 0; 0 7 M) =- (- 1)^ |0- 0 0; 0 7 M). (2-13)

Thus, the rotational levels of even 7 are of odd parity (—), while those of odd 7 are of even parity (+). 21

371-908 O - 70 - 4
2.5. Example: Parities of the Rotational Levels of a State

The rotational energy levels of a ''S state were calculated in section 1.10. The three basis set functions used to label the matrix (1-27) transform as follows when the state is a^'X + state.

o-,{2-'/^[|0+ 1 1; iyM) + |o+ 1 - l;-iyM)]} = (-i)^-H2-'/^[|o+ 1 -1; -1 y M) + |o+ 1 1; 1 y M)]} = (-i)^-H2-'/^[|o+ 1 1; 1 y M) + |o+ 1 -1; -1 y M)]}

o-„|0+ 1 0; Oy M)= (-l)-'+i|0+ 1 0; Oy M) (2-14)

o-.{2-'/^[ |0+ll;iyM)-|0+ 1 -l;-iyM)]} = (-1)^-1 {2-''^[|0+ 1 -l;-iyM)-|0+ 1 1; IJ M)]} = (-1)^{2^'/^[|0+ 11; ly M)-|0+ 1 -1; - ly M)]}.

Consequently the energy levels obtained from the upper 2X2 block of (1-27) are of odd parity

when y is even and of even parity when J is odd. The energy levels obtained from the lower righthand corner of (1—27) are of even parity when J is even and of odd parity when J is odd. Since

J = N±l for the former wave functions and J — N for the latter, rotational levels of a + state of even are of even parity; those of odd A'^ are of odd parity.

The wave functions in (2—14) all transform into themselves or into their negatives under the

operation ct;,. This is not true in general of the functions |AS2; ilJM) of the original basis set. For

this reason, the ^11 wave functions used to label the matrix (1-18) cannot be assigned a parity,

although appropriate sums and differences of such wave functions could be.

2.6. The Symmetry Operation i

The geometric symmetry operation i exists only if the diatomic molecule is homonuclear. Its effect on the various basis set wave functions of chapter 1 is quite simple [15, 16]. The operation i leaves the electronic spin functions |SS) and the rotational wave functions \J^) invariant. The

behavior of the electronic orbital wave functions under i is indicated by the subscripts g and u. For example,

i[|X,), in,), |A,), . . |n„), |A,), . . .] (2-15)

|n„), |A„), . . |n„), |A„), . . .].

It is important to recall that the molecule-fixed inversion operation i is not equivalent to the

laboratory-fixed inversion operation / (see sect. 2.2).

2.7. The Symmetry Operation C-i The symmetry operation Co also exists only if the diatomic molecule is homonuclear. Its effect on the basis set functions of chapter 1 can be determined most easily by noting that C2{y) = av{xz) -i. The effects of (Tv{xz) are given in eqs (2—11). The effects of i are given in section 2.6. We thus

conclude that the effect of C2{y) on the various basis set functions is given by the following

equations:

C2|LA) = (-1)'-A|L -A) (2-16a)

C2\SX) = {-ir-^S -1) (2-16b)

C2|ya) = (-i)'-^^|y -n).

22

(2-16c)
where (2-16c) could be written more precisely as C-zlClJM) = (— Q,JM). If L is not a good

quantum number, it is again often possible to obtain consistent answers for the symmetry properties

by arbitrarily assigning L some value. (The same value of L must be used in (2-16) as in (2-11), of course.) When A = 0, the transformation properties of the orbital part of the electronic wave

function are completely determined by the state symbol:

Co|2-)=+|X,J) C2|X,7)=-|S^)

(2-17)

C-^^u)=^\1-u) C.|2J)=-|SJ).

The transformation properties of the complete wave functions can be determined from the trans- formation properties of the individual parts as in eq (2-12) above.

It was pointed out in section 2.2 that when the symmetry operation C-iiy) acts on a complete

basis set function (corresponding to both the nonrotational and the rotational part of the problem),

then its net effect is equivalent to that obtained when the laboratory-fixed coordinates of the two

identical nuclei are permuted. Rotational states characterized as s transform into themselves under

this operation; states characterized as a transform into their negatives. We next consider an example of the determination of the parity and of the 5, a character of rotational energy levels.

2.8. Example: Symmetry Properties of the Rotational Levels in a State

The complete basis set functions for a '11 „ state can be written as sums and differences of

functions of the type |ASS; ClJM) :

2-i/2[|l„0 0; iyM)±| -1„0 0;-1 y M)]. (2-18)

Making use of (2-11) with an arbitrarily chosen value of L = 2, we find

(7,{2-'/2[|i„00; iyM)±|-l„00;-iyM)]} = T(-l)'{2-i/2[|l„00;iyM)±|-l„0 0;-iyM)]}. (2-19)

Making use of (2-16) with the same arbitrarily chosen value of L = 2, we find

C2{2-''^[|l„00;iyM)±|-l„0 0;-iyM)]} = ±(-l)'{2-'/2[|l„00; iyM)±|-l„00;-iyM)]}. (2-20)

We therefore conclude that the sum function in (2-18) is —5 for eveny and + a for oddy, and that the difference function is +a for even J and —5 for odd J. Had we arbitrarily chosen a value of L = 1 in making use of (2-11) and (2-16), we would have

concluded that the sum function was + a for even J and — s for oddy, and that the difference function was — s for even J and + a for oddy. For both values of L, we thus conclude that the rotational

levels occur in pairs for given y, one member of the pair being + a, the other being — s. Whether

the sum or difference function in (2-18) is +a for given y actually depends on our choice of phases

for the two functions | ±l„0 0;±iyM). In the absence of further computations with these wave

functions, neither phase choice introduces contradictions, and no further thought on the matter

is necessary.

2.9. Relations Between Matrix Elements

Thus far in chapter 2, we have concerned ourselves with a determination of the symmetry

properties of molecular wave functions. Symmetry operations can serve another purpose, however.

23
They can be used to obtain a relation between values of two different matrix elements. Their use- fulness here arises from the fact that the value of any matrix element is unchanged if the two wave

functions and the operator involved in the integral are subjected to a symmetry operation, since

such an operation corresponds to a change in variables everywhere in the integral. Sometimes

the relation obtained is equivalent to a selection rule. Suppose we have two wave functions ^i, ^2

and an operator L. Suppose further that these quantities obey the following transformation equations

under some symmetry operation o"

o-^2=(-l)«2ij,^ (2-21)

crL= (-1)«3 2,.

By applying the symmetry operation cr to the integrand below, we obtain

J^fL^2C?T =

J

[o-(^*I^2)](^T= (-1)"' + "^ + "^ J

^fL^2<^T. (2-22)

The integral given in (2—22) is therefore nonvanishing only if «i + «2 + 'i3 is an even integer. We

have thus obtained a selection rule. Equations (2-21) lead to a selection rule because ^i, ^2, and L all transform into some constant times themselves. The quantities ^1, ^2, and L might transform into some constant times ^3, ^4,

and L', say. When this occurs, we do not obtain a selection rule, but obtain rather a relation be- tween two different matrix elements. For example, H, Hei, and Hr of eq (1-1) are all invariant

under the symmetry operation av. Thus, by applying cr,. to both wave functions and to Hev in the

matrix element {L' A' S'1.' \Hev\LAS1,}, and by using the transformation equations (2-11), we

obtain the following equality

{L' A' S'T\Hev\LAS'^) = [±(-l)'-A'+s'-V][±(_i)L-A+s-i] S' -l'\Hev\L -A S -2). (2-23)

where the first and second factors, respectively, come from the transformation properties of the

first and second wave functions. Just as in (2-11), it is often possible to obtain consistent results,

when some of the quantum numbers in (2-23) do not have definite values, by arbitrarily assigning

them values. These values must be used throughout all calculations, however; i.e., the values

chosen here must agree with those chosen for eqs (2—11) and (2—16).

As mentioned above, Hr is also invariant under cr„. However, the individual operators occurring

in Hr can be shown to transform as follows [15, 16]:

(Tv S± = — S +

o-„S. = -S^ (2-24)

Transformation equations for L±, Lz, U and 7+, A, P can be obtained by making the obvious

substitutions everywhere in (2-24). By combining the transformation properties (2-24) with those

of the wave functions given in (2-11), various matrix elements of the Hamiltonian can be related to each other.

24
2.10. Example: L\ = L^-Ll

We now show that the matrix elements of L^in the basis set used to label (1-18) are all equal.

By applying the symmetry operation av to the wave functions and operators on the left side of the

two equalities below, we obtain

( 1 Vz V2 ; y M|I2 -Ll\ 1 1/2 V2; 3/2 y M) = + ( - 1 V2 - V2; - 3/2 JM\U-- L% - 1 V2 - V2; -^I^JM)

(1 V2-V2; V2yM|L2-L2|i 1/2 -V2;V2yM)=+(-l V2 V2;-V2yM|L2-I?J-lV2 V2;- V2yM).

(2-25)

In addition, if we assume that the matrix elements of the orbital operator L\^o not depend on the

spin quantum numbers, then we see that

(1 V2 V2; 3/2yM|L2-L||l V2V2; 3/2 y /!/)=+ (1 1/2 - V2; V2yM|L2-L||l V2- V2; V2yM). (2-26)

Equation (2-26) rests ultimately on the assumption that the nonrotating-molecule basis set functions |AS2) can be written as the product |A)|S2) of an orbital function |A) and a spin function

|5S), and on the further assumption that the same orbital function is associated with all 2S+1

spin functions corresponding to given S. These assumptions will be valid to the extent that A, S,

and S are good quantum numbers. As a consequence of (2-25) and (2-26), the four matrix elements

of in the basis set used to label the matrix (1-18) are aU equal. The equahties represented by

(2-25) are exact; that represented by (2-26) is only approximate.

2.11. The Time Inversion Operation B

The time inversion operation Q [17] (pp. 325-348) is of interest here principally in connection

with intensity calculations. The intensity of absorption or emission of hght by diatomic molecules

depends on matrix elements of the components of the dipole moment operator. These matrix

elements are usually not calculated explicitly, but are rather treated as parameters, to be deter- mined from a fit to experimental data. It sometimes happens that several such parameters occur

in the intensity expressions, which then involve, for example, the squares of sums and differences

of these parameters. It is clearly desirable to know which of the parameters are real and which are complex, since the arithmetic of real numbers is not identical to the arithmetic of complex numbers.

Time inversion is a useful tool, since it is essentially the operation of taking complex conjugates.

Indeed,

ek = k*, (2-27)

if ^ is a constant or a function of the positional coordinates of particles. However, because of the

rather special nature of spin variables, time inversion, when apphed to spin functions, is some- what more comphcated [17] (pp. 331-333).

Physically, time inversion would be expected to correspond to a transformation of variables in which the time t is replaced by —t. Thus, for example, a position coordinate x should remain in- variant under time inversion, while a velocity dx/dt or a momentum m{dxldt) should transform

into its negative. In quantum mechanics momenta are represented by operators of the form — ih{dldx) , which do not contain the time variable at all. However, in contrast to position coordinates, they do contain the pure imaginary number i. It is thus convenient in quantum mechanics

to construct a formalism in which time inversion corresponds to the taking of complex conjugates

rather than the replacing of f by — t. Arguments such as this make the following transformation equations for angular momentum

operators seem reasonable. (They are also correct [17] (pp. 329-330).)

25
dh =-Ld

dS = -Sd (2-28)

From these relations it is possible to show that when time inversion is applied to wave functions,

the signs of all angular momentum projection quantum numbers are reversed. For example,

eL,\LA)=eAfi\LA)=Afi[e\LA)]=-L,[e\LA)]. (2-29)

We thus conclude

e»|L A) |L -A)

d\Sl)^\S-l) (2-30)

e\ajM)'=^\-nj-M).

When the system being considered contains an even number of electrons, ^-=+1 [17] (p. 332).

Under these circumstances, it happens that L, S, and J are all whole nu'Sibers, so that zero is a

possible value for each of the projection quantum numbers A, X, H, and M. It is relatively easy to show that the phase factor of the wave function having a projection quantum number equal to zero can be chosen such that the function is unchanged when the time inversion operation is carried

out. In other words, it is always possible to choose phases such that

d\LO)=+\LO)

d\SO}=^\SO) (2-31)

e |oyo)=+|OiO).

When the system being considered contains an odd number of electrons, ^- = —1 [17] (p. 332). Under these circumstances, 5 and J are half-integers, so that a value of zero for X, fl, and M

is not possible. There are then no spin functions and no rotational functions which remain un- changed by the time inversion operation (see 2-30).) However, it is possible to show that one can

always choose phases consistent with those of Condon and Shortley [7] , such that

6'|S V2)=+|S -Va)

(2-32)

6>|V2y V2)=+| -V2 J -V2).

The effect of time inversion on all other functions can be obtained as follows. Our choice of

phases for the ladder operators S+ in eqs (1-13) implies that

\s V2+m) = kAs^)"'\sy2)

(2-33)

|5 V2-/i)=^2(S_)"|S V2)

where m and n are positive integers and ki and ko are positive constants. The transformation properties of components of the angular momenta given in (2-28) and eqs (2-32) and (2-33) allow us to write

26
d\S V2+m)-0/.,(S + )'»|S V2)= (-1)'«A-,(S_)'«|S -1/2) = (-1)'"|S -y2-m}

d\s y2-n) = ek-AS-)"\sy2)^{-i)"k-ds^)"\s -V2) (2-34) = (-l)"|5 -V2 + n).

Equations (2-34) can easily be shown to be equivalent to 0|S2)=(-l)^-'/^|S -2) (2-35)

for all values of 2 allowed for given half-integral S. A set of four equations similar to (2-34) holds for the functions \ClJM} when J is half-integral,

except that both laboratory-fixed (Jx^Uy) and molecule-fixed {Jx-^Uy) ladder operators must be

used. One of these equations takes the form

0|V2 + /n J + n) = dk,Ux-iJy)"'Ux + iJy)T/2 J = i-l)'"^"k,Ux + iJy)"' Ux - iJ >

) " I - V2 y - 1/2 ) (2-36) = (-l)"'+"|-V2-m J -V2-n),

where m and n are positive integers and is a positive constant. This equation and the three

analogous equations obtained by using different combinations of (Jx-^Uy)'" and {Jx±iJy)" can be

shown to be equivalent to

e\ClJM)^i-l)^+"-'\-a J -M) (2-37)

for all values of Vl and M allowed for given half-integral /. For integral values of L, S, and J we find, by using (2-31) and equations similar to (2-34) and

(2-36). that

0|L A) = (-1)^|L -A)

0|S 2) = (-l)^|S -2) (2-38)

0|ayM) = (-i)"^'"|-a j ~m).

Since an expression of the form {a\a) must always equal a real, positive number, we conclude

that i{ e\a) = e'" \ a') then ^{a | = e-'"<a' |

We consider an example of the use of time inversion in determining which matrix elements of

the dipole moment operator are real and which are not in section 3.5.

3. Calculation of Rotational Line Intensities

In chapter 3 we discuss the determination of spectral fine intensities. We shall not be con- cerned, however, with the total intensity of an electronic transition, nor with the vibrational dis- tribution of intensity in a given band system. Rather, we shall consider the rotational distribution

of intensity within a given band [1] (pp. 204-211).

Optical transitions in diatomic molecules are said to be electric dipole allowed, magnetic

dipole allowed, or electric quadrupole allowed if the transition moment matrix element is non27
vanishing when the electric dipole operator, the magnetic dipole operator, or the electric quadrupole

operator, respectively, is used [7] (pp. 79-111). (These classifications are not necessarily mutually

exclusive.) The vast majority of observed transitions are electric dipole allowed, and we shall consider only that case in this monograph. However, the considerations below can be applied to magnetic

dipole transitions after relatively minor changes: For example, all signs on the righthand side of

(3-2) must be positive, and the six signs preceding the parentheses on the righthand side of (3-4)

must be changed from +, +, —, —, —, — to —, —, +, +, —, —; these sign changes lead, of course, to some changes in the selection rules. A discussion of electric quadrupole transitions is quite differ- ent from the discussion for dipole transitions, since the quadrupole operator is a tensor of the second

rank, rather than a vector. As suggested above, the intensity of most optical transitions is governed by the value of matrix elements of the electric dipole moment operator [5] (pp. 272-282). We must therefore examine

this operator in some detail. Classically, the dipole moment of a system of charges et is a vector

quantity given by an expression of the form 2, e, r,, where r, is a vector from the point at which the' dipole moment is being defined to the charge e,. Vector operators are sometimes a source of

confusion in molecular spectroscopy, since they are often represented by their components in two different Cartesian axis systems, one fixed in the laboratory, the other fixed in the molecule.

The dipole moment operator can also be resolved into components in either of these two axis

systems. Matrix elements of the two sets of components have quite different interpretations.

We shall here follow the common, but not universal, practice of representing the laboratoryfixed components of the dipole moment operator by fJix, fJ-v, fJ-z and the molecule-fixed components

by /Xx, IJ-y, /J-z- These two sets of components are related by an equation of the form

IJLR = XsOCRslJis, (3-1)

where the direction cosine matrix [3] (pp. 10-11), [6] (pp. 285-6), [8], is found on the righthand

side of eqs (2-3). (Note that the direction cosine matrix for Hnear molecules contains only two

Eulerian angles.) The subscript R in (3-1) ranges overZ, Y, Z; the subscript s over x, y, z. For most

of the remainder of this chapter we shall discuss the determination and interpretation of matrix

elements of the quantities occurring in eq (3-1).

3.1. Laboratory-Fixed Components of the Electric Dipole Moment Operator A* The intensity of an optical transition between two states often depends on the polarization of the light passing through the sample in an absorption experiment, or on the polarization of the fight

being detected in an emission experiment. This phenomenon is reflected in the theory as follows. Theoretical calculations for experiments involving plane polarized fight with the electric vector

of the fight in the laboratory-fixed Z direction must be performed with the laboratory-fixed Z

component of the dipole moment operator [7] (pp. 90-93, 97-100), i.e., the intensity in such an experiment for a transition between an initial state i and a final state /is proportional to | (/Iaiz |i) p. Clearly, by analogy, if the light is plane polarized with the electric vector in the X or Y direction,

then the theoretical calculations must be performed with the laboratory-fixed X or Y component of

the dipole moment operator, respectively.

Theoretical calculations for experiments involving circularly polarized light travefing in the

laboratory-fixed Z direction must be performed with the combinations ^x±il^Y of the laboratoryfixed components of the dipole moment operator [7] (pp. 90—93, 97-100), i.e., intensities in such

experiments are proportional to | (/| /ix.a + i/xk | i) p or | (/|^t^ — t^ti'| f) Theoretical results for experiments involving unpolarized (ordinary) light are obtained by

averaging the results of calculations for plane polarized fight having the electric vector in each of the three laboratory-fixed directions. If, in addition, the emitting or absorbing molecules are in an

isotropic environment, then all directions are equivalent and theoretical results for experiments

using unpolarized fight can be obtained by considering plane polarized light with the electric vector

in only one of the laboratory-fixed directions [7] (pp. 90-93, 97-100).

28
The laboratory-fixed components of the dipole moment operator transform as follows [15, 16]

under the symmetry operations (Ti(xz), i, and €2(7)

CTr IJ,/{ = — /JLii

i IXR = -tXR (3-2)

C2/a« = + /A« R=X,Y,Z

These relations can be proved easily from the resuhs given in table 5. The first of the transformation

properties in (3-2) leads (see sect. 2.9) to the overall parity selection rule for electric dipole transi- tions ± The third of the transformation properties leads to the selection rule a<^ a and 5 s. The second of the transformation properties leads to no additional selection rules, since

i = aI- • C2.

3.2. Molecule-Fixed Components of ft

Molecule-fixed components of fi come into consideration when matrix elements of the dipole

moment operator are to be evaluated, because molecular wave functions are expressed in terms

of molecule-fixed coordinates and not in terms of laboratory-fixed coordinates. We shall not consider

in this monograph the numerical evaluation of matrix elements of the molecule-fixed components

of fi, since such calculations require a knowledge of many-electron molecular wave functions.

Instead, we shall treat these matrix elements as parameters, which must be determined from a fit of the calculated rotational intensity expressions to the experimental data. It is still necessary,

however, to determine precisely how many such intensity parameters can occur in the rotational

intensity expressions for a given electronic transition. Hence, it is necessary to investigate the cir- cumstances under which matrix elements of the molecule-fixed components of /x vanish.

The molecule-fixed components of /* do not involve the rotational variables. Consequently,

matrix elements of these quantities are diagonal in the rotational quantum numbers J and M, and we need only consider further matrix elements of fi in the various nonrotating-molecule basis sets. The only nonvanishing matrix elements of the molecule-fixed components of fx in the nonrotatingmolecule basis set 10) have the following form [7] (pp. 59-64):

(fH- 1 I fXx+ifJLy I Cl)

(fi — 1 I /Uj. — i/Uy I n) (3-3)

(n I /Li,

I a),

with the additional restriction (see below) that /x^ has no nonvanishing matrix elements between

0+ states and 0" states (all diatomic molecules), and that fXx, fXy, fx^ have no nonvanishing matrix

elements between g and g or between u and u electronic states (homonuclear diatomic molecules).

The two wave functions for the nonrotating molecule represented by | fl) in the third matrix

element of (3-3) may correspond to the same state of the molecule or to two different states. If these

two wave functions | fi) correspond to the same vibrational-electronic state of the molecule, then

the matrix element (fi | /it,

| fi) governs the intensity of pure rotational transitions (which are for- bidden in homonuclear molecules, of course). If the two wave functions | fi) correspond to different

vibrational states of the same electronic state, then the matrix element (fi |fi) governs the

intensity of a pure vibrational transition (which is also forbidden in homonuclear molecules). If the two wave functions | fi) correspond to different electronic states of the molecule, then the

matrix element (Cl\iXz\n) governs the intensity of an electronic transition. Each of the matrix elements in (3-3) can be related to another matrix element by applying the

symmetry operation ctv to the two wave functions and the dipole moment operator involved in the

29
integral. The transformation properties of the wave functions have been discussed in chapter 2. The transformation properties of the molecule-fixed components of the dipole moment operator

[15, 16] can be obtained immediately from table 3:

i

(Ti, ( ^tx ± l>y ) = + ( + i/Xy )

i{fMj:±ifXy)=- {/xjc±ifxy) (3-4)

iilJ'z) =— if^z)

C2il^z) =- (P-z).

The transformation properties of /Xz under cTv lead (see sect. 2.9) to the selection rule O-'^O- or The transformation properties of ft under i lead (see sect. 2.9) to the selection rule

Consider now the nonvanishing matrix elements of the molecule-fixed components of the

dipole moment operator in the nonrotating-molecule basis set | A5X). These matrix elements can

be classified as spin-allowed or spin-forbidden. Spin-allowed matrix elements can be obtained by

considering the functions | ASS) to be the product of an orbital part and a spin part, i.e.,

I AS2) = | A)|S2), (3-5)

where the quantum numbers A, S, and 2 are all perfectly good, and where the same orbital function

I A) is associated with all 2S + 1 spin functions | SS) for given S. Since the dipole moment operator

is independent of electron spin, we find

{A'S'V \tx>\ASl)={A' \iXi\A)8s's8i'^ (3-6)

where /jli represents fXx, /Xy, or yu,^. The only nonvanishing spin-allowed matrix elements of the

molecule-fixed components of the dipole moment operator in the nonrotating-molecule basis set

I ASX) thus have the form [7] (pp. 59-64):

(A+1S2 I Atx + i>y I ASS)

(A-1S2 I Atx-i/>tJ ASS) (3-7)

(ASS I At, I ASS)

with the additional restriction that (jl^ has no nonvanishing spin-allowed matrix elements between

S+ states and S" states (all diatomic molecules), and that (jlx, fJiy, fJiz have no nonvanishing matrix

elements between g and g or between u and u electronic states (homonuclear diatomic molecules).

Spin-forbidden matrix elements of the dipole moment operator are those forbidden by (3-6) and

(3-7), but allowed by (3-3).

The matrix elements in (3-7) can be related to other matrix elements in two ways: first, by

assuming that their value is independent of the spin projection quantum number S (which follows

from the factorization (3-5)); and second, by applying the symmetry operation av to the two wave

functions and the dipole moment operator involved in the integraL(see sect. 2.10). In spin-allowed transitions, the intensity of the transition comes from matrix elements involving

either the dipole moment component fx^ or the components iXjc and (Xy. The former transitions are

called parallel transitions because the nonvanishing matrix elements involve the component of

the dipole moment parallel to the internuclear axis. The latter transitions are called perpendicular

transitions because the nonvanishing matrix elements involve the components of the dipole moment

perpendicular to the internuclear axis.

30
Allowed matrix elements of the dipole moment operator in the nonrotating molecule basis set LASS) have the form

{!' A+1S2 I fxx+ifjiy I LASS)

(L' A-1 SS I /Ax-i/Lty 1 1 ASS) (3-8)

(I' ASS I At^ I LASS),

where the transition L' L must be allowed in the united atom hmit. Forbidden transitions in this

basis set are those forbidden by (3-8), but allowed by (3-7) or (3-3).

3.3. The Direction Cosine Matrix a. The elements of the 3x3 direction cosine matrix a do not involve the electronic or vibrational

variables of a diatomic molecule; they involve only the rotational angles (see eqs (2-3) above and

[3] (pp. 10-11), [6] (pp. 285-6), [8]). Consequently, matrix elements of elements of the direction

cosine matrix are diagonal in the nonrotating-molecule quantum numbers L, A, S, S. The nonvanishing matrix elements of elements of the direction cosine matrix are conveniently summarized in the form of a table [18] (p. 96).i This table, with some change in notation from [18], is presented

here as table 6. Each matrix element of a given element of the direction cosine matrix consists of

the product of three factors: one taken from the first line of table 6, one from either the second or

third lines, and one from either the fourth or fifth hues; all three factors are taken from the same

column of table 6. The choice of rows is determined by the particular element of the matrix a under

consideration. The choice of column is determined by the value of Ay.

The derivation of table 6 represents a rather elaborate exercise in group theory, operator

algebra, or generating functions. Furthermore, the derivation is slightly different for nonlinear

molecules (three Eulerian angles) than it is for linear molecules (two Eulerian angles). We shall not consider in this monograph the derivation of table 6.

Table 6. Direction cosine matrix elements {after [i8] p. 96).

The nonvanishing matrix elements ( fl' J' M' | ocrs \ fl J M) , where R=X,Y, or Z and s = x, y, or z, are given by the product

of three factors: f{J'; J) -gsW , fl'; J, fl) -hKiJ'^ 7. M). The factors /, gs, ha for a given matrix element are taken from different rows of the same column of this table. The choice of columns depends on the value of J' —J. The choice of rows depends on R and s. In all cases, the first factor /is taken from row one; the second factory's is chosen from rows

two and three; and the third factor ha is chosen from rows four and five.

Factor J'=J+l J'=J y'=y-i

/(/'; J) {4(y+i)[(2j+i)(2y+3)]'/^}-' [4y(y+i)]-> {4y[(2y+i)(2y-i)]''^}-i

gzW, H; J, ft) gAJ',n±l;J,n) or + igy(J',n±hJ, fl)

2[(y+ft + i)(y-n + i)]''^

+ [(y±ft + i)(y±n+2)]'/^ [(y+fi)(y±n + i)]''^

2[(y+n)(y-n)]'/^

±[(y+ft)(y+n-i)]''^

hzU',M; y, M)

hxW, M±l;J, M) or ±ihyU\M±l;J, M)

2[U + M+l)(J-M+l)yi' 2M

[{j^M)u±M+i)yi^

2[(y+M)(y-M)]'/^

±[{j+mu+M-i )]'/^

The elements of the direction cosine matrix transform as follows under the symmetry operations av and C2; they are invariant under i (see eqs (2-3) and sect. 2.1).

(cTy or C2) iaHx±ioiRy) =— (««j--i-ia:«y)

(oTi, or C2) (a/jz) =— (a/^^). (3-9)

' Note thai some eiarly printings of [18] contain sign errors in this table. All iMitries in the second and fourth rows should be positive.

31
3.4. Example: Honl-London Intensity Expressions for a ^H— Transition

In order to calculate rotational line intensities, it is necessary to know the correct wave functions for both upper and lower state rotational levels. The example chosen above is relatively simple,

since correct wave functions can be written very simply in terms of the basis set functions

I ASS; ClJM). The wave functions for the complete problem (rotating plus nonrotating molecule)

are given for the '2+ state and the 'IT state in eqs (3-lOa) and (3-lOb), respectively.

|0+0 0;Oi/lf) (3-lOa)

2-'/^[| 100;17A/)±|-100;-iyM)] (3-lOb)

By application of the symmetry operation cry as described in chapter 2, we find the parities of the

functions (3-lOa) and (3-lOb) to be

+ (-1)' (3-lla)

±(-1)' (3-llb)

respectively, if we arbitrarily assume a value of L=l for the state, and arbitrarily assume

correlation with a united atom state of odd parity (see eqs (2-11)).

Because we are interested in intensity formulas which are valid for unpolarized light arid for molecules in the gas phase in the absence of external fields, all three directions in space are equivalent, and it is sufficient to calculate the matrix elements of /xz, the laboratory-fixed Z component

of the dipole moment operator, to determine intensities. The Z component of ft, rather than the

Z or y component, is chosen for intensity calculations involving unpolarized Ught and molecules

in the absence of external fields, because nonvanishing matrix elements of the former obey the

selection rule AA/ = 0, whereas nonvanishing matrix elements of the latter two obey the selection

rule AM— ±l. The selection rule AM = 0 gives rise to particularly simple summation expressions,

e.g., (3-17), thus reducing the amount of algebra required to obtain the final intensity expressions,

e.g., (3-20) and (3-21).

The quantity /jlz, as given in (3-1), can be rewritten in the form

/u,z = V2(a:zj — iazy) t>i/) + V2(azx+ lazy) ()U.x — + oizzfJ-z- (3-12)

By using the selection rule AM — 0 for elements of the direction cosine matrix of the form ctzs, the

selection rule AA = ±1 for ^tx±^A^,^/, and the selection rule AA = 0 for /jlz, we see that a general

matrix element of /Az takes the form

(2-'/^[{l 0 0: U'M |±(-1 0 0;-l J'M I ] | /^z | 0+ 0 0; 0 J M) = 2-'/^(10 0; U'M I V2(azx-iazy)(/Xx+iAt«) |0+00;OiM)

±2-'l'{-lOO;-\J' M\y2{azx+iazy)itx,-i(Xy)\0^00;0JM). (3-13)

Application of the symmetry operation ctv (see sect. 2.9) indicates that the righthand side of (3-13)

is equal to zero if the upper sign is used for J' = J, or if the lower sign is used for y —J±l. If the

opposite sign choice is made in each case, the righthand side of (3-13) is equal to

2-'/KlOO; 1 J'M I (az^-iazy){tJi^+ifJLy) |0+00;OJM). (3-14)

Note that we again assume L = 1 for the '11 state, and take the lower sign choice in (2-lla), in agree- ment with the choice made in obtaining (3-llb). The fact that the matrix element given in (3-13)

vanishes for certain sign choices and for certain A/ is consistent with the parities given in (3-11)

and with the parity selection rule ±«^=P for electric dipole transitions. The matrix element (3-14) can be further simplified by recalling that the elements of the

direction cosine matrix do not contain the variables of the nonrotating-molecule problem, whereas

32
the molecule-fixed components of the dipole moment operator do not contain the rotational vari- ables. Since the complete basis set functions \AS1,;flJM) are products of a function | containing only the variables of the nonrotating-molecule problem and a function | ClJM) containing only the rotational variables, we can write (3-14) as

2-V^(l 00 I iJi^+ifJLy I 0+ 0 0) (ly M I az^-iazy \ OJM). (3-15)

The second factor in (3-15) represents a matrix element of the type given in table 6 above. The

first factor represents a matrix element which cannot be calculated from symmetry considerations

alone. Hence we shall treat it as a parameter, which is to be determined by fitting the calculated

intensity expressions to the experimental data. For simplicity we define a quantity /u-j^

ia^ = 2-'/^<100|/Lt^+i/>iy |0+00), ' (3-16)

where /Xj^ is, of course, independent of the rotational quantum numbers. In addition, we choose the

phase factors for the two wave functions | 1 00) and | 0+ 0 0) such that /u,j^ is real and positive.

(Such a choice is possible at this point, since we have not yet considered the phase of any matrix

element connecting these two states.)

Spectral line intensities are actually proportional to the square of the dipole moment matrix

elements, i.e., to the square of the quantity first given in (3-13) and later rewritten in (3-15). Fur- thermore, we are considering molecules in the absence of external fields, so that the 27+1 states having the same J but different M are all degenerate. Thus, the total intensity / is obtained by

summing over all M values for the upper state and over all M values for the lower state under

consideration. Since nonvanishing matrix elements of ixz obey the selection rule AM = 0, the sum

over upper state M values, i.e., the sum over that quantum number which would be M' , contributes

nothing until M' = M. We thus write

/oc/^i S„ I iU' M \az^-iazy\OJ M) p. (3-17)

Consider now the intensity of an R branch (7' =7 + 1). We find from table 6 that (3-17)

becomes

I{R branch) cr ^25^1 {4(y + j) [(27 + i) (27 + 3) ] '/^}

X{-2[(7 + 0 + l)(7 + 0 + 2)]'/^} {2[(7 + M+l)(7-M+l)]'/^} |^ (3-18)

where fi has been given its value of zero. Using the summation expressions

2 1 =(27 + 1)

M = - .1 ^ M2=(27 + l)7(7 + l)/3

M= -J we obtain

I{R branch) fxl (7 + 2).

In a similar fashion we can obtain

HQ branch) °^ Vs fi'i (27+1)

I(P branch) « V3 /Ltf (7 - 1 ) • (3-21)

The relative intensities given in (3-20) and (3-21) agree with the well-known Honl-London expressions for a '11 — transition in a diatomic molecule [1] (pp. 204-211).

(3-19)

(3-20)

33
3.5. Example: Rotational Intensity Distribution in a ^2" — Transition

The wave functions and symmetry properties for the rotational levels of a state have been

given in eqs (3-lOa) and (3-lla) above

The rotational energy levels of a state have been discussed in section 1.10; the symmetry

properties of the rotational levels of a ^X"*" state have been discussed in section 2.5. It is easy to

shov*^, by arguments analogous to those of section 2.5, that the parities of the rotational levels of a state are just the opposite of those of a ^X"*" state, i.e., states of even A'^ are of odd parity and

states of odd A'^ are of even parity.

The wave functions for the rotational levels of a ^X state were not determined in section 1.10. These wave functions can be determined, however, by finding the eigenvectors of the sum of the

matrices given in (1-24) and (1-27) above. We consider a '^X" state which is very near case (b); for the purposes of calculating intensities, we thus set A. = 0. The three normalized eigenfunctions

of given J then become: - [y/(2i+l)]'/^2-'/^[ I 0- 1 1; 1JM) + I 0- 1 -1;-1./M)]

+ [(y+l)/(2y+l)]'/^|0-10;0yM) (3-22a)

2-'/^[ |0-ll;liM)-|0-l -1;-UM)] (3-22b)

[(J+l)/(2y+l)]'/^2-'/^[ |0-1 1; iyM) + |o-i -1;-Uyif)]

+ [i/(2y+l)]'/M0-10;0yykf). (3-22c)

These three functions are eigenfunctions of the matrix sum (1-24) plus (1-27) when A. = 0, and

belong to the eigenvalues B{Ll) BN{N +\) , where A^ = J+1, N = J, and N= J-\, respectively.

We must next calculate all matrix elements of fXz allowed by the selection rules on J and by

the parity selection rule. (Alternatively, the parity selection rule can be used as a check on the

calculations.) The selection rule AJ — 0, ±1, and the fact that N= J+ I, J, or J — 1 in the triplet state, indicates the possibility of nine rotational branches. In standard notation, i.e., ^'^A./, these

would be called ^R, "R, ^Q, «P, ^P, «P branches. Let us consider in detail the ^R

branch. The intensity expression for such a branch depends on a matrix element between an upper state wave function of the form (3-22a) with J replaced hy U+l) everywhere (7' =7+1,

A'^'=y + 2) and a lower state wave function of the form (3-lOa) with J unchanged U" = — J). Just as in eq (3-13) above, this matrix element can be separated into parts

-[(7+l)/(2i + 3)]V22-'/^(0- 1 1; 1 i+1 M|Mz|0+00;OyM)

-[(7+l)/(2J + 3)]V^2-'/^{0-l -1;-1 7+1 M|//,z|0+00:0yM) (3-23)

+ [(7 + 2)/(27 + 3)]-/^(0- 1 0; 0 7+ 1 M I I 0+ 0 0; 07 M).

Furthermore, by applying the symmetry operation cTi (see sect. 2.9), we find that the first and second

terms in (3-23) are equal.

The next step is to replace fxz by the righthand side of (3-12). Before doing this, however, we

examine the matrix elements of the molecule-fixed components of the dipole moment operator in the nonrotating-molecule basis set under consideration. The nonrotating-molecule basis set for the

>X+ state consists of one singlet function | 0+ 0 0); that for the ^X" state consists of three triplet

functions: 1 0" 1 1 ) , 1 0" 1 0) , and |0- 1 —1). There are no spin-allowed matrix elements of the

dipole moment operator between singlet functions and triplet functions, since nonvanishing dipole

moment operator matrix elements obey the selection rule AS = 0 when S is a good quantum number

(see eqs (3-6) and (3-7)). However, if we assume that the singlet state is slightly contaminated by

34
triplet states, and that the triplet state is slightly contaminated by singlet states, then any matrix

elements allowed by the selection rules Afl = 0, ±1 are nonvanishing (see eq (3-3)). Hence the

possible nonvanishing matrix elements appear to be

<0- 1 1| Atx+I>y I 0+00)

(0- 10 I /x^ I 0+00)

^

(0-1 -1 |/Xj.-i/ay|0+ 00). (3-24)

The one remaining point to check involves the matrix element between the two states having

n = 0. Selection rules require that all matrix elements of jXz vanish between 0+ states andiO" states. We are, of course, dealing here with a X+ state and a state, so that this electronic transition is orbitaUy forbidden. However, it is already known to be spin forbidden, so this orbital forbiddenness

is of no great interest. What is of interest is whether or not in the strong spin-orbit coupling limit,

corresponding to the nonrotating-molecule basis set |fl), the Afl = 0 transition is allowed or forbidden. As might be expected, it is the transformation properties of the combined spin and

orbital parts of the wave function which determine whether a state having fl = 0 is a 0+ state or a Q- state. We note that

I 0+0 0)=+ I 0+0 0)

I 0-10)=+ I 0-10). (3-25)

Consequently, both the >2+ and the states give rise in the strong spin-orbit coupling hmit to 0+ states, and the second matrix element of (3-24) is allowed by symmetry.

Spectroscopists sometimes speak of a doubly forbidden transition. Such a label is useful,

if it is employed carefully. The degree of multiple forbiddenness is best defined to be the number of

first-order perturbations which must be carried out in succession before a given transition is made

allowed. Thus, in the particular case of a ^2" — '2+ transition, a single first-order spin-orbit per- turbation (satisfying the selection rules AS = 0, ± 1 ; Afl = 0) suffices to make the transition allowed

(e.g., the mixing o{^^~ and 'H), so that this transition is only singly forbidden. On the other hand, a ^S- — 'S+ transition is made allowed only after two successive first-order spin-orbit perturbations,

and it is therefore doubly forbidden.

Taking into account the fact that the first and second terms in (3-23) are identical, the fact that the only nonvanishing matrix elements of the molecule-fixed components of the dipole moment

operator in the basis set under consideration are given in (3-24), and the fact that /xz can

be expanded as given in (3-12), we can rewrite (3-23) in the form

-[(y+l)/(2y + 3)]'/^2-'/^(0- 1 1 l/^x + iMjO+OO) {\J+\M\ay,,-iazy\OJM)

+[(y + 2)/(2y + 3)]'/^(0-10|/x, 10+00) (Oy+IM | az, | OyM). (3-26)

For simphcity we define two quantities and /ij.

At|| = (0-10 1^,1 0+0 0)

At^ = 2-'/^(0-l 1 l/x^ + i/Lty |0+ 0 0), (3-27)

which can both be made real as follows. Since the two wave functions | 0" 1 0) and | 0+ 0 0) both

have only zero values for the angular momentum projection quantum numbers, their phases can

be chosen such that they transform into themselves under the time inversion operation 6 (see

sect. 2.11)

0|O-1O)=+|O-1O)

e I 0+0 0)=+ I 0+0 0). (3-28)

35
Applying the time inversion operation to all quantities in the first equation of (3-27) we obtain

dfJL\\ = fxf\=+{0- 10 \ fji* \ 0^0 0) = + (0- 1 0 I /u.,

I 0+ 00)= + )a||. (3-29)

Clearly, the quantity ix\\ is real under these conditions. Applying the time inversion operation in j a similar manner to the second equation in (3-27), and using transformation properties for the wave function | 0~ 1 1) obtained from eq (2-38), we find .

dfjL ^ = tx* = - 2- 'I 1-1 \ixx-iiJ,y\0+ 0 0). (3-30)

Applying the symmetry operation en to the matrix element on the righthand side of (3-30) allows one to conclude that

_2-V2(o-l -1 I |0+00) =

,

, +2-'i'{0-ll\ ijijr + ifxy\0^00) = + ix^. (3-31)

Thus, the quantity /xi is also real. (Note that the time inversion operation 9 was used together with

the reflection operation (Tv in demonstrating that /jl^^ is real. The use of both 6 and CTp will generally

be necessary when the matrix elements under consideration involve functions with nonzero values

for angular momentum projection quantum numbers.)

If we now substitute from table 6 and eqs (3-27) in (3-26), we obtain for this matrix element

of fJLz

-[(y+l)/(2y + 3)]i/Vj4(J + l)[(2y + l)(2y + 3)]i/2}-H-2)[(J + 0+l)(J + 0 + 2)]i/2

I x(2)[(J + M+l)(y-M+l)]i/2

+ [(J + 2)/(2J + 3)]i/>l|{4(i + l)[(2i+l)(2i + 3)]^/n-M2)[(y + 0+l)(y-0+l)]^/2

X(2)[U + M+l)U-M+l)yi-\ I

(3-32)

where fl has been given its value of zero. The intensity is proportional to the square of this quantity

summed over M. Thus,

/(^/? branch) cc [+^ll+^^]2(y + l)(J + 2)/3(2i + 3). (3-33)

In a similar fashion,

/(«/? branch) <=c [+ (j + i) - ^^(j + 2) ]2/3(2i + 3)

/(«^ branch)°c(+)U.^)'M2J+l)/3

(3-34)

/(«f branch) cc [+ ^^i, (j) - ^ - i ) ]2/3 (2./ - 1

)

/(«P branch) «: [+/Lt|| + /x^]V(y-l),/3(2y-l).

These resuhs were recently pubUshed by Watson [19], who corrected the resuhs previously given

by Schlapp [20]. An examination of Watson's eq (23) and eq (3-27) above shows that ixo (his

notation) = + I (this notation), but that fXi (his notation) = — /t^ (this notation).

36
I 3.6. Intensity Calculations When Closed-Form Expressions Cannot Be Obtained.

Example: Rotational Intensity Distribution in a ^A — Transition

The calculation of rotational intensity distributions when some of the pertinent expressions

cannot be written down in closed form can conveniently be divided into three parts: (i) determination of wave functions for rotationeil levels of the upper and lower electronic states of the transition,

expressed in terms of basis set functions for the upper and lower states, respectively; (ii) determinai tion of matrix elements of the dipole moment operator between functions in the upper state basis

set and functions in the lower state basis set; (iii) formation of linear combinations of basis set dipole moment matrix elements to obtain dipole moment matrix elements between upper state

final wave functions and lower state final wave functions. Each of these three steps must be per-

, formed numerically, presumably by a modern electronic computer. It is convenient in what follows

to indicate the numerical steps in matrix notation.

Step (i). Rotational energy levels and rotational wave functions for each of the two vibrationalelectronic states involved in a given optical transition can be determined as described in chapter 1. For molecules in the absence of external fields, it is thus necessary to diagonalize matrices having rows and columns labeled by a set of functions characterized by the same value of J and the same

value of M (J and M are good quantum numbers in free space), but by different values of the other

quantum numbers of the basis set (see sections 1.9 and 1.10). Furthermore, if numerical calculations

are to be performed, it is necessary to consider a number of different matrices, corresponding to dif- ferent numerical values for the quantum number 7. The quantum number M need not be assigned a numerical value, since matrix elements of the Hamiltonian operator for a molecule in free space

are independent of M[7] (p. 49). For the ^A — example considered here, we must diagonaHze two sets of Hamiltonian

matrices. One set is of dimension 8x8, with rows and columns labeled by functions |A5S: CIJM)

of the ^A upper state, the functions being characterized by a fixed numerical value of J, a fixed

algebraic value of M, and by A = ±2, 1, — ±^/2, ±V2. It is convenient to represent an individual

matrix from this set of 8 X 8 matrices by the symbol //« (J' ) , where J' is the J-value characterizing

this particular upper state Hamiltonian matrix. The other set of matrices is of dimension 6x6,

with rows and columns labeled by the functions |A52; ClJM) of the ^2+ lower state, the functions

being characterized by a fixed numerical value of 7, a fixed algebraic value of M, and by A = 0,

2 = ± ^/2. ± ^/2. ± V2. It is convenient to represent an individual matrix from this set of 6 X 6 matrices

by the symbol Hi {J"), where J" is the J-value characterizing this particular lower state Hamiltonian

matrix.

It is often desirable to perform a factorization in intensity calculations hke that used in writing

(1—27) from (1—25), since this allows parity selection rules to be taken into account at once. For

simplicity of presentation, such a factorization is not performed below.

The diagonahzation of a given upper state Hamiltonian matrix HuW) will be accomplished by

a particular transformation matrix U{J'); the diagonahzation of a given lower state Hamiltonian

matrix HiiJ") wiU be accomplished by a particular transformation matrix L{J"). As is well known

[17] (pp. 26-28), the product matrices

U-HJ')HuU')UU')

(3-35)

L-^U")HiU")LU")

are diagonal if the columns of U contain the eigenvectors of //» and the columns of L contain the

eigenvectors of Hi. The matrices U, f/-i, L, and L-\ obtained from step (i), will be used in step (iii). Step Hi). The determination and systematic handhng of basis set matrix elements of the dipole

moment operator for a -^A — transition can be earned out as foUows. The most general matrix

element of the dipole moment operator in the basis set under consideration has the form (A'S'S';

il'J'M'\iXz\A"S"l": Cl"J"M"). However, nonvanishing matrix elements of fxz satisfy the selection

37
rule A.M—0: it is thus convenient to set M' = M" = M in this general matrix element. Furthermore,

the Hamiltonian operator for a molecule in free space only mixes together basis set functions characterized by the same value of J and the same value of M (step (i)). Consequently, the transformation

from matrix elements of the dipole moment operator betw^een basis set functions to matrix elements

of the dipole moment operator between final wave functions (step (iii)) never requires adding to- gether basis set matrix elements characterized by different values of J' , different values of J", or different values of M. It is thus convenient to group the basis set matrix elements into submatrices iJ-bW , J"'. M) characterized by fixed values of J', J", and M. For the present example of a

4/\ — transition, these submatrices are of dimension 8X6; their rows are labeled by the eight

basis set functions | A'S'S'; Cl'J'M) of fixed J' and M belonging to the upper state; their columns

are labeled by the six basis set functions | A"S"S"; [l"J"M) of fixed J" and M belonging to the lower

«2+ state. We see from eq (3—12) that a matrix element of /az can be represented as the sum of three terms.

(A'S' X'; Cl'J'M\fjiz\A"S"l"; Vi"J"M) =

+ y2{A'S' V\ix^ + ilXy\I^'S" 1") {^l'J'M\ocz^-'iazy\n"J"M)

(3-36)

' +V2{A'S' S'|Atx-i>,|A"S"S") (fi7'M|az^ + iazyin'7"M)

. +{K'S' V\ix,\M'S"X") {VL'J'M\az,\CL"J"M).

The first factor in each of the three terms in (3-36) represents a quantity which can only be

calculated from a rather complete knowledge of the electronic wave functions. These quantities are analogous to the parameters m\ and ^jl ^ introduced in sections 3.4 and 3.5. Following the selection rules of eq (3-3), and making use of various symmetry arguments (chap. 2), we find that the

following independent intensity parameters /u,, must be considered for a 4^ — 62+ transition.

^,=2-'/2(23/2=^/2|Mx + t>y|0+ 5/25/2) =-2-'/^(-23/2 -3/2|/A^-i/Xy|0+5/2 _ 5/^)

/X2 = 2-'/^ (2 3/2 V2|Mx + iAty|0+ 5/28/2) =_2-'/2^_23/2 -V2|)LA^-iAt^|0+5/2 -S/g)

^3 = 2-'/. (2 3/2 - V2IMX + i^ylO^ ^/2 V2) =-2-'/^ (-2 ^/a V2|/x^- 1>,|0+ ^(2 - V2)

^4 = 2-'/^ (2 3/2 -3/2|Mx + iAty|0+5/2 - V2) = -2-'/M-2='/2^/2|Atx-i>J/|0+5/2l/2)

^3 = 2-'/^(-23/23/2|ia^+iAty|0+5/2 -3/2)=-2-'/2(23/2 -3/2|^_^_j^^|0+5/23/2) (3_37)

/A6 = 2-'/^(-2 3/2 V2|M^+i)Lty|0+ 5/2 - 8/2 )= _ 2 ^ ( 2 3/2 - V2 l/^x " 10+ 5/2 8/2 )

/Lt7 = {2 3/2 V2|At^|0+ 5/2 5/2)=+(-2 3/2 - V2 | /Lt. 1 0"^ 5/2 - 5/^)

^,={23/2 -V2|/X,|0+5/23/2)=+ (-23/21/21^,10+5/2 -S/j)

At9=(23/2 -3/21^,10+5/2 1/2)=+ <- 2 3/2 3/2!^, 1 0+5/2 -1/2).

The quantities /xi through /xe correspond to perpendicular transition moments; the quantities /Lty through /u,9 correspond to parallel transition moments. The relationships between matrix elements

in eqs (3-37) correspond to usingL = 2 and the lower sign choice in eq (2-lla). It can be shown (see

sections 2.11 and 3.5) that for this choice of phases, the parameters ixi through ju,9 are all real,

though not necessarily positive. (If, for example, a value of L = 2 and the upper sign choice had

been used in (2-lla), then the parameters /Ai through ^tg would all have been pure imaginary.)

38
The second factor in each of the three terms in (3-36) contains quantum mechanical matrix

elements (in the basis set \flJM)) of elements of the direction cosine matrix. These matrix elements can be obtained from table 6. The submatrices fXbW , J"; M) can thus be evaluated numerically when numerical values for

J' , J", M and for the /x, of (3-37) have been chosen.

It will become apparent in the description of step (iii) below that it is desirable to define a matrix /X6 {J'; J"), which has the same dimensions as iXbU',J"l M), but which is independent of the

quantum number M. It can be seen from table 6 and from (3-36) that the M-dependence of each

element of a given (i.e., fixed/', /', M) matrix fJibU\J"; M)is the same. In fact, this M-dependence

is given by the quantity hzW , M: J", M) in table 6. The M-independent matrix fJibW; J") is now

defined just like the M-dependent matrix iJLbW , J"; M), except that the M-dependent quantity

hzW, M; J", M) given in the fourth row of table 6 is replaced by the M-independent quantity

hzW; J") , where

hzU ';]") = ^hlU',M:J'\ M) (3-38)

It is thus possible to construct a simplified table 6, given here as table 7, to be used in calculating

matrix elements of elements of the direction cosine matrix, when these matrix elements are to be

used in intensity calculations for unpolarized light and for molecules in the absence of external

fields.

Table 7. A simplified table 6, for use in intensity calculations for unpolarized light andfor molecules

in the absence of external fields. Nonvanishing matrix elements (il' J' M\azs\Q,JM), where s =x, y, or z, can be replaced by the product of three factors: /(y'; J) 'gsU\ n'; J, ft) hzW; J"). The product f-hz of the first and third factors is given in row one of this table; the second factor gs is chosen from rows two and three.

Factor J'=J + l J'=J J'=J-i

fU';J)-hzU':J) [i2(y+i)]-'/. [i2y(y+i)/(2y+i)]--'^

gAJ', ft±l; y, ft) oT^igyW, Sl±l- J, ft) 2[(y+ft + i)(y-ft + i)]''^ + [(y±ft+i)(y±ft+2)]'/^

2ft [(y+ft)(y±ft + i)]''^2[(y+ft)(y-ft)]''^

±[(y+ftHy+ft-i)]''='

As an example of the use of table 7, we consider fXbiJ; J) for the ^A — •'2+ transition under dis- cussion in this section. Making use of (3-36), (3-37), and table 7, we find that iJibiJ; J) has the form

P-lCi 0 0 0 0 0

fX-zCo 0 0 0 0 — fMCe 0 0 0

0 — /Jl-oCo /A9C9 M4C4 0 0

0 0 — fJigCg 0

0 0 0 — AtsCs — AtgCs 0 0 0 0 — At2C2 — fJ^iCr 0 0 0 0 0

(3-39)

if the rows of this matrix are labeled by the functions characterized by A — + 2, S^ + ^/z, + V2, — V2, — ^/2 and A = — 2, S = + ^/2, + V2, — V2, — respectively, and if the columns are labeled by

the functions characterized by A = 0+,X = + %, 4-^/2, + V2, — V2, — ^/2, — ^/2, respectively.

39
The quantities c, are taken from table 7 to be

ci = [ (./ + V2) ( J - ( J + V2)/3J ( J + 1 ) ] C2= [(J+V2)(i-%)(i+5/2)/3J(i+l)]'/^

[(y + V2){i-V2)(.y + =5/2)/3y(j + i)]'/^

[(j+V2)(i+V2)(j+V2)/3;(y+i)]'/^

C5= [( J + V2) ( J + 3/2) (J - V2)/3J(y + 1 ) ]

'/^ C6= [(J+V2)(J + %)(y-3/2)/3J(i+l)]'/==

, C7 = + 5[(J + V2)/6J(J+1)]V^

C8 = +3[(J+V2)/6J(J+1)]'/^

C9 = +1[(J + V2)/6J(J + 1)]'^ (3-40)

It may occasionally be convenient to rewrite (3-39) as the sum of nine matrices, each matrix

depending on only one of the intensity parameters fJCi. Step {Hi). We must now transform from matrix elements of fXz between basis set functions to matrix elements of (jlz between final wave functions. This transformation can be carried out as follows [5] (pp. 208-210):

lxU'J";M)=U-HJ') fJibU'J";M)-LU"). (3-41)

The matrix jXhW , 7"; M) has been defined in step (ii). An element lying at the intersection of a given row and column of this matrix represents the matrix element of fxz between the ''A basis set function labeling the row and the "S^ basis set function labeling the column. The two transformation matrices U{J) and L{J) are defined in the text associated with (3-35). The matrix jLt(J', 7"; M)

contains matrix elements of /jlz between final wave functions. An element lying at the intersection

of a given row and column of this matrix represents the matrix element of fJLz between the *A final wave function labeling the corresponding row of U"^ (the corresponding column of U) and the

final wave function labeling the corresponding column of L. For bookkeeping purposes it is convenient to represent the element of /u,(7', /"; M) lying at the a'th row and /3'th column as At(y',y"; M)a0.

To determine the intensity I{aJ' , fij") of a transition between a rotational level aJ' of the

state and a rotational level fij" of the ^2+ state, it is necessary to square the absolute value of the

matrix element of /iz between pairs of final wave functions \otJ'M') and \fiJ"M"), and then to sum

,over all M' and M". Since nonvanishing matrix elements of M-z are diagonal in M, this double sum

reduces immediately to a single sum. In the notation of this section we can write.

. . /(«y',i3/') oc^|^(y',y";M)„0p. (3-42)

M

It is at this point that the usefulness of the matrix jxiJ'; J") becomes apparent. As mentioned in the discussion of step (ii), the M dependence of aU elements of a given (fixed J' , J", M) matrix

fJibW, J"; M) is the same, and is given by the quantity hzW, M\ J", M) of table 6. It follows from

(3-41) that the M dependence of all elements of/xiJ', J"; M) is also given by hz{J', M; J", M). It can

further be shown that the quantity hziJ'; J") of (3-38) and therefore the matrix ix(J'; J") have been

defined in just such a way that I{aJ\pj") ^^\fxU',J"iM)a,3\'^\fJi{J';J")a0\K (3-43)

' A7

i.e., if table 7 is used to calculate direction cosine matrix elements, then the summation over M

in intensity calculations is taken care of automatically.

From the discussion of this section, it is apparent that intensity expressions obtained by

squaring the appropriate element of fJi{J'; J") will in general be expressed for a ^A — "2+ transition as functions of the nine unknown intensity parameters /u.,. The values of these parameters must

40
be determined by fitting the calculated results to observed intensity data. For any particular transition, many of the (jli may be neghgibly small. For example, Klynning [21], who used essentially the

procedure described in this section, obtained good qualitative agreement between the calculated

and the observed rotational intensity distribution in a ^2 — -FI transition in SnH, even though he

considered as nonzero only two of the five possible intensity parameters for such a transition.

4. Perturbations

4.1. General Remarks

Perhaps the first question to consider in this chapter on perturbations in diatomic molecules

[1] (pp. 280-298) is the following: What is a perturbation and what is not? From an experimental

point of view, a perturbation occurs when some energy level of the molecule is found in an unex- pected position, or when a transition between some pair of energy levels is observed with an

P unexpected intensity. From a theoretical point of view, a perturbation occurs when calculated

quantities disagree with experiment because an important interaction was neglected in the calculation. Both of the preceding statements indicate that what is considered to be a perturbation and

what is not depends largely on what behavior for experimentally observable quantities is initially

I expected, or on what interaction terms in the theoretical formaUsm are initially included.

For example, one sometimes speaks of the rotational levels of a '11 state being perturbed by

the rotational levels of a nearby '5^^ state. Such an interaction leads to A-type doubling in the IT

j state and to a change in the effective B value for the 2 state. Calling this interaction a perturbation

I impUes that our experimental expectations were based on the behavior of an isolated '11 state and

' an isolated '2^ state, and that our theoretical calculations did not initially include interactions

I between these two states. On the other hand, one sometimes considers the rotational levels of a TI and a 'S"^ state simultaneously, as part of a p-complex (L= 1; A = 0, ± 1). This point of view implies that our experimental expectations are not based on the behavior of an isolated '11 and an isolated 'X^ state, and

that our theoretical calculations already include interactions between the X state and H state of

the p-complex, including those which give rise to A-type doubling in the 11 state and to a change in the effective B value of the 2 state. The p-complex can still be perturbed, but only by states exter- nal to itself, e.g., by a 'A state or by a state. Perturbations are sometimes a nuisance and sometimes a source of valuable information. If their origins cannot be understood, they are most often simply a nuisance. However, if a perturbation can be dealt with theoretically, it frequently yields information which cannot easily be obtained

in other ways.

Many techniques have been developed for deahng theoretically with perturbation problems

in molecular spectroscopy. In this chapter we do not consider all of them. Rather, we concentrate on a two-step procedure of general apphcabihty and widespread use. The two steps consist of:

(i) setting up a Hamiltonian matrix which correctly contains the effects of the perturbation(s) to be

considered, and (ii) diagonahzing this matrix on a computer. It is hoped that the explanatory

material and the three examples described below illustrate this procedure well enough to allow a diatomic-molecule spectroscopist to embark on his own perturbation calculations with relatively

little help from a professional theoretician.

The Hamihonian matrix for a perturbation calculation is set up by a procedure similar to that described in chapter 1, except that more electronic states must be included in the basis set. For

example, if a state were perturbed by a '11 state, the basis set for the perturbation calculation

would contain the six wave functions of the ^A state and the two wave functions of the '11 state. In the absence of the perturbation, one could (and would) consider the six wave functions of the

''A state by themselves, and the two wave functions of the '11 state by themselves.

Intensity matrices for perturbed states can be set up by a procedure similar to that described

in chapter 3, except that again a larger basis set must be used for the perturbed state(s).

41
4.2. Homogeneous and Heterogeneous Perturbations

Many perturbations in diatomic molecules can be characterized as either homogeneous or heterogeneous [1] (pp. 284-286). Homogeneous perturbations take place between electronic * states satisfying the selection rule AA = 0 or Afi = 0. Heterogeneous perturbations take place be- tween electronic states satisfying the selection rule AA = ± 1 or Afi = ± 1. Ambiguities can clearly

arise in this classification scheme. Is, for example, the perturbation between a case (a) ^Hi/^ state and a case (a) ^Ha/j state homogeneous (AA = 0) or heterogeneous (An = ±l)? Some measure of

consistency can be achieved by requiring the nomenclature to reflect the selection rules on fl ' when, as in cases (a) and (c), the rotational energy levels are given approximately by BJ { J -\-\)

,

and to reflect the selection rules on A when, as in case (b), the rotational energy levels are given

approximately by BN{N+1). It is interesting to note that the transition from Hund's case (a) to Hund's case (b) as J increases in a ^H state might thus be described as resulting from a heteroge- neous perturbation of the rotational levels of the ^Hi/^ state by those of the ^Ha/j state (or vice

versa).

These two types of perturbations can be described in another way. Homogeneous perturbations are those which can occur in the nonrotating molecule, i.e., perturbations caused by Hev

The rigorous selection rule is thus An = 0 for nonvanishing homogeneous-perturbation matrix

elements in the basis sets used in this monograph, with the approximate selection rules AS=0, AA=0,

AS = 0, when S, A, X are good quantum numbers in the basis set. Heterogeneous perturbations

are those which can only occur in the rotating molecule, i.e., perturbations caused by Hr. The rigor- ous selection rules are thus A.J — 0 and An = ±l for nonvanishing heterogeneous-perturbation

matrix elements in the basis sets used in this monograph, with the approximate selection rules AS = 0, and AA = ±1, AS = 0 or AS = ±1, AA = 0, when S, A, S are good quantum numbers in the basis set, as well as the approximate rule AL = 0 when L is a good quantum number in the basis

set. Matrix elements for homogeneous perturbations do not involve the rotational quantum number

J; matrix elements for heterogeneous perturbations do involve J. Many of the selection rules stated

in this paragraph can be derived by considering the operators Her and Hr, together with appropriate angular momentum commutation relations [7] (pp. 59-64).

Homogeneous perturbations arise most frequently in practice because of spin-orbit interaction

(see sect. 4.4), but they may also occur because of configuration interaction. In the latter case, the perturbations are often very large and difficult to treat accurately [22-24]. Heterogeneous

perturbations occur because of uncoupling phenomena, i.e., uncoupling of the spin angular momentum (see sect. 1.9) or uncouphng of the orbital angular momentum (see sect. 4.3) from the inter- nuclear axis because of rotation. These uncoupling phenomena can be attributed to Coriohs in- teractions in the rotating molecule.

Since the two nonrotating-molecule wave functions of a 'H state are characterized by fi = ± 1, while the nonrotating-molecule wave function of a ^2+ state is characterized by 11 = 0+, there can

be no interaction between these states in the nonrotating molecule. A homogeneous perturbation

of one of these states by the other is not possible.

However, the rotating-molecule operator Hr of eq (1-10) has in general nonvanishing matrix

elements between states with AA = ±1 and A5 = 0. Consequently, a heterogeneous perturbation

of one of these states by the other is possible. This heterogeneous perturbation can be treated

theoretically as follows. Consider a 3 X 3 Hamiltonian matrix with rows and columns labeled by the three interacting wave functions \ ASl;ilJ M) = \ 1 00; 1 J M) , \-100;-lJ M) and | 0+ 0 0; Oy M). We find, as outlined below, that this matrix has the form

4.3. Example: 'H — Heterogeneous Perturbation

Bn[JU+l)-l]+Bn(Ll)u

0 Bu[JU+l)-l]+Bu(Ll)u

-B{n\L,\^)[jU+i)yi'^ -B{n\L,\^)[jU+i)V''

0 -B{n\u\l}[jU+i)V''

-B{n\L,\i)UU+i)V''

E^+BdU+l)+Bi{Llh

(4-1)

42
where the upper lefthand 2X2 diagonal block represents the TI state by itself (arbitrarily placed at the energy origin in the nonrotating molecule), where the lower righthand 1x1 diagonal block

represents the state by itself (located at energy £'2 in the nonrotating molecule), and where the

ofF-diagonal elements represent the heterogeneous perturbation (Coriolis interaction) between

the two states. Matrix elements for the 2x2 and 1 X 1 diagonal blocks were obtained as described

in chapter 1. Off-diagonal matrix elements were obtained as follows. The first line of terms in Hr, when Hr is written as in eq (1-11), contains operators which

give rise to nonvanishing matrix elements only if An = 0. Consequently, the first line of (1-11)

cannot be responsible for an interaction between the '11 and the states. The first and third

terms of the second line of (1-11) give rise to nonvanishing matrix elements only if AS=±1.

Consequently, these two terms can also not be responsible for the interaction under consideration

here. The second term of the second line of (1-11) gives rise to nonvanishing matrix elements if AA = ± 1 and AS = AS = 0. This term thus does connect the '11 and '2+ states under consideration.

The second term of the second line of (1—11) contains four operators: J+, L+, L— Matrix

elements of 7+ and J- can be obtained from expressions of the form (1—13) and (1-14), since both

J and n are good quantum numbers in the basis set. Matrix elements oi L+ and L- cannot be found

from such expressions, since L is not a good quantum number. However, matrix elements of

L h: Still obey the selection rule AA =± 1, because A is a good quantum number. It can be shown

by using symmetry arguments involving (Ti- (see chap. 2), that

(10 0; UM |L+y_ |0+00;0yM)=±(-100;-iyM |L_y+ |0+00;OyM), (4-2)

where the + or — sign on the righthand side of (4—2) allows for various possible sign choices and

values of L in applying the transformation equation (2-lla) to the Tl state. Neither sign choice in (4—2) leads to inconsistencies elsewhere in the calculation, since no matrix elements between the

two components of the '11 state are introduced. The relative phases of the electronic orbital wave

functions | A = +l) and |A = — 1) are fixed once and only once by the choice of sign in (4-2).

The + sign was chosen in writing (4—1), where the quantity B{Yl | L+ | 2), given by

fi{n |L+ |2) ^fi(100|L+ 10+00), (4-3)

is considered to be an, unknown adjustable (real) parameter. Energy levels can be obtained by

diagonaUzing (4-1).

Actually, (4—1) can be factored by symmetry into a 2 X 2 diagonal block and a 1 X 1 diagonal

block, by using the functions 2-'/2[| + 1 0 0; + 1 J M) ± | - 1 0 0; - 1 7 A/)] as a basis set for the

Tl state, but this factorization will not be performed here. Van Vleck [25] has introduced the phrase "pure precession" in connection with heterogeneous

perturbations. From a quantum mechanical point of view, pure precession is said to occur whenever

Z- is a good quantum number. If L is a good quantum number, it is possible to use (1-13) to evaluate

the quantity (n|L+|2) occurring in (4-1). Since this quantity is really a matrix element of the form

(A = +1|L+|A=0) we find

(n|Z,+ |2) = (L, A = +1|L+|L, K=Q) = h[L{L + \)yi^ or 0. (4-4)

The value ^ [L (L + 1 ) ] occurs when the 2 and 11 states belong to the same L complex, i.e. , when

the 2 and 11 states represent different projections of the same L along the internuclear axis. The

value 0 occurs when the 2 and 11 states belong to different L complexes.

It is interesting to note that the matrix (4—1) describes several apparently rather different situ- ations. If the state and the '11 state are very nearly degenerate in the nonrotating molecule

(E\ = 0), and if L is an approximately good quantum number equal to unity (L = 1) , then the rota- tional energy levels which result from diagonalizing (4—1) correspond to those of a case (d) p- complex. (Pure case (d) occurs, from a quantum mechanical point of view, when L is a perfectly

43
good quantum number, and when all 2L + 1 states corresponding to different projections of this L

along the internuclear axis are exactly degenerate in the nonrotating molecule.) If the state and

the Tl state are separated by a distance large compared to the distance between neighboring

rotational levels {\Ei\>BJ) ^ then the rotational energy levels which result from diagonalizing

(4-1) correspond to those of a normal '2+ state and those of a normal ifl state with some A-doubling.

If the rotational energy levels of the state cross those of the '11 state for some value Jc of the

rotational quantum number (i.e., if £"1 + fiiJc (Jc + 1 ) — BuJcUc^^) for some Jc), and if the inter- action between the '2+ state and the '11 state is very small ( | (n|L+|X) |<^^) , then the rotational

energy levels resulting from diagonalizing (4-1) will correspond to those of a normal ^X"*" state and

those of a normal 'IT state, except for J values in the neighborhood of Jc. The rotational levels of

the '2+ state having J — Jc and the rotational levels of one component of the '11 state having

J ~ Jc will suffer small equal and opposite displacements, corresponding to what might be called

the usual heterogeneous perturbation.

4.4. Example: 3 A — ' n Homogeneous Perturbation

A ^ A state cannot easily interact with a ' IT state via the operator H,-, since terms of this operator

give rise to nonvanishing matrix elements only if the selection rule ilS = 0 is satisfied. To the extent

that S is a good quantum number, a heterogeneous perturbation of one of these states by the other

is not possible. Actually, of course, S is not a perfectly good quantum number and heterogeneous

perturbations between a-^A state and a 'II state can take place.

The nonrotating-molecule ^A state gives rise to multiplet components characterized by fl = ±3,

±2, ±1; the nonrotating-molecule '11 state gives rise to "multiplet components" characterized by

n = ±l. Thus a homogeneous perturbation (caused by the spin-orbit interaction term in Hev) is also possible between the ^A state and the '11 state, corresponding to an interaction between the

two multiplet components with fl = +l and between the two multiplet components with (1 = — 1. We now consider this homogeneous perturbation.

The full Hamiltonian matrix for this problem is of dimension 8x8. However, this matrix

immediately factors into two identical 4X4 diagonal blocks. We consider only one of these below,

with rows and columns labeled by the wave functions |A52; ftJM)= |2 1 1; 3JM), |2 1 0; 2JM),

|2 1 — 1; 1 JM), and |1 00; \ J M). This Hamiltonian matrix has the following form.

B^[J{J+l)-^]+B^{Ll)^ + 2A, -B^[2{J -2)U + ^)V'\ 0,

-fiA[2(J-2)(J + 3)]'/% fiA[J(J+l)-2]+fiA<Li)A, -5Ar2(J-l)(J + 2)]-/%

0, -5A[2(J-l)(J-f 2)]'/^ BJU^l)+B^{L\)^-2A,

0, 0, r,

0

0

(4-5)

£n + 5n[J(J+l)-l]+5n(Li)n

The upper left 3x3 diagonal block is just the Hamiltonian matrix for a ^A state, set up as described

in chapter 1. (In this block we have written the spin-orbit energies as /4A2, which results in three

evenly spaced components of the '^A state in the nonrotating molecule.) The lower 1 X 1 diagonal

block is the Hamiltonian matrix for a 'H state. The off-diagonal element 17 represents the spinorbit interaction between the two states with il = + 1 and is given by (see sect. 1.3) 44
T, = (2 1 -l:iyM|^^(r01rsi|100;iyM)

i = (2 1 -l|^^(n)lrs,|100) (4-6)

( = ±(-2 11|2;^(r,)lrs,|-100),

\ where the second line results from the fact that the spin-orbit interaction operator does not involve

the rotational variables, and where the third line results from symmetry arguments (see chapt. 2). The second hne of (4-6) shows that 17 is independent of J. The third line represents part of the

algebra leading to the factorization of the original 8x8 Hamiltonian matrix into two identical 4x4

diagonal blocks. The matrix element 17 can only be evaluated theoretically if the electronic wave

functions for the state and for the ^11 state are rather well known. Such information is usually

not available, so that this matrix element must be treated as an unknown adjustable (real) parameter, to be determined from a fit of calculated results to experimental data. Rotational energy levels are calculated, of course, by diagonalizing (4^5). The effect of a heterogeneous perturbation (Afl = ± 1) between the state and the ^11 state can be taken into account by placing the quantity \ [{J — 1) {J + 2)]^!'^ in the (2,4) and (4, 2) positions

of (4-5). This quantity consists of a /-dependent part, determined as in section 4.3 from (1-13) and

(1-14), and a small y-independent adjustable (real) parameter k, which is analogous to the parameter

fi(n|L+lS) in section 4.3. (The fact that tj and X can simultaneously be taken as real must, of course, be proven (see chapt. 2 and sect. 3.5)).

4.5. Van Vleck Transformations

It is sometimes desirable to take into account perturbations arising from rather distant states without actually including these states in the matrix to be diagonalized exactly. Under these

circumstances, it is convenient to use a Van Vleck transformation [26] (pp. 394-396) to correct

the matrix elements within the submatrix to be diagonalized for effects arising from interactions

with the distant states. (A Van Vleck transformation is essentially a transformation from the

original Hamiltonian to a new Hamiltonian in which first-order interactions between the states under consideration and the distant states are eliminated. For more information, the reader is referred to reference [26].) If the Hamiltonian is divided into a zeroth-order part, a first-order part,

and a second-order part

H= Ho + H, + H2 (4-7)

where the matrix of Ho is diagonal in the basis set under consideration, then the corrected matrix

elements (to second order) within the block actually to be diagonalized are given by

{i\Ho[j) + {i\H,[j) + {i\H,[j) + J^{i\H,\k){k\H,\j)[y2(E° + E])-Elll^ (^8)

In (4-8), the indices i and j are chosen from within the block to be diagonalized; the summation

index k runs over aU values outside this block. The energies Et are those calculated for each state from Ho.

4.6. Example: A-type Doubling in a 'IT State

A-type doubling in a ' H state arises from perturbations of the rotational levels of the ^ H state by

the rotational levels of a distant state [25] . In section 4.3, we considered a 'H — iX+ perturbation

problem in which the two electronic states were both included in the matrix to be diagonalized. In

45
this section we consider the same problem, but take into account the effect of the ^2 state by a Van Vleck transformation.

We thus intend to diagonahze exactly only the upper left 2X2 diagonal block of the matrix

given in (4-1). This block can be corrected, according to eq (4-8), by adding the quantity

-[fi2|<n|L,|X)p/£i]y(j+i) (4r-9)

to each of the four matrix elements in the block. Diagonahzation of the resulting 2X2 matrix yields

the desired A-doubled rotational energy levels.

£rot = firi/ (7 + 1) - Bn + fin ai)n

(4-10)

E^,^[Bn-2B^\{U\L+\X)\'IE^]J{J +1) -Bn + Bn{Ll)u

The A-doubled rotational levels of the '11 state are often represented as symmetrically split

about some mean position. Such a representation causes no difficulty when the '11 and '2 states are well separated. However, if the states are approaching case (d) coupling, it is essential to represent the probelm as indicated here, namely only one-half of the '11 rotational levels are per- turbed by interaction with the '2+ state. The other half of the levels are completely unaffected.

4.7. Centrifugal Distortion Corrections to Rotational Energy Levels

Centrifugal distortion, which can be considered from one point of view as a perturbation of

rigid-rotor rotational energy levels, may be treated in diatomic molecules as follows. We rewrite

eq (1-10), making two changes.

Hr=B{Q)h-^[Rl + Rl]

(4-11)

--B{Q)h-'[Us-L,-S.y+Uy-Ly-Syy].

First, careful attention is paid to units, and dimensionless angular momentum operators are obtained by considering h~^[R'^+ R^] rather than R'^ + Ry. Second, the quantity B is no longer taken

to be a constant, but is rather given its true functional form [3] (p. 13) in terms of the vibrational

coordinate Q= r — rg

fi((?) - ^2/2^(re + = [^2/2/^r|] [1 + C>/r,] -2

(4-12) = Be-2(Belre)Q + 3{Belrl)Q2-. • . .

Note that /x is the reduced mass mini'J (mi + m-z) of the diatomic molecule and that B{Q) has the

dimensions of energy, i.e., ML^T~^.

The next step is to apply a Van Vleck transformation (see sect. 4.5) to the problem. The complete Hamiltonian H = Hei + Hr is considered to be divided into two parts H = Ho + Hi, such that Ho = Hev and Hi = Hr. Energies of the unperturbed states are thus equal to energies of the nonrotating-molecule states. Wave functions for the unperturbed states (the basis set functions) have

one of the forms (see sections 1.4 and 1.7): \Cl; v; njM), |A5X; v; ClJM) or |LA52; v; VlJM). Since

centrifugal distortion involves an interaction between rotation and vibration, a vibrational quantum

number v is included in these basis set functions. They are still taken to be simple products, however, of the form \D.)\v)\VlJM) , \hS^)\v)\VLjM) , or \L\SX)\v)\Q.JM) , respectively. We consider,

in the submatrix to be diagonalized exactly (sect. 4.5), states characterized by fixed J and M,

by fixed vibrational quantum number v, and by fixed |fi|, or by fixed |A| and S, with 2 = S,

S — 1, . . ., — S, or by fixed L and S with A = L, L — 1, . . ., —L and 2 = S, S — l, . . ., —S,

respectively, for the three types of wave functions given above. For the purpose of calculating

centrifugal distortion corrections to the rotational energy levels, the distant perturbing states

(sect. 4.5) are taken to be states characterized by the same fixed value of J and M, by different 46
values of the vibrational quantum number v, and by electronic quantum numbers from the same

set n, A52, or LASS, respectively.

Consider now centrifugal distortion corrections for a situation in which the (2L + 1)(2S+1)

states of fixed L, S, J, M, and v having wave functions of the form |LA5S; v; HJM) he close together

in energy compared to the separation between adjacent vibrational levels. Under these circumstances the fourth term of (4-8) can be written

(LAS 2; v;njM\Hr\L\"S 2"; v"; n"JM)(LA"S 1"; v"; a"JM\Hr\LA'S VivM'JM)

IS.

where the energy separation between two states |LAS2; v; CIJM) and |LA"S2"; v"; Cl"JM) has been

approximated by the purely vibrational energy separation Ev — Ev" = {v — v")hv, and where the summation indices take on the values v" ¥^ v; A" = L, L — 1, . . .,— L; and 2" = S, S— 1, . . ., — S. Since the basis set functions are simple products of a vibrational part, and a rotational and

electronic part, and since Hr of (4-11) is also a simple product of a vibrational part and a rotational- electronic part, (4-13) can be written as the product of a vibrational part and a rotational-electronic

part. If we make a further approximation and consider only the constant and linear terms in the

series expansion (4-12) for B{Q), we find that (4-13) can be written as ^ {v\-2{Belre)Q\v"){v"\-2{BJre)Q\v}{E,-E..d-' X

(4-14)

2 (LASS; njM\fi-^[Rl+Rl] \LA"S 2"; n"JM) (LA"S 2"; n"JM\h-^[Rl+ Rl] \LA'S V; il'JM)

The sum overt;" in (4-14), i.e., the vibrational factor in the product, can be evaluated exphcitly

since [4] (pp. 67-82)

{v-\-l\Q\v)^[iv+l)hl^TTfJiVyi^

(4-15)

(v-l\Q\v) ^ [vhlinfivyi^

Representing the sum over v" by the symbol — Z)e, we find for the vibrational factor in (4-14)

-De = -mi{hvY (4-16)

where De, Be, and hv are all in units of energy, i.e., ML'^T~'^ . De can be expressed in its more common

units of cm"' by (mentally) changing the units of fie to cm"' and replacing the vibrational energy

interval hv by its equivalent in cm"'. If at the same time we drop the subscript e, we find [1] (p. 103)

L> = 4fiW. (4-17)

The sum over A" and 1" in (4-14). i.e., the rotational-electronic factor in the product, can also be evaluated explicitly. It can be shown by considering various commutation relations [7] (pp.

45-78), [8], that the operator R%+Rl, when acting on a function of the form |LA'S2'; Cl'JM)

transforms it into a linear combination of functions characterized by the same L, S, J, and M, but

by various values of A', 2', and fl' = A' + X'. Consequently, the sum in (4-14) over all values of

A" and 2" permitted for the given (fixed) values of L and S can be coUapsed, just as if the sum were

carried out over all possible basis set functions, to give for the rotational-electronic factor

(LASl; ajM\h-*[Rl-\- Riy\LA'S.l'; n'JM). (4-18)

47
Combining the vibrational factor (4-16) and the rotational-electronic factor (4-18), we rewrite

the Van Vleck correction (4-13) as

{LASt;v;njM\-Dh-*[Rl+Riy\LA'SV;v;n'JM) (4-19)

where D is a constant. The centrifugal distortion correction to the rigid-rotor rotational energy

levels for a set of states characterized by fixed L and S, and by the fact that the {2L + 1) (25+ 1)

components of differing A and 2 lie close together compared to the vibrational spacing, can thus

be determined to a good approximation by including matrix elements of the operator

-Dh-^[{J.-L.r-S,)''+Uy-Ly^S,)-y (4-20)

in the (2L + 1 ) (2S + 1 ) X (2L + 1 ) (2S + 1 ) Hamihonian matrices of fixed L, S, J, M, and v to be

diagonalized exactly. By analogy, higher order centrifugal distortion corrections can be obtained

by including matrix elements of Hh^^[R%+ Rl]'\ Kh~'^[Rl+ R'l]*, etc.

Consider as a second example of centrifugal distortion corrections a situation in which the

2(2S+1) or (2S+1) states of fixed | A|, S, J, M, and v having wave functions of the form

I ASS; f; ClJM) lie close together in energy compared to the separation between adjacent vibrational levels. Under these circumstances, the fourth term of (4-8) can be written

V V (^Sl;vMJM\Hr\A"S^"; v";n"JM) (A"SX"; v"; n"JM\Hr\A'SX'; v; n'JM)

where we have approximated the energy separation between two states \AS'^;v;ClJM) and

I A"SS"; f "; il"JM) by the purely vibrational energy separation, and where the summation indices

take on the values t;" 7^ f; A" = ± I A I ; and S" = S, S — 1, . . .,— S. The expression (4—21) can be written as the product of a vibrational factor and a rotational- electronic factor, just as (4-13) was, giving an expression identical to (4—14) except for the absence

of the quantum number L and the new significance of the sum over A". The sum over v" can again be evaluated explicitly, yielding for the vibrational factor the same quantity - De given in (4-16).

The sum over A" and X" this time does not include all functions generated when the operator

acts on a function of the form |A'SS'; Cl'JM). Consequently, the summation over A"

and 2" cannot be collapsed to give'an expression Uke (4-18). However, the sum over A" and X"

does include all functions produced when J and S act on a function of the form | A'SX'; fl'JM).

By a series of arguments it is possible to show that the centrifugal distortion correction to the

rigid-rotor rotational energy levels for a set of states characterized by fixed | A | and S, and by the

fact that the spin-multiplet components of this state lie close together compared to the vibrational

spacing, can be determined to a good approximation by including matrix elements of the operator

-Dh-'[(J^-S.y+{Jy-Syy+ (LI)]-' (4-22)

in the 2(2S+1) X2(2S + 1) or (2S+ 1) X (2S+ 1) Hamihonian matrices of fixed \A\, S, J, M,

and V to be diagonahzed exactly. When centrifugal distortion corrections of the order of DJ'^ are

negligible, (4-22) can be replaced by

-Dh-*[y-MJ^S^+JySy)r]. (4-23)

There are many other situations for which centrifugal distortion corrections could be considered, e.g., the situation corresponding to basis set functions of the form v; ClJM) , or the

situation which arises when one of the multiplet sphttings is approximately equal to a vibrational

interval. We shall not consider these here.

48
The author is extremely grateful to Dr. W. J. Balfour, Dr. G. Herzberg, Dr. Ch. Jungen, Dr.

I. Kopp, and Dr. C. M. Woodman for reading a draft of this monograph, and for communicating to the author their own comments as well as comments from some of their colleagues. This combined

criticism was invaluable in reshaping certain sections of each chapter.

5. References

[1] Herzberg, G., Spectra of Diatomic Molecules (D. Van Nostrand Co., New York, N.Y., 1950).

[2] Landau, L. D., and Lifshitz, E. M., Quantum Mechanics, translated from the Russian by J. B. Sykes and J. S. Bell (Pergamon Press, London, England. 1958).

[3] Kronig, R. de L.. Band Spectra and Molecular Structure (Cambridge Univ. Press, Cambridge, England, 1930).

[4] Pauling, L.. and Wilson, E. B., Jr.. Introduction to Quantum Mechanics (McGraw-Hill Book Co., New York, N.Y.,

1935).

[5] Dicke. R. H.. and Wittke, J. P.. Introduction to Quantum Mechanics (Addison-Wesley Publ. Co., Inc., Reading, Mass.,

1960).

[6] Wilson. E. B., Jr., Decius. J. C. and Cross, P. C, Molecular Vibrations (McGraw-Hill Book Co., New York, N.Y.,

1955).

[7] Condon. E. U., and Shortley. G. H.. The Theory of Atomic Spectra (Cambridge Univ. Press, Cambridge, England,

1951).

[8] Van Vleck, J. H., Revs.. Mod. Phys. 23 , 213 (1951).

[9] Hougen. J. T.. J. Chem. Phys. 36, 519 (1962).

[10] Hill. E., and Van Vleck. J. H.. Phys. Rev. 32, 250 (1928).

[11] Schlapp, R.. Phys. Rev. 51, 342 (1937).

[12] Kayama, K., and Baird, J. C, J. Chem. Phys. 46, 2604 (1967).

[13] Hougen, J. T., Can. J. Phys. 40, 598 (1962).

[14] Longuet-Higgins, H. C, Mol. Phys. 6, 445 (1963).

[15] Hougen, J. T., J. Chem. Phys. 37, 1433 (1962).

[16] Hougen, J. T., J. Chem. Phys. 39, 358 (1963).

[17] Wigner. E. P., Group Theory (Academic Press. New York, N.Y., 1959).

[18] Townes, C. H., and Schawlow, A. L., Microwave Spectroscopy (McGraw-Hill Book Co., New York, N.Y., 1955).

[19] Watson, J. K. G., Can. J. Phys. 46, 1637 (1968).

[20] Schlapp, R., Phys. Rev. 39 , 806 (1932).

[21] Klynning, L., Arkiv for Fysik 31, 281 (1966).

[22] Lagerqvist, A., and Miescher, E.. Helv. Phys. Acta 31, 221 (1958).

[23] Felenbok, P., and Lefebvre-Brion. H., Can. J. Phys. 44, 1677 (1966).

[24] Lewis, J. K., and Hougen, J. T.. J. Chem. Phys. 48, 5329 (1968).

[25] Van Vleck, J. H., Phys. Rev. 33, 467 (1929).

[26] Kemble, E. C. Quantum Mechanics (Dover PubUcations. New York, N.Y., 1958).

[27] Bunker. P. R., and Papousek. D., J. MoL Spectr. 32, 419 (1969).

[28] Kovacs, I., Rotational Structure in the Spectra of Diatomic Molecules, translated from the Hungarian by L. Nemes

(Adam Hilger Ltd.. London, 1969).

U. S. GOVERNMENT PRINTING OFFICE : 1970 O - 371-908

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