NBSIR 75-751

AFOSR SCIENTIFIC REPORT

AFOSR-TR- 75-15 38

THERMODYNAMICS OF CHEMICAL LASER AND HIGH TEMPERATURE SPECIES

C. W. Beckett, Project Director

Physical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

October 1975

Final Report for Agreement No.

AFOSR-ISSA- 75-0001

July 19 74-June 19 75

Approved for public release; distribution unlimited

Prepared for

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Building 410, Boiling Air Force Base

Washington, DC 20332

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part by or for the United States Government is permitted.

Previous reports describing related work have the NBS Report Nos.

6297, 6486, 6645, 6928, 7093, 7192, 7437, 7587, 7796, 8033, 8186,

8504, 8628, 8919, 9028, 9389, 9500, 9601, 9803, 9905, 10004, 10074,

10326, 10481, 10904, and NBSIR 73-280, 73-281, 74-600.

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NBSIR 75-751

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TEMPERATURE SPECIES

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July 1, 1974 - June 30, 1975

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Chemiluminescent ; diatomic molecules; dissociation energies; high

temperature; PbO; thermophys ical measurements

20 ABS T "* r T f Continue on reverse side if necessary and Identify by block number)

This report (covering July 1974 through June 1975) gives summaries (a) for

high-speed thermophysical measurements on hafnium, iron, tantalum, and

molybdenum and (b) for an interpretation of the Raman spectra of PF5 , ASF5, and VF5. Details of these activities have been or are in the

publishing process. In addition, this report includes two articles

which are current and have not started the publication process. One is

a study on the chemiluminescence of the Pb(g) + 03(g) reaction, and the

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20. other is a review of the literature for dissociation energies of diatomic

oxides and fluorides relevant to chemical lasers in the visible region.

Finally, some related bibliographies on chemical kinetics are indicated.

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FO REWARD

Structure, propulsion, and guidance of new or improved weapons

delivery systems are dependent in crucial areas of design on the

availability of accurate thermodynamic data. Data on high-temperature

(including exhaust ions) are, in many cases, lacking or unreliable.

A broad integrated research program at the "ational Bureau of Standards

has supplied new or more reliable thermodynamic properties essential

in several major phases of current propulsion development and application.

Measured were compounds of those several chemical elements important

in efficient propulsion fuels; those substances most affecting ion

concentrations in such advanced propulsion concepts as ion propulsion;

and the transition and other refractory metals (and their pertinent

compounds) which may be suitable as construction materials for rocket

motors, rocket nozzles, and nose cones that will be durable under

extreme conditions of high temperature and corrosive environment. The

properties determined extend in temperature up to 6000 degrees Kelvin.

The principal research activities were experimental , and involved

developing new measurement techniques and apparatus, as well as measuring heats of reaction, of fusion, and of vaporization; specific

heats; equilibria involving gases; several properties from fast

processes at very high temperatures; infrared spectra, matrix-isolation,

microwave, and electronic types ; and mass spectra. Some of these

techniques, by relating thermodynamic properties to molecular or crystal structures, make it possible to tabulate reliably these

properties over far wider ranges of temperature and pressure than those

actually employed in the basic investigations. Additional research

activities of the program have involved the critical review of published

chemical- thermodynamic (and some chemical-kinetic) data, and the

generation of new thermo chemical tables important in current chemicallaser research.

i

ABSTRACT

This report (covering July 1974 through June 1975) gives summaries

(a) for high-speed thermophysical measurements on hafnium, iron, tantalum,

PF5, AsF^, and VF5. Details of these activities have been or are in

the publishing process. In addition, this report includes two articles

which are current nad have not started the publication process. One is a study on the chemiluminescence of the Pb(g) + 03(g) reaction, and the

other is a review of the literature for dissociation energies of diatomic

oxides and fluorides relevant to chemical lasers in the visible region.

Finally, some related bibliographies on chemical kinetics are indicated.

ii

TABLE OF CONTENTS

Forward , i

Abstract . ii

1. High-Speed Thermophysical Measurements

(A. Cezairliyan and J. L. McClure) 1

2. Potential Functions for the vy Vibration of Pentafluorides

(S. Abramowitz, L. S. Bernstein, J. J. Kim, K. S. Pitzer,

and I. W. Levin) 2

3. A Study of the Chemiluminescence of the Pb(g) + 03(g)

Reaction

(M. J. Kurylo, W. Braun, S. Abramowitz, and M. Krauss) .... 3

4. Dissociation Energies of Diatomic Molecules Relevant to

Chemical Lasers in the Visible Region

(R. F. Krause, Jr.) 16

5. Bibliographies on Chemical Kinetics . 30

iil

1. HIGH-SPEED THE KMOPHY S ICAL MEASUREMENTS

Summary

Investigations performed during FY 75 using the millisecondresolution pulse calorimeter are summarized in the following paragraphs.

(a) Measurements were performed of heat capacity, electrical

resistivity, and hemispherical total emittance of hafnium (containing

3.12 wt. % zirconium) in the temperature range 1500 to 2400 K. Also,

measurements were made of the temperature and energy of solid-solid

(hexagonal close-packed to body-centered cubic) phase transformation

in hafni um. The transformation temperature was measured to be 2012 K

(hafnium has the highest temperature for known solid-solid phase

transformations iiji metals). The transformation energy was measured

to be 5900 J«mol . The measurements indicated decreases in heat

capacity (by about 13%) as the result of solid-solid phase transformation. The results on heat capacity are published, Reference [l ]

;

a paper on transformation data is in preparation.

(b) Measurements were performed of the radiance (brightness)

temperature (at 0.65 um) of iron and tantalum at their respective

melting points. The results yielded the values 1670 K for iron

and 2846 K for tantalum. These data in conjunction with the meltingpoints are used to obtain normal spectral emittance (at 0.65 um) of

the two metals: 0.36 for iron, 0.37 for tantalum. A paper describing

the work on iron is in press, Reference [2]; a paoer on tantalum is

in preparation.

(c) Heat capacity of molybdenum in the range 1500 to 2800 K was measured as a part of the Standard Reference Materials Program. The

results are in good agreement (within 1%) with these obtained earlier

by drop calorimetry in the overlaping temperature range (1500 to 2100 K)

.

The combination of the drop and pulse calorimetry results in the range

300 to 2800 K will yield the certification of molybdenum as a heat

capacity standard reference material.

References :

[1] Cezairliyan, A. and McClure, J. L. Simultaneous measurements of

specific heat, electrical resistivity, and hemispherical total

emittance by a pulse heating technique: hafnium-3 (wt. %) zirconium,

1500 to 2400 K. J. Res. Nat. Bur. Stand. (U.S.), 79A (Phys. and

Chem.) , 431 (1975) .

[2] Cezairliyan, A. and McClure, J. L. Radiance temperature (at 653 nm)

of iron at its melting point. J. Res. Nat. Bur. Stand. (U.S.),

79A (Phys. and Chem.), in press.

1

2. POTENTIAL FUNCTIONS FOR THE VIBRATION OF PENTAFLUORIDES

** Stanley Abramowitz, Lawrence S. Bernstein , Janice J. Kim*, Kenneth S. Pitzert, Ira W. Levin

This collaborative approach to the interpretation of the high

resolution Raman spectra of some pentafluorides has resulted in the

estimation of barriers to fluorine atom interchange for PF5, AsF^,

and VF5. A two dimensional harmonic oscillator basis set and a potential of the form

V(p,0) = (l/2)a2

-b cos(3 0)p

3

+ cp

4

was used to interpret the observed spectra. The work on PF5 has been

published (3). A complete paper on AsF^ and VF5 containing more of

the methodology and theory as well as parametric tables for other

workers in the field is being prepared.

Our estimates of the barriers for fluorine atom interchange range

between 995 to 1139 cm"1 , 755 and 864 cm"1

, and 478 and 539 cm"1 for

PF-, AsF and VF respectively (1 cm"1 = 11.96 J/mol)

.

Reference :

L. S. Bernstein, J. 7. Kim, K. S. Pitzer, S. Abramowitz, I. W. Levin,

J. Chem. Phys. 62, 3671 (1975).

* Chemical and Inorganic Materials Laboratory, Lawrence Berkeley

Laboratory, University of California, Berkeley, California

** Bell Telephone Laboratory, Murray Hill, New Jersey (formerly of

University of California, Berkeley)

*** Laboratory of Chemical Physics, National Institute of Arthritis

and Metabolic Diseases, National Institutes of Health, Bethesda,

Maryland

.

2

3. A STUDY ON THE CHEMILUMINE S CEN CE OF THE Pb(g) + 0 (g) REACTION

M. J. Kurylo, W. Braun, S. Abramowitz, M. Krauss

National Bureau of Standards

Institute for Materials Research

Washington, DC 20234

Introduction

The results of our recent studies on laser enhanced reactions have

shown that vibrational energy in a reactant molecule can reappear as excitation in a product species. For example, emissions from both NO * and SO2* produced in the reactions of NO and SO with O3 were observed"

to shift to shorter wavelength when the Oo was vibrationallv excited

using a CO2 laser-1 *-. Because of the spectroscopic complexities of

the triatomic molecules, NO2 and SO2, our intention in the present

study was to investigate a chemiluminescent reaction producing a diatomic product which would be, in principle, easier to characterize

spectroscopically . The metal atom-oxidant systems represent a class

of such reactions. A significant number of these have been inves tigated

to date because of their potential as chemical lasers. We report

herein some observations on the reaction

Pb + 0

3 * PbO + 0 . (1)

While the information obtained from studying the infrared laser enhanced

reaction component is minimized by the overall reaction complexity, it

does nevertheless provide some additional insight into this reaction

system. This information coupled with new high pressure spectroscopic

results complement the detailed low pressure investigation by Oldenborg,

Dickson, and Zare (ODZ) 3.

Experimental

The furnace, reaction cell, fast flow pumping system, CO2 laser,

and spectrometer are shown schematically in Fig. 1. Lead vapor,

produced from a resistively heated crucible containing lead metal enters

the glass reaction chamber in an Ar diluent stream. The 4% O3 in O2

effluent from a commercial O3 generator is mixed with Ar and flows

past the cell windows (to eliminate window deposits) into the cell.

There it diffusively mixes with the lead- argon flow. The temperature

of the crucible (as measured by a thermocouple probe) ranges from

900-1000 K while the temperature in the flame reaction zone varies from

500-600 K. The total pressure in the cell varies from 1 to 5 torr

with the flow through the furnace being anywhere from 20 to 50% of the

total flow.

3

The 0.5 cm diameter beam from a CO2 laser tuned to the 9.6 urn P(30)

transition is square wave chopped and traverses the flame exciting

V3, the asymme trie stretching mode of O3 (1043 cm""-'-). The chemiluminescence

from the Pb + O3 reaction is monitored through a spectrometer-photomultiplier assembly. The photomultiplier output is fed through a series of pulse amplifiers and voltage discriminator into a dual

counter, one channel of which records the "laser-on" signal and the

other the "laser-off". A printout from these counters is synchronized

with the spectrometer wavelength-scan-drive thereby facilitating the

recording of the modulation spectrum vs wavelength. The normal spectral

("laser-off") emission is automatically obtained from the "laser-off"

counter.

Results

In contrast to the spatially sharp diffusion flame observed for

the Ba + O3 (or N2O) reaction, the Pb - O3 flame is quite diffuse

(apparently reaction limited) . It is brightest in the high temperature zone at the furnace nozzle, diminishes with decreasing temperature

(increasing distance from the nozzle), and persists for some two to

three feet into the pumping system. These observations suggest a reaction rate for Pb + O3 which is considerably slower than gas

kinetic. The spectrum which we observe at several torr total pressure

partly resembles the low pressure spectrum obtained by ODZ and reproduced by us in a similar quasi-beam apparatus. The short wavelength

end of the more complete spectrum (Fig. 2) can be identified from

the low (submicron) pressure spectrum in that it is less diffuse. The

long wavelength portion between 480 nm and 595 nm is almost entirely

the new a state recently characterized by ODZ. At still longer wavelengths, our spectrum differs from that of ODZ in that we observe a series of strong lines which do not agree with a X, A -* X, or

B -* X, but rather appear to originate from a new state observed via

six weak lines by ODZ and identified as b by them. This b state is seen more intensely in our high pressure spectrum. This series of

lines in our spectrum can be fit to the expression:

v, , ltN = 16315 + 441.0 v

f - {717.7 v"

2 - 3.53 v"

2

} (v' ,v")

where the lower state constants have been taken from Rosen's compendium.^

A definite vibrational assignment for an electronic state, for

which a rotational analysis has not been observed, requires the measurement of a vibrational isotope shift. Lacking data of this type one tries to assign band heads such that both (0,v") and (v',0) levels

are observed. The assignment hypothesized by ODZ on the basis of six

observed transitions in a (v',0) transition has placed TOQ of the

b state about one vibrational quanta above TQO of the a state. In

this study 20 band heads have been observed that can be assigned to

b state. In addition to the (v',0) band heads, other bands have been

4

observed with v" = 1, 2, 3, 4, 5. The hypothesis regarding the

separation of the a and b states is supported by arguments given later.

Although considerable emission intensity is observed for the

B •*> X transition, the production of this state lies above the thermodynamic threshold for the reaction

Pb (J = 0) + 0. =» PbO(B) + 0 .

Oldenborg and Zare have suggested that production of this state results

from reaction of Pb (J = 1) present in their experiments due to an electric discharge in their furnace. The strong B X emission observed

in our high pressure experiment was found to vary markedly with the oven temperature. The observed B -> X emission increase over that emission

from either a or b -> X with increasing temperature obeyed an Arrheniustype formulation with an activation energy of some 10 kcal (i.e. nearly

equal to the energy difference between J = 0 and J = 1 multiplet com- ponents of Pb) . This observation suggests that Pb (J = 1) is produced

thermally in our furnace as opposed to its possible production via

secondary processes, eg.

PbO* + Pb (J = 0) + PbO + Pb (J = 1)

If PbO(B) is indeed produced through reaction of Pb (J = 1) with

ozone, the rate of this reaction must be extremely fast to compete

with the deactivation process

Pb (J = 1) + 0

2

+ Pb (J = 0) + 0

2

since the deactivation proceeds with a high rate constant and the O2

pressure is twenty times that of O3 under our usual experimental

conditions

.

Part of the modulation spectrum is reproduced in Fig. 3. This

spectrum represents only that component of the chemiluminescen t emission

which varies with the laser excitation of O3 (i.e. only that component

which either increases or decreases due to a change in reaction

rate with reactant vibrational excitation). Thus, if the emission

originated only from a single electronic state, the modulation spectrum

would appear as a quas i-continuum with possibly some small structure

due to sharp changes in population of one vibrational line relative

to an adjacent one. The large amount of structure in Fig. 3 indicates

the presence of emission from at least two electronic states. One

might speculate that at the longer wavelength, where the normal spectrum

consists mainly of a and b emission lines, this modulation structure

could reveal an increase or decrease of b emission lines relative to a emission lines (or vice versa). However, the peaks (or valleys) in

the modulation spectrum do not agree well with either a or b state

emission lines. More surprisingly, they agree much better with A -> X

transitions. We therefore tentatively identify the modulation spectrum

as a quasi continuum due to a -* X and b X emissions at long wavelengths

5

(possibly some B -> X at short wavelength) with structure due to A > X

emission lines superimposed throughout the entire wavelength range.

This suggests that vibrational excitation in O3 increases the production

of the a, b, and possibly the B state of PbO all to some small degree,

while increasing A state production most significantly. We must

emphasize, however, that the assignment of A state responsibility for

structure in the modulation spectrum is tentative since the modulation

data are difficult to obtain, require long integration times, and should

be obtained in the low (submicron) pressure region where only the a and B states have been observed. Nevertheless, we can speculate on the

production of PbO (A) in the laser excited reaction. It is possible

that as a result of laser excitation of O3 forming 03"^ the reaction

with Pb (J = 1) now produces PbO (B) in higher vibrational levels.

These can then be collis ionally transferred to the A state which

is seen in emission. A second equally likely possibility is that

the vibrational energy in O3 increases the rate of A state production

directly at the expense of one or more of the other reaction channels.

From the numerical value of the percent modulation in the laser

experiments (i.e. the continuum region assigned to a and b state

enhancement) we can calculate a rate constant enhancement of approximately

a factor of two. This is obtained by comparing the percent modulation

of both the Pb + 03^ and NO + 03"^ reactions in this apparatus under

identical experimental conditions.

We can obtain a very rough estimate of the quantum yield of a and b state production in the following manner. First we assume the

reaction of O3 with Pb ( J = 1) funnels entirely into PbO (B) , (i.e. quantum yield of unity) . From Fig. 2 we see that the a + b

emission intensity is comparable to the intensity of PbO (B) . However, at the oven temperature of 1000 K only 1 part in 10^ of the

Pb atoms are in J = 1. This would imply a quantum yield of a + b due

to Pb (J = 0) of about 10~5 assuming Pb (J = 0) and Pb (J = 1)

reacted with O3 with the same rate constant. The observation of the

decay of chemiluminescence over several feet in our flow tube points

to a slow rate of reaction for Pb (J = 0) while the fact that the

reaction of Pb (J = 1) must compete favorably with its deactivation

channels speaks for a very fast (collision frequency) rate of reaction

for Pb (J = 1) . Assuming a reaction efficiency of 10"^ for the Pb

(J = 0) reaction in the spectrometer observation zone increases the

quantum yield for a + b state production by a factor of 100 to 10~3.

We compare this to the ODZ upper limit of 10"^ obtained under the

assumption of a fast reaction rate for Pb (J = 0) . A slower reaction

rate would unfortunately raise their quantum yield (in greater

disagreement with our 10"-^ estimate). However our observation of a large luminescent region is not necessary contradictory to ODZ's estimate

of a fast reaction rate because of the different temperature regions

of the two studies. In the quasi-beam configuration, Pb exits the

furnace at very high temperature (>1000 K) and collides with O3

with much higher kinetic energy than in our high pressure experiments

6

(kinetic energy of 600 K at the nozzle tip and decreasing rapidly to

400 K downstream of the mixing zone)

.

Conclus ions

The low lying valence states of the Group IV oxides can be represented by the following molecular orbital configurations:

a it , E (1)

4*3 1

OTT 77 , 71" , TT (2)

2 3 * 3 V + 3 A 3 - 1 - 1. 1_+

a TT 77 , Z , A, Z , E , A, E (3)

3 3 +

The 77 and Z are the lowest energy states resulting from con- figurations 2 and 3 respectively. Oldenborg et. al. have determined

that (unlike CO where the order is reversed) the ^Z

+

is lower in PbO.

For Hund's case c coupling the ^Z+ has an 0" and 3Z component while

the ^77 has 0~

5 0+

, 1 and 2 components and the has an 0+ configuration,

Only the O"

1" and 1 are dipole permitted transitions to the 0+ ground

state.

The splitting of the 0 and 1 components is due to the second

order spin-orbit interaction and therefore proportional to the square

of the spin-orbit integrals. Assuming molecular integrals will scale

with the atomic spin-orbit parameters, the splitting in the case of

SnO would be an order of magnitude less than the value for PbO

while GeO is about two orders of magnitude less.

Oldenborg et. al. identify the two (0~, 1) components of the

3l+ as b and a respectively. This identification which can be made

by the relative intensity7 of the a and b transitions is also consistent

with theoretical considerations. Theory predicts that the 0 component

of the ^Z+ state is at a somewhat higher energy than the 1 component.

There are two mechanisms which could give intensity to the dipole

forbidden b -> X. As J increases the rotational coupling bet-ween the

a and b state increases and one expects to see the Q branches of the

b X transition in addition to the P, 0, and R branches of the a -> X

transition. However it turns out that for large spin-spin splitting

the coupling has only a slight effect even for J as large as 100

(one expects the most probable value of J to be 24 for T = 500 K) .

The intensity of the b transition may also arise from electronicrotation coupling with the 1 component of the 3-r state. This effect

goes as 8 B J (J + 1)/AE where AE is the difference in the

electronic energies of the two states and B is the rotational constant.

Since AE n

- 5000 cm~l and T ^ 500 K this effect is expected to be large

and the states are- significantly mixed. In order to treat this problem

correctly all states arising from the 3v+ and ^tt representations

7

have to be considered. It seems likely that the b state transition

probability would increase with temperature.

3 3 +

In this connection one should also consider the H -> E transitions. These are allowed transitions for both Hund's cases a and

c and should have significant intensity even though they are in the

infrared ("-5000 cm~l) . One might expect that their intensity would be

about the same as the observed a -> X and b -> X transitions.

Cur observations at high pressures are somewhat different from

those of Oldenborg et. al. In particular, we find more intensity

in the b state than they did. Their experiment was done under beam

conditions. The spectral observations were made before colliscns

took place. In our case many collisions can occur before the observation of the emitted light. We also observed that the 3 state

emission (the 1 component of the ± s more intense at high Pb

temperatures

.

Several tentative explanations of the differences between the

Oldenborg et. al. and our results can be advanced. It is possible

that under beam conditions only the lower (or even lowest) J state

of PbO is formed. This would lead to very small mixing of the b

state with the 3 (--) state and would lead to a low observed

intensity. Another possibility is pressure induced emission.

The a state may couple more strongly to the b state at the high

pressures used in our experiments, or alternatively pressure may

enhance 3 - b transitions which would result in a greater observed

intensity for the b state. The observation of the apparent formation

of the A state from the reaction of

Pb + 0

3

~ + PbO (A) + 0

2 is also not easy to explain. In order to further unravel these

questions, additional experiments must be carried out in conjunction

with a more extensive theoretical investigation of the Group IV oxides.

Table 1. Observed Band Heads and Assignments

v(obs) v(calc) assignment

24143 24116 B (4,0)

23590 23640 B (3,0)

22392 234 0 2 B (4,1)

23170 23157 B (2,0)

22957 22926 B (3,1)

22636 22665 B (1.0)

22457 22443 B (2,1)

22178 22166 B (0,0)

21954 21951 B (1,1)

21636 21660 b (12,0)

21474 21452 B (0,1)

21169 21211 b (11.0)

]1079 21065 B (5.5)

20877 20870 a (11.0)

20721 20745 B (0.2)

20614 20616 B (4.5)

20442 20445 a (10,0)

20338 20314 b (9.0)

20149 20156 a (11.1)

20024 20014 a (9,0)

19904 19867 b (8,0)

19743 19731 a (10,1)

19585 19578 a (8,0)

19467 19449 a (11,2)

19128 19138 a (7,0)

19015 19023 a (10,2)

18688 18692 a (6,0)

18515 18530 b (5,0) Sh.

18305 18323 a (10,3)

18238 18241 a (5,0)

17986 17978 a (6,1)

17790 17785 a (4,0)

17615 17630 a (10,4)

17535 17527 a (5,1)

17319 17325 a (3,0)

17212 17199 b (2,0)

17108 17052 b (8,4)

17033 17108 b (5,2)

16915 16853 b (6,3)

16858 16859 a (2,0)

9

v(obs) v(calc) assignment

16773 1675 7 b (1,0)

16614 16664 b (4,2)

16488 16485 b (2,1)

16434 1636 7 b (10,5)

16297 16315 b (0,0)

16215 16220 b (3,2)

16108 16160 b (6,4)

15773 15778 b (2,2)

15623 15601 b (0,1)

15672 15715 b (5,4)

15363 16335 b (1,2)

The calculated v have been generated from a least squares treatment of the observed data using Rosen's constants for the X state. Th

generating equations are:

a. v. , „. = 15912 + 478.4 v' - 2.5 v'

2 - {717.7 v"

2 - 3.53 v"2

b. v, . „. = 16315 + 414.0 v' - {717.7 v"

2 - 3.53 v"2

} (v f ,v")

B. v, f „. = 22166 + 502 v' - 3.8 v

f2 - {717.7 v"

2 - 3.53 v"2

} (v f ,v")

10

Table 2. Observed Band Heads for the Modulated Spectrum

v(obs.) v(calc) v' v"

17592 17599.3 0 3

18319 18299.3 0 2

19015 19006.4 0 1

19708 19720.6 0 0

18005 18041. 1 3

18748 18741. 1 2

17765 17789.3 2 4

18470. 18482.3 2 3

19216 19182.4 2 2

19885 19889.5 2 1

20644 20603.6 2 0

18925 18923.2 3 3

19611 19623.3 3 2

20 350 20330.4 3 1

17972 17984.9 4 5

18642 18670.8 4 4

19384 19363.8 4 3

20064 20063.8 4 2

20773 20770.9 4 1

21463 21485.1 4 0

19826 19803.9 5 3

20517 20504. 5 2

21191 21211.1 5 1

18783 18864.8 6 5

19554 19550.7 6 4

20226 20243.7 6 3

20947 20943.8 6 2

21737 21650.9 6 1

20024 19990.1 7 4

18493 18392.5 8 7 19015 19064.3 8 6

20434 20429.1 8 4

18854 18831. 9 7 19497 19502.9 9 6

17940 17946.8 10 9

18573 18604.5 10 8

19271 19269.2 10 7 19964 19941.1 10 6

20584 20619.9 10 5

21350 21305.9 10 4

11

References

1. W. Braun, M. J. Kurylo, A. Kaldor, and R. P. Wayne, J. Chem. Phys.

61, 461 (1974)

.

2. A. Kaldor, W. Braun, and M. J. Kurylo, J. Chem. Phys. 61_, 2496 (1974).

3. P.. C. Oldenborg, C. R. Dickson, P. N. Zare, J. Mol. Spectroscopy.

In press.

4. B. Rosen, ed., Spectroscopic Data Relative to Diatonic Molecules,

Perganon Press, New York, 1970.

12

13

(S)>un fuomqjLO) a±i SN31N

I

14

NOiivmaow

15

4 0 DISSOCIATION ENERGIES OF DIATOMIC MOLECULES

RELEVANT TO CHEMICAL LASERS IN THE VISIBLE REGION

Ralph F. Krause, Jr.

Selected values of the ground state dissociation energies of

diatonic oxide and fluoride molecules which are relevant to chemical

lasers in the visible region are assembled from the literature. When

an alkaline-earth, scandium group, rare-earth, or group IVA atom has

a bimlecular reaction with a certain oxidizer (O2, O3, N^O, or F£)

,

the enthalpy change to form the corresponding diatomic molecule

may be equivalent to an electronic transition in the visible region.

Most of these molecules have already been the subject of recent

reviews; so no attempt was made to duplicate those reviews which provide established values. The results of equilibrium studies which

have subsequently been published are included for comparison. in

those cases which appear unclear even previously published studies

are examined, and revised values are presented.

I. Introduction

Most of the diatomic oxide and fluoride molecules of interest

here have been subject to recent reviews in the literature. In a recent report [1] the dissociation energies of the scandium group and

rare-earth diatomic oxides were evaluated from published equilibrium

studies. In table 1 values of the dissociation energies of che

alkaline-earth and group IVA diatomic oxides are compared among several

compilations [2, 3, 4, 5, 6, and 7]. The most recent among these are

the new JANAF Tables [7] of the alkaline-earth oxides for which the

new assignrents [27] of the electronic states have been used. Table 2 covers the selections of several reviews on all of the fluorides

of interest here. Again the JANAF Tables [7] seem to provide the

latest values, this time for the alkaline-earth fluorides. Most of the

values selected by the JANAF Tables and reference 1 appear to be

established at least for the present. Since these reviews were performed, some other equilibrium studies have been published. Their

results are included in the present work for comparison.

In the case of those molecules which are missing from the JANAF

Tables [7] or reference 1, the relevant equilibrium studies are examined,

and values of dissociation energies are appropriately documented.

These molecules include some of the scandium group and rare-earth

fluorides and the group IVA oxides and fluorides.

16

OJO

03

<u T3

•HXo6o

(3

co J=

4JU

efl wI<uc

(4-1 o

05<u H

60M

0)gH

4J

CO •HuO

cnCO

14-1 o

009 CO rH

01 H>

ino

CO

g

cc o

•H •H

V4

CO CO & > o

CJ

CO H

EnZ 0)

m rH o o>

CM CN rH rH

+i +1 +1 +1 +i +i CO m • • • • • • m o CM 00 st o co 00 CN St ON 1

CO CO st

m en

T3

)~l CO J3O

C/3 a

4-1 0)sa

60

CO 3

vO

14-1 U

co

+iost

oo

vO

+i om

in,

CO oo

m rH

+1 +i +1 +i o 00 st rH CO m • m CM m. st vO m CO a| vO

r-«

m o o m m O CM

m • • • • • • vO cc CM vO CO VO

r«. CM st ON vO

CO CO st in vO • m m CM

a st

cu +1 +1

CD 14-4 O CU rH o m

Pi • • • on CM in m CO CO rH st

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m

TJ 4J C 4J /-n

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<Dd 0) 5 a) »-i CD CO ^O

pa pd,

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>>

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m CO CO CM CM

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60 cO

u CO

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VO

m co

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60

60

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14-1 33 •

*-> <

<4-( O

CO

St 03 • 0) VO 3

ON rH

CO>

rH

CO e o cu

1-1 14-1 st TJ

00 (LI rH 4J • CO st

=J

CU

O

U rH

CO CO

CJ

03U 0) O

4-) 0) CJ

CO

14-1 «

S3O vw in

•H

O rH

03 • U 03 CO

cu CT. 3 CM

S3 rH O CO 4-J U > <

17

Table 2. Comparison of Values of Dissociation Energies of Diatomic Fluorides Compiled by Various Reviews

D°/(kJ/mol)

Molecule Gaydon Zmbov et al. a,C Rosen Wagman et al.^ Suchard JANAF

b (ref 2) (ref 4) (ref 5) (ref 6) (ref 7)

MgF 440 ± 10 458 ± 22 444.7 440 ± 10 446.2 4- 6

CaF 525 ± 20 529 ± 7 525.6 534 ± 11 526.8 9

SrF 526 ± 10 538 ± 8 532.

1

C 526 ± 10 533.9 9

BaF 560 ± 10 584 ± 7 582.0 560 ± 10 577.0 10

ScF

YF

LaF

480 ± 100 590

610

(600 ± 50)'

585 ± 13

596.6 + 21

591.

634.2

585

596.6 ± 21

CeF (580 + 50)

PrF - (580 + 50) — _ - -

NdF 540 + 10 550 545 + 12 565.6° 567 ± 13 -

PmF - (540 + 50) - - -

SmF 5 30 527 19

n 549.3 527 + 19

EuF 5 30 523 + 17 547.

2

C 523 ± 17

GdF 590 586 + 17 648.

C

TbF (560 + 50) 527 + 18

DyF 5 30 527 + 18 549.

3

C

HoF 550 542.4 12 559.

7

C

ErF 5 70 568 + 17 584.

4

C

TmF (570 50)

YbF (5 70 + 50) LuF (570 50)

SiF 480 50 565 541.7 12 524 480 50 544 ± 11 GeF 480 100 502 481.4 + 19 484.2 480 SnF 380 + 140 477 377 461 13 PbF 290 + 50 356 352 + 8 290 50 351.3 ± 11

f^ridfrar^frrreT 2g5!

UP ^ flw

^S TM »* «d values for the group IVA

b„See footnote b_ of Table 1.

CAt 298.15 K.

Parentheses dennte an estimate.

18

II. Sources and Evaluation of Data

1. BaO

Two independent groups of investigators [28, 29] have recently

studied the vaporization of BaO(s) . They both used mass-spectrome try- on the vapor effusing from Knudsen cells. Using JANAF Tables [7],

comparable results were obtained as follows:

Temperature Range AH°(subl) / (kJ/mol) D°/(kJ/mol) Reference

2nd law 3rd law 2nd law 3rd law

1332-1681 439.3 ± 16 432.6 537.0 543.7 28

1643-1803 436.3 ± 8 428.7 540.0 547.6 29

The third law value of Dn(BaO) from reference 29 confirms the selection

made earlier by JANAF [7J, but all of the values agree within their

uncertainties. Farber and Srivastava [29] took the close agreement

between their 2nd and 3rd law values as proof of the assignment of

electronic states [27] which were adopted by JANAF [7] for the derivation

of the Gibbs free energy functions of BaO(g).

2. CaF, SrF, and BaF

Menzinger [30] calculated lower limits to the ground state

dissociation energies from his electronic chemiluminescent studies on alkaline-earth atoms reacting with fluorine. He obtained _> 5 37, 5 33,

and 592 kJ/mol, respectively, for CaF, SrF, and BaF. These results

indicate that the values selected by JANAF [7] for Eg (CaF) and Dg(BaF)

may be a little low.

3. Y0

Liu and Wahlbeck [31] measured the vapor pressure of Y20g(s)

from 2342 to 2614 K by Knudsen rate of effusion and mass spectrometry.

They calculated values for Dq(YO) from these methods to be (716.7 ± 5)

and (712.5 ± 8) kJ/mol, respectively, which confirm within their

uncertainties the value selected by reference 1.

4. NdO

Tetenbaum [32] studied the vaporization of Nd203(s) from 2155

to 2485 K by transpiration, using controlled partial pressures of

oxygen. He determined AHjq for the assumed reaction, K^O-^s) t- 2 NdC(g) +

1/2 02(g), to be (1510.8 ± 4.6) kJ/mol. Using the thermofunctions cited

in reference 1 gives D^(NdO) to be 721 kJ/mol which is about 3 kJ/mol

less than that obtained earlier [1] for the above assumed reaction,

19

but the value 695 kJ/mol was adopted [1] as a compromise on the ambiguous

presence of NdC^Cg) in the vapor phase.

5. EuO

Hildenbrand [33] has studied the vapor equilibrium for the

following two reactions from 2000 to 2250 K by mass spectrometry:

Eu(g) + BaO(g) $ EuO(g) + Ba(g)

Eu(g) + A10(g) X EuO(g) + Al(g)

He determined 2nd and 3rd law values of AHft to be (92 ± 10) and (77 ± 1])

kJ/mol, respectively, for the first reaction, and (43 ± 12) and (50 ± 11)

kJ/mol, respectively, for the second reaction. Using 548 kJ/mol [7] for

Dj3(Ba0) and 508 kJ/mol [38] for Dq(AIO) gives an average value for

Dq(EuO) of &62 ± iDkJ/mol, considerably lower than the 544 kJ/mol

adopted earlier [1]. Dickson and Zare [34] calculated Dq (EuO) _> 550

kJ/mol from their chemiluminescent study of the reaction between Eu(g)

and p£(g) , but Hildenbrand [33] reported that this data [34] could be

interpreted to yield a lower limit of 473 kJ/mol. For the present

the value for _D^(Eu0) is ambiguous.

6. ScF, YF, NdF, SmF, EuF, GdF, DyF, HoF, ErF, SiF, GeF, and SnF

Values of D° for these diatomic fluorides were calculated from

published enthalpy changes as shown in Table 3. i^ass spectrometry

has been used to study the vapor equilibria of various isomolecular

reactions [9, 14, 15, 19, 24, 25, and 26], In these studies the ratio

of ion-intensities ( I+T) have been plotted versus 1/T , and 2nd law

values of AH° were obtained from -R[d(lnKp ) /d (1/T) ]", for which the

ratio of cross sections were assumed to cancel. Only in the cases of

SiF and GeF were 3rd law values directly available. Relative enthalpy

changes of gases were generally calculated from molecular constants

provided by Rosen [4], but in some cases were estimated from Cp/R

assumed to be 5/2, 9/2, 15/2, and 10 for the gases M, MF, MF2 , and

MF3, respectively. Values of _D°(CaF) and D°(F->) which are needed in

several of the calculations are given at the bottom of the table.

Because Spedding and Henderson [13] have shown that the AH 0 for a solid transition of YF3 at 1350 K is 32.4 kJ/mol, a revised value

for the AH° of sublimation was calculated from the mass spectrometric

data [12] which had been obtained from 1256 to 14 34 K. A least squares

fit of the data, excluding the points above 1350 K, yielded the 2nd

law value of (437.6 ± 8.7;kJ/mol for AHf 2 99( subl) while the earlier

value was (418 ± 13)kJ/mol.

R = 8.3143 J.K~

1

-mol

-1

20

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23

Unfortunately there are inconsistencies in reference 15 between

the text and the plotted data involving HoF. Many of the D

01 s calculated

in Table 3 depend upon a value for D°(HoF) , but the sign and the

assignment of the values given in the text [15] for the AH° T s of reactions

17 and 18 of Table 3 appear to be in error. The values given in Table 3 for these reactions were obtained from the plotted data [15] and appear

to be more reasonable. Incidentally a sign error is believed to exist

also for the value given in another text [26] which deals with the AH°

of reaction 27 of Table 3; here again the value obtained from the

plotted data [26] was chosen.

Dickson and Zare [34] calculated DflCSmF) :> 517 kJ/mol and D^(EuF) _> 542 kJ/mol from their chemiluminescent studies of the reactions

between F2(g) and Sm(g) and Eu(g) , respectively. These values are consistent with those derived in Table 3.

7. SiO

An average value and its standard error for AHfg [SiO(g) ] may

be deduced as (-103.0 ± 3.7) kJ/mol from an analysis in the JANAF

Tables [7] of 15 independent published studies. When the value for

AHfn [Si(g) ] is taken to be 451.3 kJ/mol from Hultgren et al. [10],

Do(SiO) becomes 801.1 kJ/mol.

Hildenbrand [35] used mass spectrometry to study the equilibrium

for Ge(g) + SiO(g) X GeO(g) + Si(g) and obtained AHjgg as (134.7 ± 8)

kJ/mol, which corresponds to DQ(SiO) being (788. 7 ± 8) kJ/mol when

the revised average for I) o (Ge0) found below is used.

Zmbov, Ames, and Margrave [37] studied the reaction Si(s) +

Si02 (quartz) ^ 2 SiO (g) , from 1267 to 1487 K by mass spectrometry

and obtained a 3rd law value and standard deviation, (694.1 ± 5.4) kJ/

mol, for AH|gg. When this value is combined with the selected values

of AHf 0 for Si(g) [10], 0(g) [7], and Si02 (quartz) [7], the result

is 807.5 kJ/mol for _D°(Si0) . Supporting this value which is higher

than the average deduced from the JANAF analysis [7] is the recent ab

initio quantum mechanical calculation performed by Heil and Schaefer

[36] who predicted D^SiO) to be 804 kJ/mol.

When the values deduced from Hildenbrand [35] and Zmbov et al.

[37] are combined with the above average from the JANAF Tables [7], the

revised average value for Dq(SIO) is 800.7 kJ/mol.

8. GeO

Drowart et al. [22] studied the vaporization of Ge02(hex)

,

GeO(amorph) , and a mixture of Ce02(hex) and Ge(s) by mass spectrometry.

Correcting for the presence of (GeO)n(g) polymers in the vapor phase,

they reinterpreted other vaporization data published earlier. The

average Dq(GcO) from their analysis of nine independent experiments

24

is (653.5 ± 7.9) kJ/mol. Brewer and Rosenblatt [3] report that the

transpiration studies of Belton (1968) on Ge(s) + H2 0(g) % CeO(g) +

H2(g) yield (200.8 ± 0.4) kJ/mol for AH298 by the 3rd law method,

which corresponds to D(5(0e0) being 659.3 kJ/mol, according to the values

of the AHf 0 for H20(g) [7], 02 (g) [7], and Ge (g) [10]. When the

value from Belton (1968) is included in the average from orowart et al.

[22], the revised value for j^(GeO) is 654.1 kJ/mol.

9. SnO

Colin et al. [23] studied the vaporization of Sn02(s) by

mass spectrometry. Correcting for the presence of [SnO] 2(g) in the

vapor phase, they also reinterpreted the data from five other independent

studies published before their work and deduced the average value,

(529.3 ± 8) kJ/mol, for (SnO). Later, Hoenig and Searcy [39]

studied the vaporization of Sn02(s) from 1315 to 1660 K by Knudsen

rate of effusion and obtained (600.0 ± 4.6) kJ/mol for the AHJ98 of

Sn02(s) ^ SnO(g) + 1/2 02 (g). When this value is combined with the

values of AHf 0 for Sn(g) [10] and Sn02 (s) [23], T$(Sn0) becomes 524.3

kJ/mol. Combining this with the average value from Colin et al. [23]

gives the revised average value, 528.6 kJ/mol, for Dfl(SnO).

III. Selected Values

Selected values of the dissociation energies which are discussed

in section II are compiled in Table 4. Values which are believed to

be established from earlier reviews and some estimates are also included

to provide a complete list.

25

Table 4. Selected Values of Dissociation Energies of Diatomic Molecules

Oxides Fluorides

rioxecu±e D

o in

d

noiecuie D

o ln

d

Re f erence

kJ/mol kJ/mol

Mo-n r-igu J JO 7 MoT? AAA 7 LaU JOU 7/ jt / 7 jiU 7/ jtr J JH 7

—> uU 548 7 BaF -Jit 577 7

ScO 674 1 ScF 591

YO 711 1 YF 605

LaO *7 n n 799 1 LaF (600 ± 50) 8

CeO 795 1 CeF

n (580 ± 50) 8

PrO 745 1 PrF (580 ± 50) 8

NdO 695

b

1 NdF 563

PmO (670) 21 PmF (540 ± 50) 8

SmO 590 1 SmF 544

EuO 544 1 EuF 552

GdO 707 1 GdF 652

TbO 711 1 TbF (560 ± 50) 8

DyO 628 1 DyF 554

HoO 644 1 HoF. 566

ErO 644 1 ErF 591

TmO 565 1 TmF (570 ± 50) 8

YbO 420 1 YbF (570 ± 50) 8

LuO 678 1 LuF fs70 + sen so SiO 801 SiF 541

GeO 654 GeF 494

SnO 529 SnF 467

PbO 371 7 PbF 351 7

a -1

T l"1

Conversion factors are 4.184 J/cal, Q6.48 kJ«mol •eV"

1

, or 11.96 J«mol • cm.

^Parentheses denote an estimated value.

c Values of rare-earth fluorides are at 298.15 K.

d

See Table 3 and text for derivation ofr values without references.

26

IV. References

1. R. F. Krause, Jr., "Dissociation Energies of the Scandium Group

and Rare-Earth Gaseous Monoxides", in NBSIR 74-600, Thermodynamics

of Chemical Species Important to Rocket Technology, C. W. Beckett,

director, National Bureau of Standards, Washington, DC (October 1974).

2. A. G. Gaydon, "Dissociation Energies and Spectra of Diatomic

Molecules", Chapman and Hall Ltd., London (1968).

3. L. Brewer and G. M. Rosenblatt, "Dissociation Energies and Free

Energy Functions of Gaseous Monoxides", in Advances in High Temperature Chemistry, Vol. 2, Leroy Eyring, editor, Academic Press,

New York, (1969)

.

4. B. Rosen (director), "Selected Constants-Spectroscopic Data Relative

to Diatomic Molecules, Pergamon Press, Oxford (1970).

5. D. D. Wagman, W. H. Evans, V. B. Parker, I. Halow, S. M. Bailey,

R. H. Schumm, and K. L. Churney, NBS Technical Note 270-3 through

7, Selected Values of Chemical Thermodynamic Properties, U.S.

Government Printing Office, Washington, DC (January 1968 through

April 1973).

6. S. N. Suchard, "Spectroscopic Constants for Selected Heteronuclear

Diatomic Molecules", Vol. 2, 3, and 4, The Aerospace Corporation,

El Segundo, California (1974).

7. JANAF Thermochemical Tables, NSRDS-NBS 37, D. R. Stull and H.

Prophet (project directors), U.S. Government Printing Office,

Washington, DC (June 1971); Supplements No. 34 through 36,

H. Prophet (director), Supplements No. 37 through 42, M. Chase

(director), Dow Chemical Co., Midland, Michigan (December 1970

through December 19 74).

8. K. F. Zmbov and J. L. Margrave, "Mass Spectrometric Studies of

Scandium, Yittruim, Lanthanum, and Rare-Earth Fluorides", in

Advances in Chemistry Series 72, J. L. Margrave (editor), American

Chemical Society, Washington, DC (1968).

9. K. F. Zmbov and J. L. Margrave, J. Chem. Phys., 47, 3122 (1967).

10. R. Hultgren, P. D. Desai, D. T. Hawkins, M. Gleiser, K. K. Kelley,

and D. D. Wagman, Selected Values of the Thermodynamic Properties

of the Elements, American Society for Metals, Metals Park, Ohio

(1973).

27

11. E. Rudzitis, H. M. Feder, and W. N. Hubbard, J. Phvs. Chem. 6_9,

2305 (1965).

12. R. A. Kent, K. F. Zmbov, A. S. Kana'an, G. Besenbruch, J. D.

McDonald, and J. L. Margrave, J. Inorg. Nucl. Chem., _28> 1419 (1966).

13. F. H. Spedding and D. C. Henderson, J. Chem. Phys. 5_4, 2476 (1971).

14. K. F. Zmbov and J. L. Margrave, J. Chem. Phys.,45_, 3167 (1966).

15. K. F. Zmbov and J. L. Margrave, J. Phys. Chem., 70_, 3379 (1966).

16. R. H. Hauge, J. W. Hastie, and J. L. Margrave, J. Less-Common

Metals, 2_3, 359 (19 71).

17. W. Hubbard, private communication to D. D. Ragman, Argonne

National Laboratory, Argonne, Illinois (March 1970).

18. G. Besenbruch, T. v. Charlu, K. F. Zmbov, and J. L. Margrave,

J. Less-Common Metals, 12, 375 (1967).

19. K. F. Zmbov and J. L. Margrave, J. Inorg. Nucl. Chem. 29_, 59 (1967).

20. D. J. W. Robbins and R. F. Barrow, J. Phys. E: Atom. Molec. Phvs.,

7, L234 (1974) .

21. L. L. Ames, P. N. Walsh, and D. White., J. Phvs. Chen. 71, 2 70 7 (1967).

22. J. Drowart, F. Degreve, G. Verhaegen, and R. Colin, Trans. Faraday

Soc. , 61, 1072 (1965)

.

23. R. Colin, J. Drowart, and G. Verhaegen, Trans. Faraday Soc, 61 , 1364 (1965) .

24. T. C. Ehlert and J. L. Margrave, J. Chem. Phys., 41, 1066 (1964).

25. K. F. Zmbov, J. W. Hastie, R. Hauge, and J. L. Margrave, "Thermodynamics of Vaporization of GeF2, SnF7 , and PbF2 and Bond

Dissociation Energies of Group IVA Fluorides", in High Temperature

Technology (IUPAC Third International Symposium held in Asilomar,

California, 1967), Butterworths , London (1969).

26. K. Zmbov, J. W. Hastie, and J. L. Margrave, Trans. Faraday Soc,

64, 861 (1963)

.

27. R. W. Field, J. Chem. Phys., 60_, 2400 (1974).

28

28. K. Hilpert and H. Gerads, High Temp. Sci., 7_, 11 (1975).

29. M. Farber and R. D. Srivastava, High Temp. Sci., ]_, 74 (1975).

30. M. Menzinger, Can. J. Chem. , 5_2, 1688 (1974).

31. M. B. Liu and P. G. Wahlbeck, High Temp. Sci., 6_, 179 (1974).

32. M. Tetenbaum, High Temp. Sci., 1_ 3 37 (1975).

33. D. Hildenbrand, private communication, Stanford Research Institute,

Menlo Park, California (June 1975)

.

34. C. R. Dickson and R. N. Zare, Chem. Phys., 7_, 361 (1975).

35. D. L. Hildenbrand, High Temp. Sci., 4, 244 (19 72).

36. J. G. Heil and H. F. Schaefer, J. Chem. Phys., 56_, 958 (1972).

37. K. F. Zmbov, L. L. Ames, and J. L. Margrave, High Temp. Sci.,

5_, 235 (1973).

38. P. J. Dagdigian, H. W. Cruse, and R. N. Zare, J. Chem. Phvs.,

62, 1824 (1975).

39. C. L. Hoenig and A. W. Searcy, J. Amer. Ceram. Soc, 49_, 128

(1966).

29

5. BIBLIOGRAPHIES ON CHEMICAL KINETICS

a) NBS Special Publication 371-1 - Supplementary Bibliography

of Kinetic Data on Gas Phase Reactions of Nitrogen, Oxygen, and

Nitrogen Oxides.

b) MBS Technical Note 866 - Chemical Kinetic and Photochemical

Data for Modelling Atmospheric Chemistry.

c) Rate of reaction, and activation energy for; 0 + H£0 -> OH + OH

and 0 + H2 OH + H, where 0 is 0( 3P) , 0( 1 D) , 0( 1 S) ; chemi-ionization

reaction, Ba + OH •* BaOH* + e~; energy transfer reactions H2O + H£0 -> H2O* + H2O where * denotes vibrational excitation.

d) Short list on kinetics rate data for metallic species.

e) Chemical Kinetics of Reactions of Nitrogen Fluorides in Gas

Phase.

30