bluuebunny/crossref_metadata_2025 huggingface dataset:
Considerations on the Rotation—Vibration of Triatomic Molecules
Authors: Freed Karl F. , Lombardi John R.
Published: January 2005

In order to incorporate the variations of the moments of inertia and the reduced masses for vibration generated by large-amplitude vibrations into a treatment of the vibration—rotation interaction of polyatomic molecules, we investigate the use of the instantaneous moments of inertia to define the rotating reference frame. This technique does not suffer from the disadvantage of tying the treatment to any equilibrium configuration and treats all vibrating molecules as asymmetric tops.

The case of large-amplitude bending in triatomic molecules is used to illustrate the general features of such a method. An adiabatic approximation is used to separate the stretching from the bending vibrations, exhibiting the important aspects of the coupling of these modes. The properties of angular momenta are utilized, and the resulting equations for the bending vibration are a set of coupled differential equations which are analogous to the algebraic equations encountered in the treatment of the rotational motion of asymmetric-top molecules.

As a simple test of the validity of the Hamiltonian, the l-type doubling parameter is calculated for a number of molecules in good agreement with experiment.
Use of INDOR in a Four-Spin 19F Spectrum for the Assignment of Lines and Relative Signs of Coupling Constants
Authors: Baker Edward B.
Published: January 2005

The NMR spectra of the mixed erythro and threo isomers of trans-CFClBrCFBrCF=CFCl were obtained. The assignment of lines and the relative signs of the coupling constants were determined by the INDOR 19F–19F double-resonance method, using a systematic procedure suitable for this nonsymmetrical nearly first-order four-spin system. Three of the six coupling constants have signs opposite to the other three. Numerical values of shifts and coupling constants are given, as well as higher-order corrections.
Semiempirical Calculation of EPR Spin—Spin Coupling Constants
Authors: Petersson G. A. , McLachlan A. D.
Published: January 2005

Orbitals of the extended Hückel theory have been used to calculate some EPR spin—spin coupling constants. The results are similar to those obtained by the hyperconjugation approach. The computer programs used make the method easy to apply, and the results show substantial agreement with experiment.
On the Statistical Mechanics of Nucleation Theory
Authors: Lothe Jens , Pound G. M.
Published: January 2005

Some statistical-mechanical aspects of the theory of homogeneous nucleation in condensation of droplets from supersaturated vapor are considered, but no attempt is made to dispense with the standard capillarity approximation. It is shown that the major differences between current approaches of this type lie in the replacement factor, which describes the contributions arising from the fact that net motions of the cluster as a whole in the stable condensed phase are unavailable to it in the vapor. Calculation of this factor depends on the model assumed for the bulk liquid. Arguments are presented to show that the Dunning model of free translation and rotation of clusters in the bulk liquid is unrealistic and yields replacement factors which are much too low, i.e., ∼10−16. A method is proposed for determining the replacement factor for condensation from measurements of nucleation rate in freezing. It is concluded that the original Lothe—Pound replacement factor of 10−3 is not greatly in error. Thus the factor of disagreement between theory and observation, at least in the case of water vapor and the vapors of certain polar and asymmetric molecules, stands at approximately 1014−1017, the theory predicting homogeneous nucleation rates which are much too high.
Ferrimagnetic Resonance in Manganese Acetate Tetrahydrate
Authors: Cowen J. A. , Johnston G. T. , Van Till H.
Published: January 2005

The electron paramagnetic resonance spectrum of Mn(C2H3O2)2·4H2O below the transition temperature at 3.2°K has been interpreted by use of the ferrimagnetic resonance equations developed by Smit and Beljers. The equations, suitably modified to include rhombic anisotropy, represent accurately the experimental results at high fields but deviate systematically from the data at low fields where multidomain structure is presumably important. The experiments were performed at a number of frequencies between 5 Mc/sec and 11.5 kMc/sec using rf and microwave techniques. The theory agrees well with the early data of Abe who used a less satisfactory theoretical model.
Vibronic States of a Dimer
Authors: Bierman Arthur
Published: January 2005

Equations representing the vibronic states of an excited dimer are developed, using adiabatic strong-coupling state vectors; the equations can be uncoupled using an approximation scheme and the problem reduces to solving a one-dimensional, two-parameter Schrödinger equation with a complicated potential. It is analyzed qualitatively and analytic solutions are derived for a certain domain in the parameter space. The vibrations are shown to fall into two classes: distorted oscillator functions of a single-well Hamiltonian, and vibrations belonging to a double-well potential. Some properties of the double well are investigated and some qualitative conclusions are drawn. The theory is also examined in the weak-coupling limit and shown to yield the correct answer.
Paramagnetic Resonance of Alkyl Nitroxides
Authors: Adams J. Q. , Nicksic S. W. , Thomas J. R.
Published: January 2005

Methods of preparation and the electron paramagnetic resonance spectra are given for the three prototype alkyl nitroxide free radicals: H2NO, CH3(H)NO, and (CH3)2NO. The sigma—pi interaction theory of Karplus and Fraenkel for 13C, which has been applied extensively to 14N hyperfine splitting, is examined critically by the application of linear regression to 41 14N-containing free radicals. Three different sets of 14N sigma—pi interaction parameters are found which reproduce the experimental 14N coupling constants to an equally high degree of accuracy. Consequently, the prediction of experimental coupling constants within the order of magnitude of solvent effects (5%—10%), should not be considered a satisfactory test of a set of theoretical 14N interaction parameters, since the parameter values can fall within a wide range and still fulfill the test requirement. The parameter SN does not appear to play an important role in the sigma—pi interaction of these radicals.
Formation of Excited Molecular States in the Positive Column of a Helium Discharge
Authors: Teter M. P. , Robertson W. W.
Published: January 2005

Excited molecular states are shown to be populated in the positive column of a glow discharge in helium by electron excitation of metastable molecules. At higher pressures in the range 2–20 torr, metastable molecules are formed primarily by three-body conversion of metastable atoms, Hem+2 He→He2m+He. The rate coefficient is low because of the repulsive potential between Hem and He. At lower pressures a second mechanism is effective, He(2p,3P)+2He→He2(2pπ,3Πg)+He followed by He2(2pπ, 3Πg)→He2(2s,3Σu+)+hν. The metastable molecule is thus formed indirectly by a three-body reaction involving the 2p, 3P state, where the absence of a repulsive potential indicates a large cross section which together with a relatively high population at lower pressures results in a significant contribution to the molecular metastable concentration. The data permit derivation of the three-body coefficient for the conversion of He (2p, 3P) to He2(2pπ, 3Πg).
Helium Recombination in a Radio-Frequency Discharge
Authors: Stafford B. , Durham J. , Schlüter H.
Published: January 2005

The recombination in a steady-state, plasmoidlike radio-frequency helium plasma is investigated spectroscopically. In an ionization—recombination balance only the radiative-collisional recombination rate involving the abundant atomic ions is of importance. This atomic recombination rate is investigated, yielding reasonable agreement with theory. The small contributions due to molecular recombination are studied for various conditions. Lower limits for the molecular collisional-radiative recombination coefficient are evaluated and found to be consistent with theory. Evidence for generally strong contributions of Hornbeck—Molnar processes to the molecular ion formation and for fast vibrational relaxation is reported.
Vibrational Spectrum of Methaneselenol
Authors: Harvey Albert B. , Wilson M. Kent
Published: January 2005

The infrared spectra of CH3SeH, CH3SeD, CD3SeH, and CD3SeD have been investigated from 250 to 4000 cm−1 in both the vapor and the solid states. The Raman spectra of all four compounds in the liquid phase have been recorded. A complete vibrational assignment and normal-coordinate calculation have been carried out. In CD3SeH a large repulsion exists between the methyl wagging mode and the Se–H bending vibration. The far-infrared spectrum of CH3SeH has a broad band centered at about 145±40 cm−1, which is assigned to the torsional vibration. A height of 320±40 cm−1 for the barrier to internal rotation is calculated from the torsional frequency.
Calculation of the Dielectric Constant of a Fluid by a Cluster Expansion Method
Authors: Jepsen D. W.
Published: January 2005

A method is developed for calculating the static dielectric constant of a fluid of polar but, at the present state of the treatment, nonpolarizable molecules by cluster expansion methods. The dielectric constant is obtained as the inverse of the coefficient in the Coulomb potential between two charged spherical probes immersed in the fluid. This yields a new derivation of the Kirkwood expression for the dielectric constant in terms of the moment of a sample inside a larger sphere produced by a fixed molecular dipole, showing that the sample may be taken infinite in size without shape-dependence problems. The Onsager result (for nonpolarizable molecules) is obtained from a partial summation of the diagrams without any use of classical electrostatics.
Anomalously Long Radiative Lifetimes of Molecular Excited States
Authors: Douglas A. E.
Published: January 2005

Certain excited states of molecules have been found to have anomalously long radiative lifetimes compared with the values calculated from integrated absorption coefficients. Using the properties of the stationary states, four mechanisms which may account for such anomalous lifetimes in collision-free molecules are presented. Although the four mechanisms bear a one-to-one relationship to those which have been discussed using semiclassical concepts, certain consequences of the more exact treatment do not appear in the semiclassical approach. In particular, it is shown that the two mechanisms which are most important for polyatomic molecules lead to highly perturbed energy levels and correspondingly complex spectra. The spectra of NO2, SO2, and CS2 are discussed as examples of the proposed mechanisms.
Tunnel Model for Rigid Disks near Close Packing
Authors: Salsburg Z. W. , Rudd W. G.
Published: January 2005

The partition function for the ``tunnel'' model of rigid disks has been evaluated in the asymptotic limit of the reduced ``volume'' τ=A/A0→1 (A0 is the close-packed value of the closed system's area A). In this model all the disks are held fixed on the sites of a triangular lattice except those in one row. In the limit τ→1 the Helmholtz free energy per particle has the form F/NkBT∼ln(λ/σ)−2ln(τ−1)+C+···,in which λ is the mean thermal de Broglie wavelength, σ is the disk diameter, and C is a constant. From a free-area single-particle theory one obtains C=0.14384, while a recent cell-cluster expansion carried through fourth order gave C=0.14370. The value of C for the tunnel model is evaluated in this article with the result C=0.15356.
Kinetics of Polymer Pyrolysis from Surface Regression Rates
Authors: Houser T. J.
Published: January 2005

A method of calculating polymer pyrolysis kinetic parameters from surface-regression-rate data and temperature-profile measurements has been presented. This method is based on the assumption that bond rupture, rather than evaporation and/or diffusion of the monomer, is the rate-limiting step. To apply this method it is first necessary to determine the temperature profile, then the rate is calculated as a sum of the rates of a series of thin isothermal slabs, each at the appropriate temperature along the profile. Because the slope of the temperature profile increases as the regression rate increases, the reaction-zone thickness decreases correspondingly. Thus, the size of the reaction zone is inversely proportional to the rate. This characteristic leads to activation energies about double those obtained by conventional calculations. The results of several sample calculations are tabulated. In addition, certain weaknesses in the desorption-rate-limited mechanism previously proposed are pointed out.
Order—Disorder Phenomena. V. Pippard Equations and the Phase Diagram for Ammonium Chloride
Authors: Renard Rémi , Garland Carl W.
Published: January 2005

The Pippard equations and the phase diagram for NH4Cl are analyzed in terms of a compressible Ising model. It is demonstrated that the Pippard relations are completely consistent with the occurrence of a first-order transition in the immediate vicinity of a lambda point. In addition, the substantial variation in the slope of the order—disorder transition line with pressure can be well represented by this Ising model.
Free-Radical Formation by Ultraviolet Irradiation in Single Crystals of Cysteine HCl
Authors: Box Harold C. , Freund Harold G. , Budzinski Edwin E.
Published: January 2005

Electron spin resonance spectroscopy was used to study free-radical formation in single crystals of cysteine HCl exposed to ultraviolet irradiation. Irradiation at 77°K and momentary warming produced the free radical S–CH2–CH(NH3Cl)COOH. Further irradiation at 77°K using longer wavelengths causes a quantitative transformation of the paramagnetic absorption which is attributed to the formation of the radical ion −S–CH2–CH–NH3Cl. The transformation is reversible and, upon warming, the initial radical is reconstituted.
Nuclear Magnetic Resonance of 7Li and 1H in Solid Lithium Imide, Lithium Amide, and Lithium Nitride
Authors: Haigh Paul J. , Forman Richard A. , Frisch Ronald C.
Published: January 2005

Nuclear magnetic resonance lines of 7Li nuclei have been observed in solid, polycrystalline samples of LiNH2, LiNH, and Li3N. Proton resonance lines have also been observed in the first two samples. In LiNH2 there is a splitting of the 7Li resonance line due to first-order quadrupole effects which indicates a coupling constant of 119±4 kHz (estimated limit to maximum error) at room temperature. In Li2NH no quadrupole effects are seen which is in agreement with the fact that the lithium nuclei in this compound are situated in sites of cubic symmetry. In Li3N both first- and second-order quadrupole effects have been observed simultaneously. From these data 7Li coupling constants for Li3N have been found to be 296±4 and 200±10 kHz at room temperature. An unexplained line was also observed at low magnetic fields for this compound. These effects were found to vary with temperature and with the purity of the sample.
Non-Gaussian Behavior of the Polyethylene Chain of Finite Length
Authors: Nagai Kazuo
Published: January 2005

The second and fourth moments 〈r2〉 and 〈r4〉 of the end-to-end distance r of the polyethylene chain of length n are calculated, based on expressions previously derived. The model employed is the familiar three-state [trans (θ(T)=0°), gauche (θ(G)=120°), and gauche prime (θ(G′)=−120°)] rotational—isomeric model. An approximate expression for the reduced distribution function Pn(ξ)=4π〈r2〉*32ξ2Wn(〈r2〉*12ξ), which is readily obtainable from 〈r2〉 and 〈r4〉, is calculated as a function of the reduced end-to-end distance ξ=〈r2〉*−½r. Here Wn(r) is the distribution function in the usual sense and 〈r2〉*=nlin lim n→∞n−1〈r2〉.(An asterisked quantity denotes the corresponding Gaussian analog.) Approximate expressions for Wn(0)/Wn*(0) and 〈r−1〉/〈r−1〉* are also calculated as functions of n.

For the GG′-eliminated trans-preferred chain with reasonable values for parameters, which approximates the polyethylene chain: (i) 〈r2〉 and 〈r4〉 are smaller than their respective Gaussian analogs, the deviation being more pronounced with the latter, (ii) the approximate distribution function becomes sharp and somewhat shifted toward ξ=0 compared with the Gaussian distribution, and (iii) as n decreases from infinity, both Wn(0)/Wn*(0) and 〈r−1〉/〈r−1〉* first decrease gradually from unity, then pass through minima, and subsequently increase rapidly, largely exceeding unity. The GG′-eliminated gauche-preferred chain behaves like the chain just mentioned above in all respects. For the gauche-preferred independent-rotation chain, the opposite trends result for Items (i) and (ii) above; for Item (iii), both quantities monotonously decrease as n decreases. It is shown that for the polyethylene chain, Kuhn's equivalent random chain does not correctly predict (considerably underestimates) the departure from the Gaussian distribution.
Electron-Impact Ionization Cross Sections. II. Classical Calculations
Authors: Stafford Fred E.
Published: January 2005

Absolute cross sections for single ionization of the atomic elements have been calculated using the classical method of Gryzinski. They are in good agreement with experimental cross sections. They differ significantly, however, from certain of the values of Otvos and Stevenson.
Polarization of the Lowest-Energy Allowed Transition of β-Ionylidene Crotonic Acid and the Electronic Structure of the Polyenes
Authors: Parkhurst Lawrence J. , Anex Basil G.
Published: January 2005

Polarized specular reflection spectra have been obtained for two faces of the all-trans β-ionylidene crotonic acid single crystal in the 700-to-220-mμ region. Visual inspection of these results leads to the assignment of the absolute polarization of the lowest allowed transition, which is believed to be the first accurate absolute assignment for a polyene-related compound. The polarization is confirmed by the integrated intensities of the absorption bands derived through a Kramers—Kronig analysis of the reflection data. The relationship between the experimental polarization and that predicted by various zero-order theories that have been applied to the polyenes is examined and the effect of refinements in these theories on the direction and magnitude of the transition moment explored.
Variations in the Efficiency of Fluorescence Excitation in Anthracene
Authors: Wright W. H.
Published: January 2005

The efficiency with which fluorescence is excited in single crystals of anthracene has been reported to vary considerably with both the excitation wavelength and the condition of the crystal. These variations are analyzed quantitatively in this paper. It is assumed that the variations are principally the effect of a layer of anthraquinone which forms when the crystal is abrased or aged in the atmosphere. A model which takes into account selective absorption of incident radiation in this surface layer of anthraquinone, specular reflectance by the bulk crystal, quenching of the fluorescence via nonradiative exciton decay at the surface, and the absorption characteristics of the bulk crystal (including reabsorption and re-emission of part of the fluorescence) is used to obtain a theoretical fluorescence excitation spectrum. This spectrum agrees very well with the corresponding experimental spectrum.
LCAO SCF Computations for Ammonia
Authors: Kaldor U. , Shavitt I.
Published: January 2005

A series of LCAO SCF calculations with Slater-type atomic orbitals were performed for the NH3 molecule over a range of bond lengths and angles (keeping at least C3v symmetry). In addition to a minimal basis set (with two different values for the hydrogen 1s orbital exponent), two types of extended basis sets were examined for the planar and the experimental equilibrium configurations. One of these consisted of a ``double zeta'' set on N and one 1s function on each H (with exponent 1.20), while the other involved a minimal set augmented by four ``bond orbitals''−2s and 2p functions centered on N but having their maximum densities in the bond regions.

The results for the minimal basis set are typical for this type of calculation, giving good ionization energies and an inversion barrier height which is twice the experimental value. The extended basis sets, though giving fairly low total energies, resulted in an excessive concentration of charge on the nitrogen atom, with a high dipole moment and a zero, or even small negative, inversion ``barrier''. Direct calculations were also performed for the NH3+ ion in the pyramidal and planar configurations.
On the Helix—Coil Transition in Heterogeneous Polymers
Authors: Crothers D. M. , Kallenbach N. R.
Published: January 2005

A general theory for the helix—coil transition of block copolymers of regularly alternating sequence, applicable in principle to both nucleic acids and polypeptides, is developed. The stability of helical regions of the molecule is permitted to vary depending on the nature of the monomers in that section. Calculations are reported for DNA-like molecules. The approximation is then made that a molecule of random sequence can be treated as a linear combination of block copolymers of varying block size, and on this basis the average length of correlations in the state of neighboring base pairs in a natural DNA which is undergoing transition is estimated to be about 1400 base pairs. An approximate model for DNA melting developed on these grounds yields a transition curve in satisfactory agreement with experiment.
Pseudopotentials, the Sizes of Atoms and Their s—p Splittings
Authors: Austin B. J. , Heine Volker
Published: January 2005

The pseudopotential has made an important contribution to our understanding of the electronic structure of solids. It is here applied to free atoms. The strongly attractive real potential inside the atomic core of completed shells is nearly all cancelled off in the pseudopotential, resulting for instance in 1s-like nodeless pseudowavefunctions for the valence electron in all the alkali atoms. This gives a natural means of comparing atoms from different rows of the periodic table. The method is applied to the sizes of atoms, in particular to the increase of size with row in the periodic table, and the effect of a soft core as in the noble metals. It explains various qualitative trends with row and valency through the periodic table. It also gives an illuminating picture of the s—p energy differences, which determine in large measure the tendency to covalency and other effects in solids.
Energy Transfer in Ion—Molecule Reactions in the Methane System
Authors: Giardini-Guidoni Anna , Friedman Lewis
Published: January 2005

Cross sections for the reactions of CH4+ with CH4 have been measured by determining the pressure-dependent loss of CH4+ in the mass spectrum of methane produced by electron impact in the mass-spectrometer ion source. CH4+ loss occurs by reaction of low-velocity ions in the source and via collision-induced dissociation of CH4+ in the mass-spectrometer analyzer tube. The velocity dependence of source reactions permits a quantitative separation of these two processes. Experimentally determined cross sections have the magnitude and velocity dependence predicted by the Langevin cross section. This is in sharp contrast to results obtained by earlier workers who studied the reactionCH4++CH4→CH5++CH3.Here values of the phenomenological cross section usually fall below theoretical values and a reciprocal kinetic-energy dependence rather than velocity dependence had been proposed. Some of this earlier work has been repeated and confirmed in this paper. The difficulty in interpretation of the earlier work is shown to come from the assumption that CH5+ production is the only channel of reaction available in collisions of CH4++CH4. This assumption was based on the fact that CH5+ production appears to be the only reaction exothermic with ground-state CH4+ ions. We have shown that both internal excitation and energy transfer from kinetic to internal energy contributes to the production of excited CH5+ ions which can decompose into CH3+ and H2. This reaction makes up a major fraction of the ``missing'' cross section in the CH4+–CH4 reactions and the kinetic-energy transfer process involved in producing CH3+ accounts for the peculiar velocity dependence observed in the CH5+ production reaction. Thus it is demonstrated that the Gioumousis—Stevenson assumption of unit collision efficiency taken with the classical Langevin cross section provides an accurate description of these ion—molecule reactions.
Far-Infrared Spectra of Trimethylene Sulfide and Cyclobutanone
Authors: Borgers Tom R. , Strauss Herbert L.
Published: January 2005

The far-infrared spectra of trimethylene sulfide and cyclobutanone have been investigated. The positions of the trimethylene sulfide bands observed are well fitted by the potential function derived from the microwave studies of Harris et al. However, some of the band shapes cannot be explained in detail although the anomalies are probably due to Coriolis terms in the Hamiltonian. The positions of the lower frequency bands of cyclobutanone can be fit approximately by a quartic—quadratic potential with a small barrier. Although this potential function fits the rotational constants measured by Sharpen et al., it does not predict the positions of the higher frequency bands correctly. Furthermore the band shapes cannot be explained on the basis of the simple potential function, and it is suggested that the low-frequency out-of-plane vibration of cyclobutanone is perturbed by a higher frequency vibration.
Nuclear Magnetic Relaxation of n-Alkanes in the Rotating Frame
Authors: Douglass D. C. , Jones G. P.
Published: January 2005

The rotating-frame nuclear magnetic relaxation time T1ρ has been measured for 10 normal alkanes ranging from C4H10 to C94H190. Data were obtained within the range −200° to +70°C for C94H190, C40H82, and C6H14. The high-temperature region is characterized by a process of high activation energy ascribed to chain rotation while the low-temperature region exhibits a T1ρ minimum arising from coupling of the entire spin system to the methyl-group rotation via spin diffusion. This information and the analogous T1 data yield an activation energy of 2.6 kcal/mole for the methyl-group rotation. The remaining compounds were examined in the vicinity of the T1ρ minimum at −190°C and relaxation times characterizing the intrinsic methyl relaxation and spin-diffusion process have been extracted from the data for three rf field strengths. Theoretical estimates of the spin-diffusion coefficients at low fields and methyl-proton relaxation times are in satisfactory agreement with the observed quantities.
Swarm Determination of the Cross Section for Momentum Transfer in Ethylene and in Ethylene Mixtures
Authors: Christophorou L. G. , Hurst G. S. , Hendrick W. G.
Published: January 2005

Data are reported on the temperature dependence of electron-swarm drift velocities w in ethylene. The analysis of the data gave a velocity-independent cross section for momentum transfer, σE for ethylene at thermal electron energies. Using a constant σE, we investigated the dependence of wE/wM on gas concentration, where wE is the drift velocity in ethylene and wM that in mixtures of ethylene with a polar impurity P. Three regions of concentration were distinguished: initial, intermediate, and final. The initial region can provide information about the velocity dependence of σP, the intermediate (better suited for experimentation) can easily provide σP if the velocity dependence of σP is known, and the final region provides a new way to measure the drift velocity for pure polar compounds P by simply measuring wE/wM in mixtures of P with ethylene. The latter is extremely important when P cannot be studied alone.
Group-Theoretical Classification of the Atomic States of gN Configurations
Authors: Wybourne B. G.
Published: January 2005

The terms of the gN configuration of equivalent electrons (or nucleons) are classified in both LS and jj coupling using the methods of group theory. It is suggested that the electronic structure of the gN configurations, which should be important in an element of atomic number Z=126, could be most readily calculated in terms of the jj coupling scheme.
Relaxation of Moments Derived from a Master Equation
Authors: Shuler Kurt E. , Weiss George H.
Published: January 2005

Previous work on the relaxation of moments derived from a master equation has been extended to master equations for joint probability distributions and to nonlinear master equations involving product probability distributions. Necessary and sufficient conditions on the transition probabilities are given to obtain closed moment equations. The special case of a simple one-term exponential relaxation of the moments is also discussed.
Electronically Excited NO by Photodissociation of NO2 and NOCl
Authors: Welge Karl H.
Published: January 2005

The photodissociations of NO2 and NOCl yielding electronically excited NO are observed by the fluorescence of NO(γ) and NO(β) bands: NO2→ lim hνNO(A 2Σ+;v=0)+O(3P),NO2→ lim hνNO(B 2Π;v=0,1,2,3)+O(3P),NOCl→ lim hνNO(A 2Σ+;v=0,1,2)+Cl(2P0).

The dissociations of NO2 are observed at wavelengths 1165, 1236, and 1295 Å but not at 1470 Å and longer wavelengths, and the dissociation of NOCl is found at wavelengths from 1165 Å up to about 1600 Å. The NO(A 2Σ+) and NO(B 2Π) molecules from NO2 and NOCl are strongly rotationally excited.
Transition Probabilities for H2, D2, N2, N2+, and CO
Authors: Fowler R. G. , Holzberlein T. M.
Published: January 2005

The previously observed quenching of the a 3Σg+ state of the hydrogen molecule by neutral collision has been confirmed by a study of deuterium. Lifetimes have also been measured for what are believed to be the B 2Σu+ state of N2+; the C 3Πu state of N2; and the b 3Σ, d 3Π, and B 1Σ states of CO.
Emission Polarization and Circular Dichroism of Hexahelicene
Authors: Weigang O. E. , Turner J. A. , Trouard P. A.
Published: January 2005

Measurements of the relative polarization of electronic transitions and their circular dichroism are reported for hexahelicene (phenanthro[3,4-c]phenanthrene). A new assignment scheme for the electronic spectrum is proposed in the light of these measurements.

The longest wavelength 1Lb (375–425 mμ) transition shows evidence of mixed polarization in the fluorescence which would arise from vibronic coupling in the emission process. Its circular dichroism, which may contain ``forbidden'' character, is negative for the dextrorotatory molecule.

The 1La (325–375 mμ) transition has a strong positive circular dichroism which determines the optical activity at visible wavelengths. The success of previous chirality calculations for hexahelicene hinges on a satisfactory treatment of this transition and the rather complex detailed polarization of its excited state. Discrepancies between predicted optical activity by free-electron and by Hückel MO calculations are noted. The 1Bb (280–325 mμ) transition has a weak circular dichroism whose sign is difficult to determine from experiment; calculations predict that it should have the same sign as 1La. What appears to be the 1Ba (or 1Cb) (230–280 mμ) transition has a stronger negative circular dichroism.

Because of the out-of-plane distortion of the molecule, relative transition polarizations somewhat peculiar for polynuclear aromatics are expected. These account for certain data in the emission polarization, phosphorescence lifetime, and phosphorescence—fluorescence relative quantum yield.
Competitive Unimolecular Decomposition Processes in Chemically Activated Systems
Authors: Tardy D. C. , Rabinovitch B. S. , Larson C. W.
Published: January 2005

A general formulation of the rates of competitive unimolecular reactions is given, and various relationships between the rates and between the energetics of competing processes are described. The relative and absolute rates of decomposition, C–C–C–Ċ–C–C*→ lim kEp·CH3+1−C5H10→ lim kEb·C2H5+1−C4H8,have been measured at pressures from 0.004 to 90 mm Hg for vibrationally excited 3-hexyl radicals, chemically activated by the addition of hydrogen atoms to trans−3-hexene at 300°C. Use of a quantum-statistical model for kE in the general relations provides good theoretical agreement with the experimental data at all pressures. The results can also be used to lead to either of two alternative conclusions: that D0(CH3–H) — D0(C2H5–H) is 5.4 kcal mole−1, or that the difference in critical energies for addition of methyl to 1-pentene, E0m, and of ethyl to 1-butene, E0e, is E0m—E0e= −0.4 kcal.
Optical Spectrum of Re4+ in Single Crystals of K2PtCl6 and Cs2ZrCl6 at 4.2°K
Authors: Dorain Paul B. , Wheeler Robert G.
Published: January 2005

The optical spectrum of Re4+(5d3) has been studied at 4.2°K in single crystals of K2PtCl6 and Cs2ZrCl6. The spectrum is characterized by extremely narrow lines in the region from 8500 to 36 000 cm−1. Vibrational structure is observed which is assigned to the odd modes of the ReCl62− complex. A crystal-field analysis of the 0–0 transitions fits the observed eight electronic transitions with an rms deviation of 209 cm−1 for the parameters A = 0, B = 327 cm−1, C = 1818 cm−1, V = 30 347 cm−1, and λ5d=2392 cm−1. Magnetic-field measurements of the excited states give g values which are compared with theory. A discussion of the intensities of the transitions is presented which demonstrates that some absorptions observed are magnetic-dipole transitions. Finally, several conclusions about the study of optical spectra of dn systems are presented.
Mössbauer-Effect Studies of Weak Nuclear Quadrupole Interactions in 119Sn
Authors: Stöckler Hans A. , Sano Hirotoshi , Herber Rolfe H.
Published: January 2005

Nuclear quadrupole interaction in some tin compounds has been investigated by the use of the Mössbauer effect. Stannic oxide shows a partially resolvable quadrupole splitting of 0.49±0.06 mm/sec at 1 atm pressure. The origin of the nuclear quadrupole interaction is attributed to the characteristic rutile crystal structure in SnO2. The absence of resolvable quadrupole splitting in stannic bromide and diphenyltin indicates that interpretations concerning the electric field gradient tensor based on the Mössbauer effect in 119Sn alone are not always sufficient to warrant definitive conclusions about molecular structures. It has been shown that this restriction is largely due to the relatively broad total natural linewidth for 119Sn, I=32+. Quadrupole-splitting and isomer-shift data for some phthalocyaninotin complexes indicate that simple correlations between chemical parameters, e.g., electronegativities, and Mössbauer parameters are not generally valid.
Mössbauer Effect in MnFeO3
Authors: Banks E. , Kostiner E. , Wertheim G. K.
Published: January 2005

The Mössbauer spectrum of MnFeO3 was measured at 4.2°, 78°, and 300°K. At 300°K the spectrum consists of two overlapping sets of resonances having isomer shifts of +0.496 and +0.478 mm/sec and quadrupole splittings of 1.161 and 0.666 mm/sec, respectively. At 4.2°K the isomer shift increases to +0.61 mm/sec and a magnetic hyperfine splitting, corresponding to an effective field of 469 kOe, is observed. The distribution of Fe3+ ions over the available cation positions was determined from an analysis of the resonance curves.
On the Existence of Conformers of Cyclobutyl Monohalides. II. Temperature Dependence of the Infrared Spectra of Bromocyclobutane and Chlorocyclobutane
Authors: Rothschild Walter G.
Published: January 2005

The infrared spectra of bromocyclobutane and chlorocyclobutane vapor between 250 and 3100 cm−1 are reported as a function of temperature between 30° and 172°C. The spectra of the low-temperature (−185°C) solids are also given. The data are described in terms of two conformers which are present in each halide. The energy difference between the two conformers of bromocyclobutane was measured to be about 1 kcal/mole. The conformers differ by their average dihedral angle: The more stable conformer is in a bent ring conformation (``equatorial''), the less stable one is in an essentially planar ring conformation. The sets of energy levels of the ring-puckering motion of the two conformers are contiguous, there is no tunneling between the conformers. The conformations are sufficiently different as to lead to two widely separated carbon—halogen stretching fundamentals for each cyclobutyl halide. One stretching fundamental is based on the equatorial, the other (towards higher wavenumbers) is based on the planar ring conformation. The data were evaluated with the help of (1) calculations of the dipole moment as a function of the ring conformation, (2) computer calculations of vapor band envelopes including rotation—vibration interactions, (3) quantum-mechanical computer calculations of the energy levels, probability distribution, transition moments, infrared intensities, and average dihedral angles of the ring-puckering mode, and (4) some simple, qualitative considerations of the contributions of exchange interactions to the measured and calculated energy differences between the two conformers.
Charge-Exchange Cross Sections for Argon Ions in H2 and D2 below 1 keV
Authors: Amme Robert C. , McIlwain John F.
Published: January 2005

Charge-exchange cross sections for Ar+ ions incident on hydrogen and deuterium have been measured over the energy range of 30 to approximately 1000 eV. The argon-ion beam was formed by electron bombardment and electrostatic acceleration. Ionizing electron energy was nominally 18 eV, although the results appeared to be insensitive to this parameter. The measured cross sections for Ar++H2→Ar+H2+ as a function of ion energy are compared with the results of other investigators, which are in rather poor agreement. The measurements confirm that the cross section exhibits a maximum at approximately 180 eV incident-ion energy. Theoretical calculations by Gurnee and Magee and by Karmohapatro are discussed for this reaction. The cross section for Ar++D2→Ar+D2+ was also found to exhibit a maximum, but at approximately 85 eV, which is nearly the same center-of-mass energy as for the H2 target. The possible influence of ion—molecule reactions on the charge-transfer cross sections is discussed.
Theory of Spin Relaxation via Quantum-Molecular Systems: Resonance Effects
Authors: Freed Jack H.
Published: January 2005

A relaxation equation is derived for the spin-density matrix on the assumptions that (1) the spins are weakly coupled to the molecular degrees of freedom which are described quantum mechanically and (2) the relaxation of the molecular systems is well represented by a general time-independent relaxation matrix which can include effects of weak as well as strong collisions. The irreversibility of the spin relaxation is achieved with no further assumptions, and is given in detail in terms of the normal modes of relaxation of the molecular systems. It is also pointed out that the present formulation inherently includes the assumption of resonant-type interactions between molecular systems and spins.
Raman and Infrared Spectra of 3-Methyleneoxetane
Authors: Durig J. R. , Morrissey A. C.
Published: January 2005

The infrared spectrum of 3-methyleneoxetane in both the gaseous and liquid states has been recorded from 4000 to 250 cm−1 and the far-infrared spectrum of the vapor has been recorded down to 70 cm−1. The Raman spectrum of the liquid has been recorded and depolarization values measured. The limited number of polarized Raman lines suggests the molecule is planar and has point-group symmetry C2v. The assignment of the 27 fundamental vibrations, based on band positions, band contours, and depolarization values, is made. The interesting ring-puckering vibration is assigned to a strong band in the infrared spectrum of the vapor which has a Q branch at 122 cm−1 with additional Q branches at 117 and 113.5 cm−1. These latter bands are assigned as the upper-state transitions of the ring-puckering vibration and indicate that the potential function governing the vibration is only slightly anharmonic.
Mass-Spectrometric Study of Photoionization. IV. Ethylene and 1,2-Dideuteroethylene
Authors: Botter R. , Dibeler V. H. , Walker J. A. , Rosenstock H. M.
Published: January 2005

Photoionization-efficiency curves for ions of the molecule and of selected fragments of C2H4 and CHDCHD are reported for the wavelength region from onset of ionization to 600 Å. The minimum threshold of ionization is 10.50 eV for both molecules. In addition, certain features of the ion-yield curves are discussed and identified as resulting from vibrationally excited states or from autoionization from Rydberg levels leading to electronically excited states. Threshold measurements of the acetylene ions permit calculation of heats of dissociation and indicate that the C2H2+ ion is formed without excess activation energy. Of the vinyl ions, the threshold for the C2H3+ ion is the most clearly defined and leads to a C2H3–H bond-dissociation energy of 100 kcal·mole−1.
Electric-Field Fluctuations and Spin Relaxation in Liquids
Authors: Kivelson Daniel
Published: January 2005

The intermolecular potentials in liquids are modulated by the motion of the molecules, and the electron spin in an s=½ molecule senses this variation in electric field through spin—orbit coupling. The three most important nonrotational mechanisms, the Van Vleck direct process, the Van Vleck—Raman process, and the Orbach process have been studied. The Raman process is negligible compared to the direct process unless ω02τc2≪1, where ω0 is the Zeeman frequency and τc is a mean correlation time for intermolecular fluctuations; under these conditions the direct process is proportional to the applied field squared and the Raman process is independent of field, but both processes appear to be of minor importance. If ω02τc2≫1, the Raman effect is negligible; the direct process is independent of applied field and probably not important. Even at ω02τc2≈1, the direct process is probably insignificant compared to other mechanisms such as spin—rotational relaxation. The Orbach process is significant if (δ0nℏ/kT) is not too large, i.e., less than six, where δ0nℏ is the excitation energy to the first excited state; this condition holds for symmetrical radicals in solution, i.e., for radicals that are orbitally degenerate in the gas phase. This mechanism may explain the anomalous ESR linewidths of Cu(H2O)6++, benzene− and coronene−. The Orbach process is independent of applied field and is proportional to τc−1; this correlation time can be related to the correlation time that enters into expressions for ir and Raman vibrational relaxation. In the absence of a low-lying excited electronic state, the Orbach process vanishes and rotational relaxation mechanisms dominate.
Charge Exchange between Homonuclear Diatomic Molecules and Protons
Authors: Breene R. G.
Published: January 2005

The impact-parameter method was adapted to the calculation of proton—homonuclear-diatomic-molecule charge transfer. In doing so, matrix elements of the Brinkman—Kramers type are calculated and subsequently adjusted through the Mapleton ratio. The cross section is actually determined by finding a sort of hard-sphere radius. The relative importance of transfer from various molecular orbitals and to various hydrogen orbitals is considered. Transfers to hydrogen orbitals having principal quantum numbers of less than 5 must be considered. Transfers from the various molecular orbitals are considered. The calculations for O2 and N2 agree reasonably well with experiment, and the program may be readily applied to other homonuclear diatomics having a similar electronic structure.
Irreversibility and Information in Mechanical Systems
Authors: Hobson Arthur
Published: January 2005

Concepts from information theory are used to investigate the relation between measurement and irreversibility in finite, closed classical systems having known Hamiltonians. The effects of measurement upon the predictions of the observer are found for the cases of a single measurement, two simultaneous measurements, and two measurements at different times. The physical basis of coarse graining and the significance of the increase in the coarse-grained entropy are discussed. An evolution equation is obtained describing the probability distribution of the measured quantities, and it is shown that this equation is non-Markoffian. Despite this non-Markoffian behavior, the distribution may behave irreversibly and may approach equilibrium. Thus, irreversible behavior is obtained without continual coarse graining. The condition for irreversible behavior is given for the case of a single measurement. The thermodynamic entropy is defined for general nonequilibrium situations.
Thermal Conductivities of Some Polyatomic Gases at Moderately High Temperatures
Authors: Choy P. , Raw C. J. G.
Published: January 2005

Thermal conductivities of nitrous and nitric oxides, carbon tetrafluoride, silicon tetrafluoride, sulfur hexafluoride, and boron trifluoride have been determined at moderately high temperatures, ranging from about 60° to about 400°C in different cases. The experimental results (including the collision numbers for rotational relaxation) are compared with the results of several different approximate theories of heat conduction in polyatomic gases, and with values obtained for several modifications of the rough-sphere model. Generally, the Mason—Monchick theory is found to give good agreement with experiment if the average rotational collision number in the temperature range is treated as an adjustable parameter. For N2O and SF6, the modified Eucken expression based on the diffusion mechanism theory of heat conduction was successful in accounting for the experimental results over the entire temperature range.
Optical Spectra of Tripositive Curium−244 in Lanthanum Trichloride
Authors: Gruber John B. , Cochran William R. , Conway John G. , Nicol Ann T.
Published: January 2005

Absorption and emission optical spectra are reported for Cm3+ in LaCl3 between 3000 and 8000 Å. These data are explained in terms of the Racah, spin—orbit, and configuration-interaction parameters: E0=4939.29 E1=4621.47, E2=17.70, E3=399.42, ζ=2841.66, α=57.08, β=−4761.60 cm−1, respectively. Crystal-field splitting of the ground state S728 is observed.
Ultraviolet Spectroscopy of Irradiated Polyethylene
Authors: Bodily David M. , Dole Malcolm
Published: January 2005

When polyethylene has been irradiated with high-energy radiations ultraviolet absorption bands having peaks at 229, 236, 245, 258, 264, 275, 285, 288, 307, 323, 340, 359, and 396 mμ can be observed. The bands are most clearly distinct at liquid-nitrogen temperature, but some can only be observed after the decay of free radicals on admission of oxygen at room temperature. The assignment of these bands to conjugated polyene and polyenyl free-radical groups is given and discussed.