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15 Jun 1983

Volume 78, Issue 12, pp. 7019-7507

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Visible spectroscopy of matrix isolated HCO: The 2A″(Π)←X  2A′ transition

L. J. van Ijzendoorn, L. J. Allamandola, F. Baas, and J. M. Greenberg

J. Chem. Phys. 78, 7019 (1983); http://dx.doi.org/10.1063/1.444745 (10 pages) | Cited 14 times

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The absorption spectrum of the 2A″(Π)←X 2A′ transition of HCO has been measured in Ar, Kr, Xe, CH4, CO, and N2 matrices and in the mixtures CO/H2O/NH3/CH4 (10:4:3:2) and (3:5:2:1). In the noble gas matrices, the spectrum appears with an alternating intensity pattern opposite that found in the gas phase. In a 10 K matrix, the transition originates from the K″=0 level instead of the K″=1 level and the ΔK=±1 selection rule is still important. Transitions into states normally strongly predissociated in the gas phase are measured indicating the importance of the cage effect. In the molecular matrices, the intensity alternation disappears and a strong underlying broad absorption is observed. This behavior is explained in terms of the complete breakdown of the ΔK=±1 selection rule. This interpretation is supported by calculations, using the Pople–Longuet–Higgins formulation, which show that the Renner effect is significantly greater in the molecular matrices then in the noble gas matrices. Upon warmup from 10 to 15 K the absorbance of HCO increases in Ar, Kr, and CH4 matrices indicating the release of H atoms which react with a low activation energy with CO. Simultaneously, the controversial infrared absorptions assigned either to matrix isolated hydrogen atoms or ArnH+ (KrnH+) disappear.
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33.20.Kf Visible spectra

Infrared gas phase intensity measurements, polar tensors, and effective charges of cis‐difluoroethylene and its deuterated modifications

R. O. Kagel, D. L. Powell, John Overend, Mozart N. Ramos, A. B. M. S. Bassi, and Roy E. Bruns

J. Chem. Phys. 78, 7029 (1983); http://dx.doi.org/10.1063/1.444746 (9 pages) | Cited 7 times

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All gas phase fundamental vibrational intensities for cis‐difluoroethylene and its deuterated modifications have been measured. Isotopic invariance, G sum rule, and quantum chemical information have been used in selecting preferred experimental values for the dipole moment derivatives and polar tensor elements of cis‐C2H2F2 and C2D2F2. The G sum rule has been used to determine individual intensity values for the ν11, ν12, overlapped band system of the dihydrogen molecule. Values of the polar tensor elements for cis‐difluoroethylene are compared with those found previously for vinylidene fluorine. The hydrogen effective charge value of 0.17 e of cis‐difluoroethylene is much smaller than the value of 0.29 e for vinylidene fluoride. The polar tensor of cis‐difluoroethylene is shown to provide an excellent estimate of the vibrational intensities of trans‐difluoroethylene.
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33.20.Ea Infrared spectra
33.70.Fd Absolute and relative line and band intensities

Effect of anion substitution on the electron spin resonance of Cr5+ in calcium phosphate apatite

J. H. Pifer, S. Ziemski, and M. Greenblatt

J. Chem. Phys. 78, 7038 (1983); http://dx.doi.org/10.1063/1.444747 (6 pages) | Cited 3 times

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Electron spin resonance studies have been made at 4.2 K of CrO3−4 substituted for PO3−4 in three calcium phosphate apatites. The hydroxyapatite studied has a hexagonal structure stabilized by Cl− impurities. The fluorapatite is confirmed by the ESR to undergo a phase transition involving small structural changes to a noncentrosymmetric phase. But the ESR shows that the distortion of the PO3−4 tetrahedra is remarkably similar to that of the hydroxyapatite. Chlorapatite has a centrosymmetric monoclinic structure and has a considerably more distorted PO3−4 tetrahedron. Comparison of the results with previously studied hexagonal barium and strontium apatites shows that cation substitution has a much stronger effect on the PO3−4 structure than does anion substitution.
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76.30.Fc Iron group (3d) ions and impurities (Ti-Cu)

Electronic reorganization in the photoelectron spectra of transition metal compounds

Michael C. Böhm

J. Chem. Phys. 78, 7044 (1983); http://dx.doi.org/10.1063/1.444748 (21 pages) | Cited 33 times

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The validity of Koopmans’ theorem in a series of 16 transition metal compounds with a large variety of 3d centers (Ti, Cr, Mn, Fe, Co, Ni, and Zn) is investigated. The reorganization energies are determined by means of the Green’s function method employed in a semiempirical INDO Hamiltonian. A self‐energy approximation is used that allows a fragmentation of the calculated Koopmans’ defects into relaxation increments as well as into correlation parameters that take into account the loss of pair correlation in the electronic ground state and the modification of the pair correlation in the cationic hole state. The magnitude and the importance of these parameters are studied as a function of the 3d occupation pattern, the oxidation state of the transition metal center, the nature of the orbital wave functions and the one‐particle energies. It is demonstrated that pair relaxation energies in the various hole states are by no means negligible in comparison to the relaxational corrections that lead to the most pronounced deviations from IKv,j (IKv,j=− ϵj). The limitations of purely relaxational models (e.g., ΔSCF approach) are analyzed in detail. The gradual modifications of the calculated Koopman’s defects within the 3d series are rationalized. The most pronounced reorganization energies are encountered in d6–d8 complexes. The magnitude of relaxation and correlation is reduced with a decreasing and an increasing number of 3d electrons. The physical background leading to the breakdown of Koopman’s theorem in 3d derivatives is compared with the results of recent studies in various molecular species (e.g., small molecules, organic lone‐pair systems).
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33.60.+q Photoelectron spectra
79.60.-i Photoemission and photoelectron spectra

Deuterium magnetic resonance in the discotic columnar mesophases of hexaalkyloxytriphenylenes: The conformation of the aliphatic side chains

D. Goldfarb, Z. Luz, and H. Zimmermann

J. Chem. Phys. 78, 7065 (1983); http://dx.doi.org/10.1063/1.444749 (8 pages) | Cited 27 times

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Deuterium NMR spectra of chain perdeuterated hexa‐pentyloxy, ‐hexyloxy, ‐heptyloxy, and ‐octyloxy triphenylene (THE5, THE6, THE7, and THE8) were studied as functions of temperature in the mesophase region. The deuterium quadrupole splittings exhibited several characteristic features, in particular a steplike decrease in the splitting along the alkyl chain and an even–odd alteration in the methyl splittings within the homologous series. The results are interpreted in terms of possible conformational distributions of the alkyl chain. It is found that there is bending of the alkyl chains out of the aromatic plane, and a considerable degree of chain disorder.
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76.60.Es Relaxation effects
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Radiative lifetimes of trapped molecular ions: HCl+ and HBr+

Cecilia C. Martner, Jürgen Pfaff, Neil H. Rosenbaum, Anthony O’Keefe, and Richard J. Saykally

J. Chem. Phys. 78, 7073 (1983); http://dx.doi.org/10.1063/1.444750 (4 pages) | Cited 25 times

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Excitation spectra in the A 2Σ+−X 2Π system of HCl+(0,0) and HBr+(1,0) as well as radiative lifetimes of selectively excited rotational levels in the A 2Σ+ electronic states have been measured in a RF ion trap using laser induced fluorescence and time‐resolved single‐photon counting techniques. Coherent ultraviolet radiation was generated by frequency mixing of a Nd:YAG/dye laser system. For HBr+ the lifetimes of four rotational levels adjacent to the v′=1 predissociation limit at J′=25/2 were measured. No statistically significant variation with J was found. The average of these four values (3.89±0.15 μs) is therefore reported as the effective lifetime of the v′=1 level of A 2Σ+ HBr+. For HCl+ the lifetime measurements likewise indicate no strong variation with J. The average lifetime of the v′=0 level of the A 2Σ+ state of HCl+ is 3.4±0.4 μs. Measurements of radiative lifetimes of molecular ions by the RF ion trap technique eliminates the determinate error that results from drift of ions out of the detection volume in conventional experiments.
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33.50.Dq Fluorescence and phosphorescence spectra
33.70.Fd Absolute and relative line and band intensities

Ab initio infrared and Raman spectra

Donald R. Fredkin, Andrew Komornicki@f@f, Steven R. White@f@f, and Kent R. Wilson

J. Chem. Phys. 78, 7077 (1983); http://dx.doi.org/10.1063/1.444751 (16 pages) | Cited 22 times

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We discuss several ways in which molecular absorption and scattering spectra can be computed ab initio, from the fundamental constants of nature. These methods can be divided into two general categories. In the first, or sequential, type of approach, one first solves the electronic part of the Schrödinger equation in the Born–Oppenheimer approximation, mapping out the potential energy, dipole moment vector (for infrared absorption) and polarizability tensor (for Raman scattering) as functions of nuclear coordinates. Having completed the electronic part of the calculation, one then solves the nuclear part of the problem either classically or quantum mechanically. As an example of the sequential ab initio approach, the infrared and Raman rotational and vibrational‐rotational spectral band contours for the water molecule are computed in the simplest rigid rotor, normal mode approximation. Quantum techniques are used to calculate the necessary potential energy, dipole moment, and polarizability information at the equilibrium geometry. A new quick, accurate, and easy to program classical technique involving no reference to Euler angles or special functions is developed to compute the infrared and Raman band contours for any rigid rotor, including asymmetric tops. A second, or simultaneous, type of ab initio approach is suggested for large systems, particularly those for which normal mode analysis is inappropriate, such as liquids, clusters, or floppy molecules. Then the curse of dimensionality prevents mapping out in advance the complete potential, dipole moment, and polarizability functions over the whole space of nuclear positions of all atoms, and a solution in which the electronic and nuclear parts of the Born–Oppenheimer approximation are simultaneously solved is needed. A quantum force classical trajectory (QFCT) molecular dynamic method, based on linear response theory, is described, in which the forces, dipole moment, and polarizability are computed quantum mechanically, using gradient techniques step by step along a classical trajectory whose path is determined by these quantum forces. We believe the QFCT method to be a more practical ab initio route to spectral band contours for large molecules, clusters, and solutions, and it can be equally applied to equilibrium and nonequilibrium systems. It is pointed out that a similar ab initio QFCT molecular dynamic approach could be used to compute other types of spectra, e.g., electronic absorption, as well as other parameters such as transport properties and thermodynamic functions and their quantum corrections. For parameters not depending on momenta, a parallel ab initio Monte Carlo approach would use electronic energies and other parameters of interest generated quantum mechanically, and ‘‘classical’’ trial moves of the nuclei.
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33.20.Ea Infrared spectra
33.20.Fb Raman and Rayleigh spectra (including optical scattering)
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.20.-t Molecular spectra

ESR and ENDOR study of photoexcited triplets of cyano‐substituted porphyrins

Hans van Willigen and T. K. Chandrashekar

J. Chem. Phys. 78, 7093 (1983); http://dx.doi.org/10.1063/1.444752 (6 pages) | Cited 9 times

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An ENDOR study has been made of the photoexcited triplets of some cyano‐substituted zinc tetraphenylporphyrins randomly oriented in a rigid matrix. The results demonstrate that the ENDOR method can be applied in studies of short‐lived triplets (triplet lifetime < 1 ms) in amorphous solids. The application is possible because of the fact that the ESR signal intensity is strongly enhanced by electron spin alignment. As a consequence of the spin alignment, ENDOR spectra can appear in absorption or emission depending on the field setting employed. Hyperfine data derived from the spectra reflect the effect of CN substitution on the electron spin distribution.
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33.35.+r Electron resonance and relaxation
33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)

Electron spin‐lattice relaxation in the phosphorescent state of xanthione

M. R. Taherian and A. H. Maki

J. Chem. Phys. 78, 7099 (1983); http://dx.doi.org/10.1063/1.444740 (9 pages) | Cited 4 times

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Spin‐lattice relaxation in the phosphorescent T1 state of xanthione has been measured at 1.0 K in a dilute n‐hexane matrix. Direct excitation of the T1z←S0 transition with a laser pulse of 10−8 s width was followed by observation of the phosphorescence decay monitoring either a T1z or T1y emission band. These bands are optically resolved in the n‐hexane Shpol’skii matrix because of the large zero‐field splitting, D=−15.5 cm−1. Depopulation of T1z by the spin‐lattice relaxation processes, T1z∼≳T1x, T1y was found to be complete within 10−7 s, the time resolution of the measurement. The relaxation process, which at 1.0 K requires the spontaneous creation of a lattice phonon of 15.5 cm−1, is unusually efficient. Thus, when the T1 state is populated by intersystem crossing using either S1←S0, or S2←S0 optical pumping, lack of observed T1z phosphorescence at 1.0 K does not imply that the lower T1x and T1y sublevels are selectively populated. Observation of T1z emission at higher bath temperatures, ∼3.3 K, following a T1z←S0 pulse reveals a peculiar initial rise in intensity followed by an eventual decay. This behavior is explained in terms of lattice heating which accompanies the radiationless T1∼≳S0 decay. The initial increase in intensity results from a thermal pumping rate from the lower energy sublevels which exceeds the overall decay rate of the T1 state. Nonexponential decay of the T1y emission is explained by the same model. We find that the T1z emission intensity vs time cannot be explained quantitatively by a uniform lattice temperature during the decay, but that temperature gradients must be present.
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31.70.Hq Time-dependent phenomena: excitation and relaxation processes, and reaction rates
33.50.Dq Fluorescence and phosphorescence spectra

Vibronic coupling theory of infrared vibrational transitions

Laurence A. Nafie and Teresa B. Freedman

J. Chem. Phys. 78, 7108 (1983); http://dx.doi.org/10.1063/1.444741 (9 pages) | Cited 49 times

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The theory of vibronic coupling is developed for infrared vibrational transitions. It is shown that the lowest order nonadiabatic Born–Oppenheimer correction terms contain an important adiabatic component which may be used to describe infrared transition intensity for imaginary Hermitian operators, such as the momentum and angular momentum operators. This previously unrecognized source of adiabatic infrared intensity forms a complement to the traditional Herzberg–Teller vibronic coupling expressions, which are active for the position operator, and resolves the paradox of vanishing electronic intensity for momentum operators in the Born–Oppenheimer approximation. Expressions for infrared absorption and vibrational circular dichroism are derived that utilize only ground electronic state wave functions; LCAO wave functions are used in these expressions to provide a more detailed description of these new momentum intensity contributions.
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33.20.Ea Infrared spectra
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.20.-t Molecular spectra

Photoelectron angular distributions of the N2O outer valence orbitals in the 19–31 eV photon energy range

C. M. Truesdale, S. Southworth@f@f, P. H. Kobrin, D. W. Lindle, and D. A. Shirley

J. Chem. Phys. 78, 7117 (1983); http://dx.doi.org/10.1063/1.444742 (7 pages) | Cited 10 times

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Photoelectron asymmetry parameters, partial cross sections, and branching ratios for the X, A, B, and C states of N2O+ were measured using synchrotron radiation in the photon energy range 19–31 eV. Vibrationally averaged data are reported for all four states, as well as vibrationally resolved data for the A and C states. The data are compared with a multiple scattering calculation, (e, 2e) dipole measurements, and similar data on CO2. The N2O and CO2 results show remarkable state‐by‐state similarity in their asymmetry parameters.
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33.60.+q Photoelectron spectra

Multiphoton ionization of nitrogen dioxide: Four photon spectroscopy of the npσu Rydberg series

Bennett H. Rockney@f@f, Gregory Hall, and Edward R. Grant

J. Chem. Phys. 78, 7124 (1983); http://dx.doi.org/10.1063/1.444743 (8 pages) | Cited 12 times

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Mass resolved multiphoton ionization (MPI) of NO2 reveals resonance‐enhanced production of NO+2 in the region from 490 to 540 nm. From an analysis of the MPI spectrum, we assign these resonant features to four photon transitions to high‐lying members of the npσu Rydberg series. A high resolution scan of one of these features shows clear rotational structure, the assignment of which yields a rotational constant for the linear upper state (B′=0.440±0.005 cm−1).
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33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Wz Other multiphoton processes
33.20.Sn Rotational analysis

Intermolecular coupling in HOD solutions

J. Wiafe‐Akenten and R. Bansil

J. Chem. Phys. 78, 7132 (1983); http://dx.doi.org/10.1063/1.444744 (6 pages) | Cited 11 times

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Intermolecular coupling of OD oscillators from HOD in H2O has been investigated by Raman spectroscopy. The data indicate that at concentrations of HOD greater than 10 mol % the effects of intermolecular coupling of OD ⋅⋅⋅ OD pairs become noticeable. Difference spectra show a characteristic derivativelike feature with an increase of intensity around 2400 cm−1 as intermolecular coupling increases. The peak frequency of the OD stretching vibration in HOD decreases from 2525 cm−1 at infinite dilution to 2500 cm−1 in 50 mol % HOD, while the width of the OD stretching band increases from 150 cm−1 (infinite dilution) to 178 cm−1 (50 mol % HOD). Depolarization measurement indicates that the band at ∼2500 cm−1 is polarized. By comparing these difference spectra with the spectrum of OD oscillators from D2O we suggest that the major features of the D2O spectrum in the liquid state can be obtained by considering intermolecular coupling of OD oscillators.
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78.30.C- Liquids
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
61.20.Qg Structure of associated liquids: electrolytes, molten salts, etc.
33.20.Fb Raman and Rayleigh spectra (including optical scattering)

A self‐consistent eikonal treatment of electronic transitions in molecular collisions@fa@f)

D. A. Micha

J. Chem. Phys. 78, 7138 (1983); http://dx.doi.org/10.1063/1.444753 (8 pages) | Cited 106 times

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We develop an eikonal treatment of electronic transitions in many‐atom collisions, in which classical nuclear trajectories are self‐consistently coupled to quantal electronic transitions. The treatment starts with a discussion of the electronic representations required to assure that Hamiltonian matrices are Hermitian. The amplitudes of wave functions are found to satisfy coupled equations which are expanded in powers of a local de Broglie wavelength. Time‐dependent equations are transformed to derive a Hamiltonian formalism that couples nuclear positions and momenta with electronic amplitudes. Cross sections are obtained from flux conservation and also from T‐matrix elements.
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34.50.Fa Electronic excitation and ionization of atoms (including beam-foil excitation and ionization)

Fluorescence lifetimes of single vibrational levels in HSO (math 2A′)

Masahiro Kawasaki, Kazuo Kasatani, Shigeo Tanahashi, Hiroyasu Sato, and Y. Fujimura

J. Chem. Phys. 78, 7146 (1983); http://dx.doi.org/10.1063/1.444754 (7 pages) | Cited 10 times

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Fluorescence lifetimes of single vibrational levels of the first excited state of HSO and DSO have been measured under effusive flow conditions following excitation by a pulsed dye laser. The lifetimes show a systematic decrease with v′3 (S–O stretch) from 74 μs (v′3=1) to 26 μs (v′3=8) for HSO. For DSO, the lifetimes are longer than the corresponding ones of HSO. These results are interpreted in terms of a second‐order coupling model (math 2A′→X 2A′′→continuum) in which the final states are those of H+SO dissociation continuum. An analytical expression is given to compute the vibrational energy dependence of nonradiative rates assuming an energy dependent linewidth from the dissociation.
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33.50.Dq Fluorescence and phosphorescence spectra
33.70.Fd Absolute and relative line and band intensities

Rotational energy transfer in HF

Roger L. Wilkins and Munson A. Kwok@f@f

J. Chem. Phys. 78, 7153 (1983); http://dx.doi.org/10.1063/1.444755 (6 pages) | Cited 7 times

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A rotational nonequilibrium model has been developed to simulate the infrared double‐resonance experimental technique designed to study rotational relaxation of HF gas in the v=1 and higher vibrational states. State‐to‐state rate coefficients for this rotation‐to‐translation relaxation model have been obtained from a surprisal analysis and are found to scale as an inverse power of the rotational energy transferred. Phenomenological rates for the rotational energy transfer in the v=1 state for J=0 to J=7 with Δ J =+1, +2, +3, and +4 are found to be in excellent agreement with the reported phenomenological rates from available experiments for both the v=1 and v=2 states. It appears, therefore, that the state‐to‐state rate coefficients for rotational relaxation of HF are insensitive to the v state. Angular momentum statistics corresponding to conservation of mj are found to give better fits to the data than those with mj assumed to be completely randomized.
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34.50.Ez Rotational and vibrational energy transfer
33.20.Ea Infrared spectra

Intermolecular multiple scattering of electrons. I. Theory

Lawrence S. Bartell and Anding Jin

J. Chem. Phys. 78, 7159 (1983); http://dx.doi.org/10.1063/1.444756 (6 pages) | Cited 4 times

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Coherent intramolecular multiple scattering by free molecules has been investigated extensively in the literature. Scant attention has been paid to incoherent intermolecular multiple scattering, however, a potentially serious problem under some circumstances. Therefore, a treatment of this problem has been carried out, taking advantage of simplifications afforded by the predominance of forward scattering. Explicit expressions of elementary form are derived for the differential cross sections corresponding to double, triple, and higher scatterings, and for the fractional contribution of each to the total intensity. Illustrative calculations are presented for electrons diffracted at various sample pressures encountered in a recent diffraction study of collisionally assisted laser pumping of SF6.
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34.80.Bm Elastic scattering

Intermolecular multiple scattering of electrons. II. Observed effects for SF6

Anding Jin and Lawrence S. Bartell

J. Chem. Phys. 78, 7165 (1983); http://dx.doi.org/10.1063/1.444757 (4 pages) | Cited 2 times

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A theory of intermolecular multiple scattering of electrons by vapor molecules is tested by comparing predicted effects with effects observed over a wide range of sample densities. It is found that the theory, which contains no adjustable constants and is based on small angle approximations, gives a good account of experimental observations. The degree to which experimental structure refinements are degraded by multiple scattering is also examined. It is found that derived internuclear distances are disturbed very little even when the mean number of scatterings per electron is as high as 2 and the interference features are washed out by a factor of 2. Apparent amplitudes of vibration are influenced more significantly but are still correctable with fair precision.
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34.80.Bm Elastic scattering
51.90.+r Other topics in the physics of gases (restricted to new topics in section 51)

IR multiple photon dissociation of C2HCl3: Molecular elimination vs bond fission and efficient dissociation of the C2Cl2 producta)

J. F. Caballero and C. Wittig

J. Chem. Phys. 78, 7169 (1983); http://dx.doi.org/10.1063/1.444758 (6 pages) | Cited 5 times

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The primary step in the IR multiple photon dissociation (IRMPD) of C2HCl3 is molecular elimination of HCl, even with laser fluences as high as 102 J cm−2. A large amount of atomic chlorine derives from the secondary photolysis of the vibrationally excited C2Cl2 produced concomitantly with HCl in the molecular elimination step. This C2Cl2 is dissociated very efficiently ([Cl]/[HCl]=0.6±0.2), since it absorbs radiation readily and is born with considerable vibrational excitation. We point out that the ultimate production of C2 molecules almost certainly involves the IRMPD of C2Cl, which has a low lying A 2A′′ electronic state that facilitates such optical excitation.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Wz Other multiphoton processes

The dynamics of infrared photodissociation of van der Waals molecules containing ethylene: An experimental study

D. S. Bomse@f@f, J. B. Cross@f@f, and J. J. Valentini@f@f

J. Chem. Phys. 78, 7175 (1983); http://dx.doi.org/10.1063/1.444759 (16 pages) | Cited 24 times

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Infrared (∼950 cm−1) predissociation of ethylene clusters has been studied using a crossed laser beam–molecular beam apparatus equipped with a moveable detector. van der Waals molecules undergo dissociation following absorption of a single infrared photon. Angular distributions, obtained for product molecules (C2H4)n, n=1–3, all show nearly exponentially decreasing product flux with increasing scattering angle. A product flux contour map has been generated for the photolysis reaction (C2H4)2 → C2H4+C2H4. Two isotropic center‐of‐mass distribution functions yield excellent agreement with experimental results. One is a function of reaction kinetic energy E, with P(E)=exp(−E/80 cm−1); the other is a function of product velocity (momentum) u, with U(u) =u exp(−u/9×103 cm/s). The latter distribution is characteristic of a dissociation pathway with a barrier in the exit channel. Such a barrier could result from centrigufal effects. It is argued that isotropic product scattering can be consistent with a direct dissociation reaction occurring on the subpicosecond time scale. It is not possible to determine, unambiguously, if dissociation is direct or if the reaction proceeds through a long‐lived intermediate. Experimental results indicate that only a small fraction of the energy available to products appears as translation, with the remainder appearing as rotational excitation of C2H4. Results are compared with other experimental studies on infrared predissociation of van der Waals cluster as well as a recent theoretical analysis of (C2H4)2 dissociation.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
36.40.-c Atomic and molecular clusters

Wigner method studies of ozone photodissociation

Maurice G. Sheppard and Robert B. Walker

J. Chem. Phys. 78, 7191 (1983); http://dx.doi.org/10.1063/1.444760 (9 pages) | Cited 87 times

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Ultraviolet absorption spectra (180–300 nm) out of vibrationally excited ozone are calculated using the classical Wigner method in conjunction with both ab initio and empirically adapted potential energy surfaces. Final state rovibrational distributions following photodissociation at 266 nm are calculated using a Wigner density in combination with classical trajectories.
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33.20.Lg Ultraviolet spectra
33.80.Gj Diffuse spectra; predissociation, photodissociation
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.20.-t Molecular spectra

Electron attachment to the perfluoroalkanes n‐CNF2N+2 (N=1–6) and i‐C4F10 a)

S. M. Spyrou, I. Sauers, and L. G. Christophorou

J. Chem. Phys. 78, 7200 (1983); http://dx.doi.org/10.1063/1.444761 (17 pages) | Cited 73 times

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Parent and fragment negative ion formation from seven perfluoroalkanes [n‐CNF2N+2 (N=1–6) and i‐C4F10] under low‐energy (0–10 eV) electron impact has been studied using a time‐of‐flight mass spectrometer. For CF4, C2E6, and C3F8 only fragment anions were observed, F− being the most abundant. For n‐C4F10 a weak parent ion was observed, but again the predominant ion was F−. For i‐C4F10, n‐C5F12, and n‐C6F14 the parent negative ions were the most abundant with relative cross sections peaking at 0.6 eV and autodetachment lifetimes from 10 to 100 μs depending on the molecular size and electron energy. In addition to the parent negative ion and the F− ion, fragment negative ions of the form CNF−2N+1, CNF−2N, CNF−2N−1 (with N=1–6) have been detected. The relative cross sections for all observed negative ions have been measured and corrected for the finite width of the electron pulse using an unfolding procedure. The positions of the dissociating negative ion states (NISs) shift to lower energies with increasing size of the molecule. Possible fragmentation mechanisms of the NISs leading to the production of the observed fragment negative ions have been suggested and discussed. From the appearance onsets of a number of fragment negative ions, various C–C and C–F bond dissociation energies, heats of formation, and electron affinities EA of certain fragments have been determined and are reported. It was found that the EA of perfluorocarbon radicals increases with increasing size of these radicals, and similar behavior is indicated also for the perfluoroalkane molecules themselves. The separation times of the dissociating fragments and the autodetachment lifetimes of the extremely short‐lived (∼10−15 s) and dissociating NISs of CF4 and C2F6 were estimated. The intensity of the parent anion as a function of molecular size and geometry is discussed, and the relative intensities of the fragment negative ions are rationalized on the basis of the present work and relevant data from electron swarm studies.
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34.80.Gs Molecular excitation and ionization
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

First quantum corrections to second virial coefficients for anisotropic interactions: Simple, corrected formulaa)

Russell T Pack

J. Chem. Phys. 78, 7217 (1983); http://dx.doi.org/10.1063/1.444762 (6 pages) | Cited 83 times

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A simple formula for the first quantum correction to the second virial coefficient, valid for the interaction of any like or unlike combination of atoms, diatomics, spherical top, or symmetric top molecules is given. It is found that the commonly used formulas of Wang Chang contain an error that omits an anisotropic contribution. The sizes of the different contributions to the quantum correction are discussed. As examples, calculations for He–SF6 and He–CO2 are reported.
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34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
34.90.+q Other topics in atomic and molecular collision processes and interactions (restricted to new topics in section 34)

The generalized mobility curve for alkali ions in rare gases: Clustering reactions and mobility curves

M. Takebe

J. Chem. Phys. 78, 7223 (1983); http://dx.doi.org/10.1063/1.444763 (4 pages) | Cited 13 times

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The measurements of mobility values for alkali ions in rare gases at room temperature over a wide range of E/N were completed for all 25 combinations. The experimental mobility curves were compared with a generalized mobility curve calculated from a model potential consisting of an inverse 8th power repulsive term and 6th and 4th power attractive terms, which took into account the core size, and potential well depths being determined for all the ion–gas combinations except for the cases of Rb+–He and Cs+–He from the relation between the observed maximum mobility and the ion energy. Experimental generalized mobility curves for alkali ions in rare gases were obtained using these well depths. It was found that all the experimental mobility curves were unified into a single curve using the model potential including the core size. The rate coefficients were measured for backward clustering reaction: Li+Ar–Ar, Li+Kr–Kr, and Li+Xe–Xe, using a drift tube. It was found that the activation energy is roughly half the well depth, comparing the well depths 0.550 eV for Li+–Ar, 0.710 eV for Li+–Kr, and 0.901 eV for Li+–Xe, with the activation energies obtained by Arrhenius plot for the backward reactions 0.34 eV for Li+Ar, 0.45 eV for Li+Kr, and 0.49 eV for Li+Xe.
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51.50.+v Electrical properties (ionization, breakdown, electron and ion mobility, etc.)
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies

Dissociative attachment in HCl, DCl, and F2

J. Norman Bardsley and J. M. Wadehra

J. Chem. Phys. 78, 7227 (1983); http://dx.doi.org/10.1063/1.444764 (8 pages) | Cited 83 times

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Resonant scattering models, using nonlocal decay widths, are developed for dissociative attachment of slow electrons to diatomic molecules. Cross sections are obtained for HCl and DCl in several initial rotational and vibrational states, and the dependence of the average attachment cross section upon the rotational and vibrational temperature is examined. For F2 the cross section for ground state molecules agrees well with experiment above 0.2 eV but shows no zero energy peak. The attachment cross section is higher for vibrationally excited molecules, but the enhancement is much less than that found in H2 and HCl.
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34.80.Gs Molecular excitation and ionization
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