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15 Jun 1987

Volume 86, Issue 12, pp. 6579-7251

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Theory of local mode excitation in polyatomics by frequency‐modulated lasers

Xie Bo‐Min and Ding Jian‐Qiang

J. Chem. Phys. 86, 6579 (1987); http://dx.doi.org/10.1063/1.452403 (5 pages) | Cited 3 times

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We investigate in this paper the possibility of local model (LM) excitation in polyatomic molecules by frequency‐modulated (FM) lasers. The interaction between the LM and all other background‐forming vibrational modes is treated as a perturbation, whose characteristics can be drawn from the experimental LM overtone line shapes. An integral expression describing the LM energy absorption process is obtained by which one can devise the effective experimental way to excite the LM of polyatomics by FM lasers.
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33.20.Tp Vibrational analysis
33.80.-b Photon interactions with molecules

Saturation and nonlinear electromagnetic field effects in the picosecond resonance Raman spectra of β‐carotene

P. J. Carroll and L. E. Brus

J. Chem. Phys. 86, 6584 (1987); http://dx.doi.org/10.1063/1.452404 (7 pages) | Cited 12 times

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The ground state resonance Raman spectra of β‐carotene, in fluid isopentane at 293 and 119 K, show saturation and near‐Lorentzian broadening as a function of fluence, using intense ∼30 ps visible laser pulses. There are no lines assignable to transient species. A two‐pulse, two‐color pump‐and‐probe Raman experiment shows that the broadening is due to high optical field, and not due to unrelaxed internal excitation in the molecule. The broadening is a manifestation of nonlinear resonance Raman scattering, previously predicted (but not observed) in molecules when the Rabi energy becomes larger than the vibrational dephasing linewidth. Our data can be semiquantitatively explained using a model by Dick and Hochstrasser. The saturation represents population loss from the vibrationally relaxed ground electronic state, and is consistent with lowest excited singlet lifetime on the order of 10 ps.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)
33.70.Jg Line and band widths, shapes, and shifts
33.70.Fd Absolute and relative line and band intensities
78.30.C- Liquids

Iterative maps for broadband excitation of transverse coherence in two level systems

H. Cho and A. Pines

J. Chem. Phys. 86, 6591 (1987); http://dx.doi.org/10.1063/1.452405 (11 pages) | Cited 3 times

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An iterative scheme has been used to derive a pulse sequence, compensated for off‐resonance and rf inhomogeneity pulse errors, which implements a π/2 rotation of the spin density operator around a well‐defined axis in the transverse plane. A fixed point analysis is applied to this and other iterative schemes revealing the source and nature of the compensation. Contrasting features of the different schemes are uniquely revealed by this analysis. General considerations for the construction of iterative schemes with other stable fixed sets are considered.
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07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques
33.25.+k Nuclear resonance and relaxation
76.60.-k Nuclear magnetic resonance and relaxation

Proton and carbon‐13 relaxation and molecular motion in glassy bisphenol‐A polycarbonate

John J. Connolly, Paul T. Inglefield, and Alan Anthony Jones

J. Chem. Phys. 86, 6602 (1987); http://dx.doi.org/10.1063/1.452406 (14 pages) | Cited 3 times

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An interpretation of proton and carbon‐13 spin–lattice relaxation in glassy polycarbonate is developed which is consistent with the geometry, time scale, and amplitude determined from chemical shift anisotropy line shape collapse. The line shape data indicate π flips and libration about the same axis as the predominant motions. A correlation function incorporating these motions is developed to quantitatively interpret the proton spin–lattice relaxation data and the line shape collapse. The π flip process is described as an inhomogeneous distribution of correlation times using the Williams–Watts fractional exponential. An apparent activation energy of 46 kJ/mol is determined with the fractional exponent remaining constant at 0.15. The librational motion is described by the Gronski formalism where the amplitude increases with the square root of temperature; and the rotational diffusion constant, linearly with temperature. Rotational diffusion constants fall in the range of 108 to 109 s−1 which is comparable to those observed in solution in sterically hindered polycarbonates. The librational motion only contributes to spin–lattice relaxation at the higher temperatures so that only an order of magnitude estimate of the restricted rotational diffusion constant results. This correlation function is then applied to carbon‐13 T1 data taken at various positions across the chemical shift anisotropy line shape on an isotopically enriched system. Little change in spin–lattice relaxation with position is observed which is consistent with the broad distribution of π flip correlation times. The rate of carbon‐13 spin–lattice relaxation is also fairly well predicted. Comparisons are made with magic angle sample spinning spin–lattice relaxation both in the laboratory and rotating frame. The former is fairly well approximated by the correlation function while the latter requires a significant spin–spin contribution to be reconciled with the rest of the interpretation.
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76.60.Es Relaxation effects
76.60.Cq Chemical and Knight shifts

Density fluctuation dynamics in a screened Coulomb colloid: Comparison of the liquid and bcc crystal phases

Lise K. Cotter and Noel A. Clark

J. Chem. Phys. 86, 6616 (1987); http://dx.doi.org/10.1063/1.452407 (6 pages) | Cited 15 times

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Inelastic light scattering from particle number density fluctuations was carried out on a suspension of 0.109 μ diam sulfonated polystyrene microspheres exhibiting coexisting colloidal liquid and body‐centered‐cubic single crystalline phases. Comparison of the first cumulants of the decay of the density–density correlation function in the two phases reveals that the wave vector dependence in the liquid exhibits many features in common with that for longitudinal 110 lattice vibrations in the crystal.
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82.70.Dd Colloids
61.25.H- Macromolecular and polymers solutions; polymer melts
78.35.+c Brillouin and Rayleigh scattering; other light scattering

Electron spin resonance (ESR) investigation of the structure of methyl radical trapping sites in methanol glass

T. Doba, K. U. Ingold, A. H. Reddoch, W. Siebrand, and T. A. Wildman

J. Chem. Phys. 86, 6622 (1987); http://dx.doi.org/10.1063/1.452408 (9 pages) | Cited 7 times

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Measurements are reported of ESR spectra of methyl radicals trapped in methanol glasses. In these spectra, forbidden lines appear as satellites of the lines of the methyl quartet as a result of dipolar coupling of the unpaired electron with protons of neighboring methanol molecules. The relative intensity of the satellites is used to study the structure of the sites where the radicals are trapped. Comparison of intensities observed in CH3OH, CH3OD, CHD2OD, CD3OH, and CD3OD indicates a structure that is locally similar to the (disordered) β‐phase crystal structure of methanol, with the methyl radical replacing a methanol molecule and occupying a position close to its methyl position. The resulting methyl–methyl distances are compared with those deduced from the observed rate constants of the hydrogen abstraction reaction taking place at the trapping sites. If volume changes due to cooling and phase transitions are taken into account, the distances obtained in the two experiments are found to be compatible. This confirms earlier conclusions that methanol glass has many structural features in common with the β‐phase crystal.
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76.30.Rn Free radicals
61.43.Fs Glasses
61.43.-j Disordered solids
61.66.Hq Organic compounds

Velocity modulation infrared laser spectroscopy of negative ions: The ν3, ν3+ν1−ν1, ν3+ν2−ν2, and ν3+2ν2−2ν2 bands of cyanate (NCO−)

Martin Gruebele, Mark Polak, and Richard J. Saykally

J. Chem. Phys. 86, 6631 (1987); http://dx.doi.org/10.1063/1.452409 (6 pages) | Cited 23 times

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We have measured 132 transitions in the ν3 (CN stretching) fundamental and the corresponding bending and stretching hotbands of the cyanate anion, ranging from P(52) to R(66), with a tunable diode laser using the velocity modulation technique. The spectra were fit to the standard linear triatomic molecule rotation–vibration Hamiltonian, yielding effective molecular parameters for the (0000), (0110), (0200), (1000), (0001), (0111), (0201), and (1001) states. The equilibrium rotational constant was determined to be Be =0.385 933(116) cm−1. A comparison with condensed phase results is presented. As previously oberved for N− 3, cyanate is detected with high vibrational excitation in the bending mode, most likely as a consequence of the formation mechanism.
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33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis

The effect of vibrational state mixing on the predissociation lifetime of ν1 excited OC–HF

K. W. Jucks and R. E. Miller

J. Chem. Phys. 86, 6637 (1987); http://dx.doi.org/10.1063/1.452410 (9 pages) | Cited 50 times

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Sub‐Doppler resolution infrared spectra have been obtained for the ν1 band of OC–HF. For most of the observed rovibrational transitions the linewidths are found to have a Lorentzian component of 190±10 MHz FWHM, presumably resulting from the vibrational predissociation of the complex. In several cases, however, perturbations, due to either anharmonic or Coriolis coupling between the vibrational state corresponding to the excited HF stretch and other vibrational states of the molecule, have been observed in the spectrum. Where these perturbations are present the width of the transitions vary with the relative contributions from the two states involved. This is explained in terms of a simple perturbative treatment of the coupling in conjunction with a Golden Rule treatment of the vibrational predissociation process. Stark measurements have also been performed in order to determine the dipole moment of the complex in the vibrationally excited state, namely μ1=2.545±0.008 D.
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33.20.Ea Infrared spectra
33.70.Jg Line and band widths, shapes, and shifts
33.57.+c Magneto-optical and electro-optical spectra and effects
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

The spectroscopy of the group Vlb transition metal hexacarbonyls using the electron impact method

C. F. Koerting, K. N. Walzl, and A. Kuppermann

J. Chem. Phys. 86, 6646 (1987); http://dx.doi.org/10.1063/1.452411 (8 pages) | Cited 5 times

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The electron energy‐loss spectra of Cr(CO)6, Mo(CO)6, and W(CO)6 were measured at impact energies of 25, 50, and 100 eV and at scattering angles from 0° to 90°. The differential cross sections (DCS’s) were obtained for several features in the 3–7 eV energy‐loss region. The symmetry‐forbidden nature of the 1A1g→1A1g,2t2g (π)→3t2g(π∗) transition in these compounds was confirmed. Several low energy excitations were assigned to ligand field transitions on the basis of the energy and angular behavior of their associated DCS’s. No transitions which could clearly be assigned to singlet→triplet excitations involving metal orbitals were located in these molecules. In addition, a number of states lying above the first ionization potential were observed for the first time. Several of these excitations seem to correspond quite well to some of the transitions observed in free CO.
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34.80.Gs Molecular excitation and ionization
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

The lowest triplet states of anthraquinone and chloroanthraquinones: The 1‐chloro, 2‐chloro, 1,5‐dichloro, and 1,8‐dichloro compounds

Kumao Hamanoue, Toshihiro Nakayama, Yōichi Kajiwara, Tetsuji Yamaguchi, and Hiroshi Teranishi

J. Chem. Phys. 86, 6654 (1987); http://dx.doi.org/10.1063/1.452412 (6 pages) | Cited 12 times

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From the measurements of phosphorescence spectra and triplet–triplet absorptions of the title compounds, it has been proposed that the lowest triplet states (T1) of the α‐chloro compounds are of mixed nπ∗–ππ∗ or ππ∗ character, while the nπ∗ triplet states are the lowest ones for anthraquinone and 2‐chloroanthraquinone (the β‐chloro compound). Compared with anthraquinone and the β‐chloro compound, much shorter lifetimes of the T1 states and small phosphorescence quantum yields were obtained for the α‐chloro compounds. These results have been interpreted in terms of the modification of the geometrical molecular structure by the interaction of the carbonyl group with chlorine atom(s) at the α position(s), causing the T1 states to be of ππ∗ character with short lifetimes.
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33.50.Dq Fluorescence and phosphorescence spectra
33.20.Lg Ultraviolet spectra
33.70.Fd Absolute and relative line and band intensities

The temperature dependence of the EPR spectrum of Cu2+ in ZnTiF6⋅6H2O between 4 and 160 K

R. S. Rubins and John E. Drumheller

J. Chem. Phys. 86, 6660 (1987); http://dx.doi.org/10.1063/1.452413 (5 pages) | Cited 1 time

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The EPR spectrum of Cu2+ in the low temperature monoclinic phase of ZnTiF6⋅6H2O was studied at 9.24 GHz between 4 and 160 K. Assuming the Jahn–Teller spectra to have axial symmetry, g∥ was found to vary from 2.472±0.004 at 4 K to 2.331±0.004 at 158 K, while g⊥ varied from 2.097±0.004 to 2.173±0.004 over this range. From the temperature variations of the g values, the lowering of the energy Δ of one potential well associated with the Jahn–Teller distortion compared to the other two was found to be approximately 143 cm−1. It is shown that an apparent decrease in Δ with increasing temperature above 100 K could be due to the presence of an excited vibronic state about 170 cm−1 above the ground state. The variation of ‖A∥‖ from (107±1)×10−4 cm−1 at 4 K to (53±3)×10−4 cm−1 at 158 K agreed with predictions based on the temperature variation of the g tensor.
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76.30.Fc Iron group (3d) ions and impurities (Ti-Cu)

The far infrared spectra of IBr charge–transfer complexes

Roseanne J. Sension and Herbert L. Strauss

J. Chem. Phys. 86, 6665 (1987); http://dx.doi.org/10.1063/1.452414 (4 pages) | Cited 7 times

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The IBr infrared band is examined as a function of temperature both in neat decane and in decane containing benzene or other complexing solutes. It is concluded that the spectra provide no evidence for a fast complexing reaction, contrary to previous reports.
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33.20.Ea Infrared spectra
82.53.-k Femtochemistry

The ultraviolet absorption spectrum of the math 1A″2←math 1A1 transition of jet‐cooled ammonia

V. Vaida, M. I. McCarthy, P. C. Engelking, P. Rosmus, H.‐J. Werner, and P. Botschwina

J. Chem. Phys. 86, 6669 (1987); http://dx.doi.org/10.1063/1.452415 (8 pages) | Cited 56 times

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The math←math absorption spectra of NH3 and ND3, recorded in a cold molecular jet, are presented. Vibrational band progressions resolvable up to v′2=14 appear. No other vibrations are present, either alone or in combinations. Relative band intensities for v2 progressions are recorded, and the homogeneous lifetime broadenings of vibrational levels of the math state are reported. The FWHM linewidths span 34–293 cm−1 over all bands of NH3 and 30–135 cm−1 over the v′2=2 through 14 bands of ND3. In general, the rate of dissociation increases nonlinearly with vibrational energy. The band intensity alternation, previously observed only in matrix spectra below 15 K, has been observed in these very cold gas phase samples.
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33.20.Lg Ultraviolet spectra
33.70.Jg Line and band widths, shapes, and shifts
33.20.Tp Vibrational analysis

Theoretical A 1A″2–X 1A1 absorption and emission spectrum of ammonia

P. Rosmus, P. Botschwina, H.‐J. Werner, V. Vaida, P. C. Engelking, and M. I. McCarthy

J. Chem. Phys. 86, 6677 (1987); http://dx.doi.org/10.1063/1.452416 (16 pages) | Cited 76 times

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Potential energy, electric dipole moment, and electronic transition moment surfaces have been calculated for the A and X states of NH3 from CASSCF and CEPA electronic wave functions. Anharmonic vibrational term values, Franck–Condon factors, and A–X radiative transition probabilities for the symmetric stretching and bending modes of NH3 and ND3 have been evaluated. The theoretical absorption spectra at room and low temperatures agree well with experimental data. The symmetric stretching mode in the A state has only small intensities in the A–X absorption spectrum. Emission rates from various initial vibronic levels of the A state are given. The ab initio electric dipole moment surfaces for the ground state of NH3 have been used to compute transition moments, which are in good agreement with experimental data.
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33.20.Lg Ultraviolet spectra
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
82.20.Kh Potential energy surfaces for chemical reactions
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.20.Tp Vibrational analysis

Dissociation of NH3 to NH2+H

M. I. McCarthy, P. Rosmus, H.‐J. Werner, P. Botschwina, and V. Vaida

J. Chem. Phys. 86, 6693 (1987); http://dx.doi.org/10.1063/1.452417 (8 pages) | Cited 73 times

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Potential energy, dipole moment, and electronic transition moment surfaces for the lowest dissociative pathways of the singlet math and math states of NH3 yielding NH2 (math 2B1,math 2A1) +H(2S) products have been calculated using complete active space MCSCF ab initio wave functions. The math state dissociation proceeds via a minimum barrier at the following planar geometry: αHNH =113°, rNH =1.042 Å (in the NH2 fragment), and RNH =1.323 Å (in the dissociation coordinate). The barrier height is calculated to be 3226 cm−1 with an expected accuracy of about 300 cm−1. The barrier height increases with increasing out‐of‐plane angle. Close to the barrier there are strong variations of the shapes of the dipole moment and transition moment surfaces. The minimum energy path through the math–math conical intersection follows planar geometries. Along this pathway the angle αHNH decreases, but the distance rNH in the NH2 fragment hardly changes. The crossing distance RcNH of the math and math states in planar structures depends strongly on αHNH and varies from about 1.68 Å (60°) to infinity (180°). The photodissociation process NH3(math) →NH2(math 2B1) +H(2S) is discussed on the basis of the calculated potential energy surfaces.
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33.20.Lg Ultraviolet spectra
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
82.20.Kh Potential energy surfaces for chemical reactions

Electron‐impact spectroscopy of various diketone compounds

K. N. Walzl, I. M. Xavier, and A. Kuppermann

J. Chem. Phys. 86, 6701 (1987); http://dx.doi.org/10.1063/1.452418 (6 pages) | Cited 9 times

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The spectra of the diketone compounds biacetyl, acetylacetone, acetonylacetone, 1,2‐cyclohexanedione, and 1,4‐cyclohexanedione have been investigated by the technique of low‐energy variable‐angle electron energy‐loss spectroscopy. With this method low‐lying, spin‐forbidden transitions have been observed. The energy difference between the lowest spin‐allowed and spin‐forbidden n→π∗ excitations in the acyclic diketones is found to be 0.35 eV, on average, which is nearly the same as that of comparable acyclic monoketone compounds; in 1,2‐cyclohexanedione, however, this energy difference is 0.84 eV, more than twice as large. This discrepancy in the magnitude of the n→π∗ singlet–triplet splittings may be attributed to differing amounts of overlap between the initial and final orbitals.
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34.80.Gs Molecular excitation and ionization

van der Waals clusters of pyridazine and isoquinoline: The effect of solvation on chromophore electronic structure

J. Wanna and E. R. Bernstein

J. Chem. Phys. 86, 6707 (1987); http://dx.doi.org/10.1063/1.452369 (10 pages) | Cited 10 times

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van der Waals clusters of pyridazine and isoquinoline with CH4, NH3, H2O, and CH3OH are generated in a supersonic molecular jet expansion and investigated by two‐color time‐of‐flight mass spectroscopy. As is the case for the other diazine systems, no spectra could be observed for pyridazine (H2O)n or (CH3OH)n clusters. Both chromophore molecules are reported to have close lying, vibronically coupled S1 and S2 excited states: nπ∗ for pyridazine and nπ∗ (S1) and ππ∗ (S2) for isoquinoline. Cluster spectra for pyridazine methane and ammonia clusters do not favor the presence of two nπ∗ transitions in the S1←S0 transition region but rather suggest that the ‘‘S2 origin’’ is a vibronic feature of the S1←S0 transition. Isoquinoline clusters that are only weakly or not at all hydrogen bonded (CH4 and NH3) display a complicated spectrum indicative of S1 (nπ∗)–S2 (ππ∗) vibronic coupling and not the usual shifted isolated molecular spectrum. Isoquinoline clusters with substantial hydrogen bonding (H2O and CH3OH) display relatively simple spectra indicative of only a single electronic transition S2 (ππ∗)←S0 in the region and no interstate vibronic coupling. These results are compared and contrasted with each other and the spectra of the other diazine clusters. Potential energy calculations are also employed to help understand the clustering in these systems.
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36.40.-c Atomic and molecular clusters
33.20.Kf Visible spectra
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
31.70.Dk Environmental and solvent effects

Orbitally correlated crystal field parametrization for lanthanide ions

Y. Y. Yeung and D. J. Newman

J. Chem. Phys. 86, 6717 (1987); http://dx.doi.org/10.1063/1.452370 (5 pages) | Cited 28 times

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Analogous to the treatment of the spin‐correlated crystal field, a new type of lanthanide correlation crystal field, which depends on the relative orientations of the individual 4f electron orbital angular momenta and the total orbital angular momentum, is postulated. Associated sets of parameters have been obtained for TmCl3⋅6H2O, Er3+:LaCl3, and Pr3+:LaCl3 from the quadratic moments of the J‐multiplet energy levels. The results show that this field is noticeable in the Pr3+:LaCl3 spectrum in which the particularly well known deviance of the 1D2 multiplet found in conventional crystal field fitting is largely eliminated.
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71.70.Ch Crystal and ligand fields

An investigation of the hydrogen‐bonded dimer H3N⋅⋅⋅HBr by pulsed‐nozzle, Fourier‐transform microwave spectroscopy of ammonium bromide vapor

N. W. Howard and A. C. Legon

J. Chem. Phys. 86, 6722 (1987); http://dx.doi.org/10.1063/1.452371 (9 pages) | Cited 23 times

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The ground‐state rotational spectrum of a dimer of ammonia and hydrogen bromide has been detected by using the technique of Fourier‐transform microwave spectroscopy in a Fabry–Perot cavity to examine a supersonically expanded gas pulse composed of ammonium bromide vapor entrained in argon. The spectroscopic constants B0, DJ, DJK, χ(14N), χ(79Br), and χ(81Br) have been determined (where appropriate) for the four symmetric‐top type isotopic species (14NH3, H79Br), (14NH3, H81Br), (15NH3, H79Br), and (15NH3, H81Br) and for the first of these the values are as follows: B0=3226.862(1) MHz, DJ=9.0(2) kHz, DJK=142.2(6) kHz, χ(14N)=−3.183(8) MHz, and χ(79Br)=361.245(6) MHz. The spectroscopic constants have been interpreted in terms of a hydrogen‐bonded dimer of C3v symmetry, having r(N⋅⋅⋅Br)=3.255 Å and the hydrogen‐bond stretching force constant kσ=13.4 N m−1. A detailed analysis has demonstrated that χ(79Br) is consistent with a model of the dimer in which only a small electrical rearrangement in the HBr subunit occurs on dimer formation (as opposed to proton transfer to NH3) and that this can be viewed as the transfer of ∼0.le from H into the 4pz orbital of Br.
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33.20.Bx Radio-frequency and microwave spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Dj Interatomic distances and angles

T–V energy transfer and the exchange reaction of H(D)+HF at 2.2(2.1) eV: Vibrational state distributions by time and wavelength resolved infrared fluorescence

Lisa M. Cousins and Stephen R. Leone

J. Chem. Phys. 86, 6731 (1987); http://dx.doi.org/10.1063/1.452372 (7 pages) | Cited 10 times

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The product state distributions for hot atom collisions of H(D)+HF are measured by the laser photolysis–infrared emission technique. The vibrational distribution of the HF T–V transfer process and exchange reaction product at 2.2 eV is 0.81±0.08, 0.16±0.02, and 0.03±0.01 corresponding to v=1–3, respectively. The HF and DF distritubions resulting from D+HF collisions at 2.1 eV are 0.65±0.09, 0.25±0.05, and 0.10±0.02 for HF(v=1–3) and 0.55±0.09, 0.25±0.04, 0.14±0.02, and 0.06±0.01 for DF(v=1–4). It is found that H atoms are 3.0 times more efficient than D atoms at exciting HF vibrations for the same kinetic energy. Although the vibrational distributions are similar, the D+HF T–V channel deposits approximately two times as much energy in the HF molecules as the vibrational exchange channel leaves in the DF molecules. The results are compared to recent three‐dimensional quasiclassical trajectory calculations and classical infinite‐order‐sudden calculations (accompanying paper) and are also considered in light of some simple models. The agreement between experiment and theory is excellent. The theoretical results show that significantly different mechanisms are resonsible for T–V energy transfer on the reactive and unreactive portions of the potential energy surface.
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34.50.Ez Rotational and vibrational energy transfer
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Rp State to state energy transfer
34.50.Lf Chemical reactions

A quasiclassical trajectory study of final state distributions in collisions of fast H(D) atoms with HF(DF)

George C. Schatz

J. Chem. Phys. 86, 6738 (1987); http://dx.doi.org/10.1063/1.452373 (7 pages) | Cited 7 times

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This paper presents a detailed theoretical study of the reactive and nonreactive final vibrational state distributions obtained in collisions of translationally hot H atoms with HF (and isotopic counterparts D+HF, H+DF, and D+DF). The potential surface used is surface No. 5 of Brown, Steckler, Schwenke, Truhlar, and Garrett, and it is characterized by a high barrier (1.9 eV) to F atom transfer. Cross sections and other dynamical information were generated using the quasiclassical trajectory (QCT) method, and we also did classical infinite‐order‐sudden (CIOS) calculations to characterize vibrational excitation mechanisms. Perhaps our most important results refer to the nonreactive final state distributions, where we find that collision of H with the F atom end of HF gives a broad vibrational distribution spread over many states while collision with the H atom end of HF gives a narrow distribution in which v′=1 is the only significant excited product. For D+HF, only the first collision mechanism is important, while for H+HF, H+DF, and D+DF, the second mechanism makes the dominant contribution to v′=1, and the first mechanism is the major contributor to v′>1. This leads to nonreactive vibrational distributions for H+HF, H+DF, and D+HF in which v′=1 is much larger relative to v′>1 than in D+HF. Comparison of these results with experiment for H+HF and D+HF indicates excellent agreement. Reactive distributions are also studied, and we find that the variation of these distributions with isotope can be explained in terms of a Franck–Condon overlap model. Comparison of the reactive final state distribution for D+HF with experiment indicates excellent agreement. Rotational excitation is examined for both reactive and nonreactive collisions, and we find that while the nonreactive rotational excitation is sensitive to which end of the molecule is struck, the reactive rotational distribution is controlled by kinematic propensities.
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82.20.Fd Collision theories; trajectory models
82.20.Rp State to state energy transfer
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
34.50.Ez Rotational and vibrational energy transfer

CARS spectroscopy of O2(1Δg) from the Hartley band photodissociation of O3: Dynamics of the dissociation

James J. Valentini, Daniel P. Gerrity, David L. Phillips, Jong‐Chen Nieh, and Kevin D. Tabor

J. Chem. Phys. 86, 6745 (1987); http://dx.doi.org/10.1063/1.452374 (12 pages) | Cited 71 times

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Rotationally and vibrationally resolved CARS spectra of the O2(1Δg) photofragment produced by the photodissociation of O3 at 17 wavelengths between 230 and 311 nm are reported. The spectra are taken under collision‐free conditions, therefore, they reveal the nascent rotational and vibrational state distributions of the O2(1Δg) photofragment. At all photolysis wavelengths studied the vibrational distribution peaks very sharply at v=0, although all energetically allowed vibrational states are observed. The rotational state distributions are narrow, and peak typically at high J. The rotational distribution shifts to lower J as the photolysis wavelength increases. These observations imply vibrationally adiabatic, rotationally impulsive energy release in the dissociation. The shape and width of the rotational distributions can be completely accounted for by the spread in the O3 thermal rotation and zero‐point vibration contributions to the O2(1Δg) photofragment angular momentum. The most striking observation about the O2(1Δg) photofragment quantum state distribution is an apparent propensity for even‐J states. Experiments with 18O enriched ozone indicate that this propensity is observed only for 16O16O, not for 18O16O, and by implication not for 17O16O. We show that this is the consequence of a selective depletion of only odd‐J rotational states of 16O16O(1Δg) by a curve crossing to O2(3Σg), but an equal depletion of both even‐J and odd‐J rotational states of 18O16O and 17O16O(1Δg) by the curve crossing. The odd‐J selectivity for 16O16O is a consequence of the restriction of 3Σg to only odd‐J states, due to the requirement of even nuclear exchange symmetry for this homonuclear species with spin‐zero nuclei. As a result of the different curve crossing behavior, the quantum yield for 3Σg is twice as great for 18O16O and 17O16O as it is for 16O16O, and this imposes a mass‐independent
isotopic fractionation in the photodissociation: the O2(1Δg) fragments are depleted of 17O and 18O, while the O2(3Σg) fragments are enriched in these isotopes.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
42.65.Dr Stimulated Raman scattering; CARS
42.65.Es Stimulated Brillouin and Rayleigh scattering
33.20.Vq Vibration-rotation analysis

Mass‐independent isotopic fractionation in nonadiabatic molecular collisions

James J. Valentini

J. Chem. Phys. 86, 6757 (1987); http://dx.doi.org/10.1063/1.452375 (9 pages) | Cited 30 times

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Symmetry and parity constraints associated with nonadiabatic collisions are shown to result in selection rules that can produce mass‐independent isotopic fractionation between molecular electronic states coupled by a nonadiabatic process. Nonadiabatic transitions from Π or Δ molecular electronic states to ∑ electronic states can result in isotopic fractionation for atoms occurring in equivalent positions in the molecule if the common isotope of those atoms is a spin‐zero nucleus. The Π or Δ state becomes depleted of the rare isotopes of those atoms while the ∑ state is enriched in the rare isotopes. Chemical processes that distinguish between the Π or Δ and ∑ states can convert this isotopic fractionation between electronic states of the same chemical species to a fractionation between different chemical species. Such nonadiabatic‐collision‐induced isotopic fractionation might explain observations of mass‐independent isotopic fractionation in electrical discharges, as well as in meteoritic samples. Nonadiabatic‐collision‐induced isotopic fractionation also may provide an explanation for observations of isotopic enrichments in the earth’s atmosphere.
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34.50.Gb Electronic excitation and ionization of molecules
82.20.Tr Kinetic isotope effects including muonium

Total scattering, surface ionization, and photoionization of a beam of H3 metastable molecules

James F. Garvey and Aron Kuppermann

J. Chem. Phys. 86, 6766 (1987); http://dx.doi.org/10.1063/1.452376 (16 pages) | Cited 17 times

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In a previous paper we reported a technique for generating an intense hyperthermal beam of hydrogen atoms and metastable H3 molecules. From the flight time of the H3 species between the source and detector we estimated that its lifetime exceeds 40 μs and that it must therefore be in the 2 p 2A″2 excited Rydberg state. In this paper we report experiments utilizing this novel source of H3 molecules. Beam‐gas attenuation measurements indicate that the H3–Ar cross section is roughly ten times larger than the H–Ar cross section for translational energies in the 1 to 10 eV range. This observation is consistent with the assignment of the H3 to that excited state, which has a much larger effective radius than a ground state hydrogen atom. The temperature dependence of the surface ionization of H3 by heated tungsten and platinum filaments is used to obtain effective ionizational potentials of this species. These potentials suggest that upon interaction with a metal surface, the metastable state decays to the repulsive 2 p 2E′ state which then surface ionizes to produce H+3. The production H+3 and H+ when the H3 beam is irradiated with UV light from a high pressure mercury lamp was also observed and is attributed to the relatively low ionization potential (∼3.7 eV) of the 2 p 2A′′2 metastable state of H3.
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34.50.Gb Electronic excitation and ionization of molecules
33.80.Eh Autoionization, photoionization, and photodetachment

Product formation in ion–ion recombination reactions involving the molecular anions of SF6 and perfluoromethylcyclohexane

C. A. Valkenburg, L. A. Krieger, and E. P. Grimsrud

J. Chem. Phys. 86, 6782 (1987); http://dx.doi.org/10.1063/1.452377 (10 pages) | Cited 3 times

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Experiments are described in which resonance electron attachment reactions A+e→A−, where A is SF6 or perfluoromethylcyclohexane (C7F14), are made to occur under conditions in which the observed depletion of A is affected not only by the electron attachment reaction, but also by the detailed nature of the subsequent recombination of the molecular anions with positive ions, p++A−→neutrals. It will be shown that the composite rate coefficients thereby measured, along with knowledge of the electron attachment coefficients, allow the efficiencies of molecular regeneration by ion–ion recombination, p++A−→A+neutrals, to be determined for A−=SF−6 and C7F−14. It is shown that SF−6 is converted by recombination primarily to unknown neutral species which do not undergo further electron attachment. It is also shown that C7F−14 is converted by recombination primarily back to the molecular species, C7F14.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies
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