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15 Dec 1992

Volume 97, Issue 12, pp. 8835-9494

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Collision‐induced transitions between A 1Σu+ and b 3Πu states of Na2: The ‘‘gateway’’ effect of perturbed levels

Li Li, Qingshi Zhu, A. M. Lyyra, T.‐J. Whang, W. C. Stwalley, R. W. Field, and M. H. Alexander

J. Chem. Phys. 97, 8835 (1992); http://dx.doi.org/10.1063/1.463971 (7 pages) | Cited 22 times

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We present here the best qualitative and quantitative illustration to date of the perturbation ‘‘gateway’’ effect in collision‐induced transitions between two mutually perturbing electronic states. The gateway effect, as described by Gelbart and Freed [Chem. Phys. Lett. 18, 470 (1973)], is a suggestion that all collision‐induced transfer of population between two electronic states proceeds through a small number of isolated‐molecule eigenstates which are of mixed electronic character, the ‘‘gateway levels,’’ and that the rates for such gateway‐mediated processes are related to the mixing fractions in the gateway levels. The gateway levels here are the Na2 A 1Σu+ v′=26∼b 3Π2u v′=28 J′=16e,a‐symmetry levels which are significantly mixed owing to an extremely small spin–orbit perturbation matrix element (the neighboring J′=15 and 17e,s‐symmetry levels are essentially free of mixing). A cw optical–optical double resonance (OODR) scheme is used to PUMP a single parent level and PROBE single daughter and granddaughter levels.
The oscillator strengths for the PUMP and PROBE transitions are derived, respectively, from the A 1Σu+←X 1Σg+ (26,4) band and the 2 3Π2g←b 3Π2u (28,28) subband. The qualitative observation of the gateway effect is that whenever an a‐symmetry A 1Σu+ v′=26 parent level is selected, b 3Π2u v′=28 daughter and granddaughter levels are observably populated, but when an s‐symmetry A 1Σu+ v′=26 parent is selected, essentially no population is detected in b 3Π2u v′=28 daughter and granddaughter levels (i.e., no perturbation, no interelectronic state transfer). The quantitative observation of the gateway effect is that when a J′=12 (or 14)e,a parent is selected, the most efficiently populated rotational levels of the other electronic state are granddaughter levels centered about the J′=16e,a gateway daughter level rather than about the J′ value of (or minimum energy gap relative to) the parent level.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
34.50.Gb Electronic excitation and ionization of molecules

High‐resolution and symmetry‐resolved N and O K‐edge absorption spectra of NO

Nobuhiro Kosugi, Jun‐ichi Adachi, Eiji Shigemasa, and Akira Yagishita

J. Chem. Phys. 97, 8842 (1992); http://dx.doi.org/10.1063/1.463359 (8 pages) | Cited 46 times

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High‐resolution electron‐yield and ‘‘symmetry‐resolved’’ ion‐yield spectra have been measured at the N and O K edges of a 2Π open‐shell molecule, NO. Several Rydberg transitions converging to the 3Π and 1Π ionization thresholds are found. The core‐to‐valence and core‐to‐Rydberg transitions are interpreted with the help of ab initio self‐consistent‐field configuration‐interaction and frozen‐core calculations for the core‐excited states with three open shells. It is found that the equivalent‐core model (N∗O=O2) breaks down in discussing the state ordering of the three N 1s–2pπ∗ excited states, 2Δ, 2Σ−, and 2Σ+. It is important to consider explicitly the core hole and the exchange repulsion between the core and π∗ electrons.
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33.60.+q Photoelectron spectra
31.15.V- Electron correlation calculations for atoms, ions and molecules

The hyperfine structure of (DCCD)2

R. L. Bhattacharjee, J. S. Muenter, and L. H. Coudert

J. Chem. Phys. 97, 8850 (1992); http://dx.doi.org/10.1063/1.463360 (14 pages) | Cited 5 times

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An experimental and theoretical study of the quadrupole‐coupling hyperfine structure of the nonrigid (C2D2)2 dimer is carried out. This dimer exhibits a large amplitude interconversion motion which splits rotational levels into three sublevels. The quadrupole‐coupling hyperfine pattern arising from the four deuterium atoms depends on the symmetry species of the tunneling sublevel. For nondegenerate sublevels, the hyperfine structure is especially interesting since the dimer behaves as if the quadrupole coupling were identical for all four deuterium atoms and the effective hyperfine Hamiltonian is completely symmetrical. The symmetry group used to classify the hyperfine levels is, therefore, the permutation group of four objects S4. For the other tunneling sublevels, which are doubly degenerate, the dimer behaves as if the two monomer units were inequivalent. Prior to the diagonalization of the hyperfine Hamiltonian, symmetry‐adapted nuclear spin wave functions in S4 are set up and allow us to select hyperfine levels whose symmetry is compatible with the tunneling symmetry species. This formalism is used to analyze the hyperfine patterns of three rovibrational transitions in (C2D2)2, which were recorded under high resolution. The components of effective quadrupole‐coupling tensors are thereby determined. These tensors are related to the eQq of an isolated DCCD monomer to obtain vibrationally averaged angles for large amplitude bending motions within the dimer.
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33.15.Pw Fine and hyperfine structure

Methyl group dynamics in the crystalline alanine dipeptide: A combined computer simulation and inelastic neutron scattering analysis

G. R. Kneller, W. Doster, M. Settles, S. Cusack, and J. C. Smith

J. Chem. Phys. 97, 8864 (1992); http://dx.doi.org/10.1063/1.463361 (16 pages) | Cited 25 times

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The dynamics of the methyl groups in the crystalline alanine dipeptide is examined over a wide range of temperatures using elastic and inelastic neutron scattering experiments and molecular dynamics (MD) simulations of the full crystal. Neutron scattering spectra are calculated from the simulations and directly compared to the experimental profiles. The N‐ter and C‐ter methyl groups in the molecule have low rotational barriers, i.e., ≲1 kcal/mol. They undergo rotations that are activated on time scales faster than ≊1 ns at temperatures as low as 50–100 K and are on the picosecond time scale at 150 and 300 K. At 300 K the rotational motion becomes strongly diffusive in the simulation. In contrast, the side‐chain methyls possess a significant intramolecular intrinsic torsional barrier, ≊3 kcal/mol. As a result, their dynamics consists of librations and rare jumps between wells. The simulations are further analyzed to characterize in detail the motions giving rise to the calculated scattering. Using a quaternion‐based method the simulated methyl dynamics is decomposed into rigid‐body rotational and translational components. The decomposed motions and their contributions to the calculated neutron profiles are examined.
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87.15.H- Dynamics of biomolecules

Detection and characterization of gas‐phase GaCl using resonance enhanced multiphoton ionization

David V. Dearden, Russell D. Johnson, and Jeffrey W. Hudgens

J. Chem. Phys. 97, 8880 (1992); http://dx.doi.org/10.1063/1.463362 (6 pages) | Cited 5 times

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We report resonance enhanced multiphoton ionization (REMPI) spectra of gas‐phase gallium monochloride (GaCl) produced with laser light tuned between 330 and 430 nm. The spectra originate from one, two, and three photon resonances with electronic states that reside between 29 500 and 80 000 cm−1. We have assigned five Rydberg series comprised of 20 new electronic states. Each state exhibits a short vibrational progression. A least‐squares fit of the unperturbed Rydberg state origins yields the adiabatic ionization potential, IPa(GaCl)=80 540±10 cm−1. We have also identified bands that originate from one‐photon resonances with the previously known a 3Π state. We conservatively estimate the REMPI detection sensitivity for GaCl to be 109 molecules cm−3 (laser pulse)−1.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.60.+q Photoelectron spectra

Photodissociation spectroscopy of Mg+–H2O and Mg+–D2O

K. F. Willey, C. S. Yeh, D. L. Robbins, J. S. Pilgrim, and M. A. Duncan

J. Chem. Phys. 97, 8886 (1992); http://dx.doi.org/10.1063/1.463363 (10 pages) | Cited 71 times

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Mg+–H2O ion–molecule complexes are produced in a pulsed supersonic nozzle cluster source. These complexes are mass selected and studied with laser photodissociation spectroscopy in a reflectron time‐of‐flight mass spectrometer system. An electronic transition assigned as 2B2←X 2A1 is observed with an origin at 28 396 cm−1. The spectrum has a prominent progression in the metal‐H2O stretching mode with a frequency (ω′e) of 518.0 cm−1. An extrapolation of this progression fixes the excited state dissociation energy (D′0) at 15 787 cm−1. The corresponding ground state value (D″0) is 8514 cm−1 (24.3 kcal/mol). The solvated bending mode, and symmetric and asymmetric stretching modes of water are also active in the complex, as are the magnesium bending modes. A second electronic transition assigned as 2B1←X 2A1 is observed with an origin at 30 267 cm−1 and a metal stretch frequency for Mg+–H2O of 488.5 cm−1 (ΔG1/2). Spectra of both excited states are also observed for Mg+–D2O. Partially resolved rotational structure is analyzed for both isotopes, leading to the conclusion that the complex has a structure with C2v symmetry. This study was guided by ab initio calculations by Bauschlicher and co‐workers, which provide accurate predictions of the electronic transition energies, vibrational constants, and dissociation energies.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
33.15.-e Properties of molecules
31.15.V- Electron correlation calculations for atoms, ions and molecules

The rotationally resolved 1.5 μm spectrum of the HCN–HF hydrogen‐bonded complex

E. R. Th. Kerstel, H. Meyer, K. K. Lehmann, and G. Scoles

J. Chem. Phys. 97, 8896 (1992); http://dx.doi.org/10.1063/1.463364 (10 pages) | Cited 5 times

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We have measured the overtone spectrum of the CH stretching mode in HCN–HF. The vibrational predissociation rate is approximately twice that previously determined for fundamental excitation, whereas the complexation induced frequency shift is only marginally larger than that of the fundamental spectrum. These results are discussed in terms of a first‐order perturbation theory treatment as set forth by LeRoy, Davies, and Lam [J. Phys. Chem. 95, 2167 (1991)]. We suggest that the frequency shift observed here might not only be due to complexation, but also to a long‐range anharmonic interaction.
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33.20.Ea Infrared spectra
33.15.Fm Bond strengths, dissociation energies
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Rotational–translational coupling and asymmetric line shapes in the high resolution stimulated Brillouin gain spectra of liquid carbon disulfide

W. T. Grubbs and Richard A. MacPhail

J. Chem. Phys. 97, 8906 (1992); http://dx.doi.org/10.1063/1.463365 (9 pages) | Cited 6 times

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We have measured high resolution stimulated Brillouin gain spectra of liquid carbon disulfide at a number of temperatures between 158 and 301 K. As the temperature decreases the shape of the Brillouin peak becomes increasingly asymmetric, with the intensity skewed away from the center of the spectrum. We show that this unusual asymmetry results from the coupling between rotational and translational motions. Rotational–translational coupling is well known to give rise to line shape distortions in the depolarized light scattering spectra of nonspherical molecules (e.g., the ‘‘Rytov dip’’), but the observation of line shape distortions (the asymmetry) in the polarized Brillouin lines has not been noted before. In the latter case, the asymmetry in the Brillouin peak can be traced to a cross correlation between the collective molecular orientation variable and the number density fluctuation and can be viewed as an acoustically induced birefringence. A calculation using the theory of rotational–translational coupling and experimental values for the relevant parameters reproduces quantitatively the observed asymmetry in the Brillouin lines over the entire temperature range.
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42.65.Es Stimulated Brillouin and Rayleigh scattering
33.70.Jg Line and band widths, shapes, and shifts

Calculations of absorption and emission spectra: A study of cis‐stilbene

David C. Todd, Graham R. Fleming, and John M. Jean

J. Chem. Phys. 97, 8915 (1992); http://dx.doi.org/10.1063/1.463366 (11 pages) | Cited 13 times

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Absorption and emission spectra are calculated by modeling cis‐stilbene as a system of 12 displaced harmonic oscillators. We are able to obtain good agreement with the peaks of both the room temperature absorption and low temperature emission spectra using parameters from resonance Raman experiments by adjusting the position of the zero–zero transition energy (E0,0) and slightly adjusting the displacements along the normal modes. The best fit value for E0,0 is 29 000 cm−1. Using the displacements along the 12 degrees of freedom, and a normal mode description based on semiempirical quantum force field calculations (QCFF/PI), we determine a ‘‘relaxed’’ excited state geometry which is twisted a maximum of approximately 37 deg along the ethylenic torsional coordinate relative to the relaxed ground state geometry. An extension of the spectral calculations is described which allows for one or more of the modes to be anharmonic and vibrationally unrelaxed. We apply this extension to show that cis‐stilbene emission can not be originating from a geometry with a 90 deg twist along the ethylenic coordinate. Comparison of our results with a recently obtained room temperature solution phase emission spectrum suggest that this emission originates from vibrationally unrelaxed molecules.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.20.-t Molecular spectra
82.30.Qt Isomerization and rearrangement

A study of phenylacetylene and styrene, and their argon complexes PA–Ar and ST–Ar with laser threshold photoelectron spectroscopy

J. M. Dyke, H. Ozeki, M. Takahashi, M. C. R. Cockett, and K. Kimura

J. Chem. Phys. 97, 8926 (1992); http://dx.doi.org/10.1063/1.463367 (8 pages) | Cited 22 times

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In this work, the molecules styrene (ST) and phenylacetylene (PA), as well as their argon complexes ST–Ar and PA–Ar, have been investigated with (1+1′) resonance enhanced multiphoton ionization (REMPI) threshold photoelectron spectroscopy (TES). The first adiabatic ionization energies of ST, PA, ST–Ar, and PA–Ar have been measured as 68 267±5, 71 175±5, 68 151±5, and 71 027±5 cm−1, respectively. For both ST–Ar and PA–Ar, the first photoelectron band shows structure in the lowest frequency van der Waals (vdW) bending mode in the ground ionic state, with νvdW being measured as 15 cm−1 in each case. For each molecule excitation to a particular vibrational level of the S1 state followed by ionization, allows structure in that mode to be observed in the threshold photoelectron spectrum. This has been achieved for three modes in both styrene and phenylacetylene. The experimental ionic vibrational frequencies thus obtained, have been compared with those known for the S0 and S1 states.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.60.+q Photoelectron spectra

Suppression of sodium nuclear magnetic resonance double‐quantum coherence by chemical shift and relaxation reagents

Robert B. Hutchison, James J. A. Huntley, Haoran Jin, and Joseph I. Shapiro

J. Chem. Phys. 97, 8934 (1992); http://dx.doi.org/10.1063/1.463320 (7 pages)

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An investigation into the signal suppression behavior of the paramagnetic shift and relaxation reagents, Dy(P3O10)27− and Gd(P3O10)27−, with regard to their use in the nuclear magnetic resonance spectroscopic study of sodium has been performed. Measurements of T1 and T2 relaxation time constants of sodium in normal saline, Krebs–Henseleit buffer, and human blood serum, as a function of concentration of these reagents showed that, although closely coupled in the saline and K–H buffer environments, in plasma T1 and T2 become decoupled, transverse relaxation dominating in comparison to longitudinal relaxation. Linewidth measurements further suggest that relaxation in the plasma milieu is controlled primarily by inherent T2 relaxation, rather than by field inhomogeneity or diffusion effects. Quantitative single‐quantum (1Q) and double‐quantum (2Q) intensity measurements, biexponential T2 relaxation measurements, and parametric studies of the preparation time of the 2Q pulse sequence, were obtained in suspensions of bovine serum albumin and human erythrocytes. The observed suppression of sodium 2Q coherence by paramagnetic shift and relaxation reagents was found to exhibit a complex behavior in albumin solutions, involving the biexponential T2 decay to be expected during the preparation time of the 2Q filter pulse sequence, as well as the optimum preparation time for production of the double‐quantum coherence itself. The controlling factor for both of these effects is the biexponential amplitude function in the expression for the transverse magnetization observed following application of the 2Q pulse sequence. This in turn is determined entirely by the values for the slow and fast components of biexponential relaxation in sodium, which themselves depend upon the concentration of the macromolecular binding sites for quadrupolar interaction. A similar behavior has been observed in suspensions of human erythrocytes.
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87.61.-c Magnetic resonance imaging

Influence of broken conformational symmetry on the surface enrichment of polymer blends

Glenn H. Fredrickson and James P. Donley

J. Chem. Phys. 97, 8941 (1992); http://dx.doi.org/10.1063/1.463969 (6 pages) | Cited 22 times

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We construct an expression for the free energy of an incompressible binary blend of homopolymers in the vicinity of a hard reflecting surface. Expressed as a functional of the concentration profile away from the surface, we find a contribution to the free energy that is linear in the gradient of the concentration and with a range that extends from the surface a distance of order of the radius of gyration. This term, which was overlooked in previous investigations of wetting phenomena in polymer blends, derives from conformational restrictions on the polymers in the surface layer. An important feature of this nonlocal surface term is to favor an excess of the more flexible component at the surface. In addition, it is capable of influencing the detailed shape of the concentration profile and modifying wetting and prewetting behavior.  
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61.25.H- Macromolecular and polymers solutions; polymer melts
82.60.Lf Thermodynamics of solutions

Theoretical aspects of higher‐order truncations in solid‐state nuclear magnetic resonance

M. Goldman, P. J. Grandinetti, A. Llor, Z. Olejniczak, J. R. Sachleben, and J. W. Zwanziger

J. Chem. Phys. 97, 8947 (1992); http://dx.doi.org/10.1063/1.463321 (14 pages) | Cited 42 times

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Recent experimental developments of high‐resolution NMR in solids (for example, double rotation and dynamic‐angle spinning) address the reduction of second‐order line broadening effects, particularly in systems involving quadrupolar nuclei such as 23Na, 17O, 27Al, and 14N. However, some aspects of the theoretical description of these systems have not been clearly understood; in particular, the various procedures available to truncate the interactions give incompatible results. We present a general framework, based on static perturbative methods, which provides a natural procedure to derive the correct Hamiltonian for higher‐order effects in irreducible tensor form. Applications of this method to coherent averaging techniques (sample motion or radio‐frequency irradiation) are described and compared to previous treatments based on average Hamiltonian theory.
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76.60.Gv Quadrupole resonance
76.20.+q General theory of resonances and relaxations

Laser spectroscopy of the CaOH A 2Π–X 2Σ+ (020)–(000) band: Deperturbation of the Fermi resonance, Renner–Teller, and spin–orbit interactions

Mingguang Li and John A. Coxon

J. Chem. Phys. 97, 8961 (1992); http://dx.doi.org/10.1063/1.463322 (9 pages) | Cited 19 times

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The A 2Π–X 2Σ+ (020)–(000) band of CaOH has been observed using cw dye laser excitation, and rotationally analyzed. The measured line positions, in combination with data obtained in our laboratory for the A 2Π–X 2Σ+ (100)–(000) band, have been employed in a global deperturbation of the A(100) 2Π and A(020)κ 2Π and μ 2Π vibronic states that simultaneously takes account of Fermi resonance, the Renner–Teller effect, and spin–orbit coupling. All the measured line positions are reproduced to within the measurement accuracy of 0.005 cm−1. The Renner–Teller and Fermi resonance parameters in the CaOH A 2Π state have been determined: ϵω2=−36.569 19(97) cm−1 and W1=10.300 58(54) cm−1.
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31.50.Df Potential energy surfaces for excited electronic states
33.20.Sn Rotational analysis

Exploiting quantum interference effects for the determination of the absolute orientation of an electronic transition moment vector in an isolated molecule

David F. Plusquellic and David W. Pratt

J. Chem. Phys. 97, 8970 (1992); http://dx.doi.org/10.1063/1.463323 (7 pages) | Cited 19 times

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Quantum interference effects have been observed in the O00 band of the fully resolved S1←S0 fluorescence excitation spectrum of the hydrogen bonded complex of trans‐2‐naphthol with ammonia (t‐2HNA). The effects owe their origin to the simultaneous existence of (1) ab hybrid band character in the spectrum; (2) perturbations in the spectrum that are produced by a coupling of overall rotation with the hindered internal rotation of the attached NH3 group; and (3) low barriers along the torsional coordinate in both electronic states. Analysis of the resulting interference patterns makes possible an unambiguous determination of both the magnitude and the sign of the orientation of the transition moment vector with respect to the a inertial axis in t‐2HNA, θTM=−66±2°. Comparison of this result with that for the S1←S0 transition of the bare molecule shows that the orientation of its transition moment in the molecular frame is unaffected by complex formation. Additionally, symmetry‐based arguments are given that may be used to predict the existence of quantum interference effects of this type in other systems.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.50.Dq Fluorescence and phosphorescence spectra
03.65.-w Quantum mechanics

Temperature dependence of nuclear shielding and quadrupolar coupling of noble gases in liquid crystals

Juhani Lounila, Oliver Muenster, Jukka Jokisaari, and Peter Diehl

J. Chem. Phys. 97, 8977 (1992); http://dx.doi.org/10.1063/1.463324 (9 pages) | Cited 14 times

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A theoretical model for nuclear magnetic resonance (NMR) observables of noble gas atoms in nematic and smectic A liquid crystal environments is developed. It is used to account for the behavior of the nuclear shielding of 129Xe (σ) and the quadrupolar coupling of 21Ne (B) measured in 1‐butyl‐c‐4‐(4′‐octylbiphenyl‐4‐yl)‐r‐1‐cyclo‐hexan‐carbonitrile (NCB 84). The analysis provides detailed information on the orientational and positional order of the liquid crystal solvent and of the solute atoms. In the nematic phase, the decrease of σ with decreasing temperature is attributed to the increase of the density and orientational order parameter S of the solvent. The former factor determines the variation of the isotropic part of σ, while the latter governs the change in the anisotropic part (the anisotropy of the shielding tensor Δσ is of the order of −10 ppm). The prominent increase of σ in the smectic A phase is ascribed to the tendency of the atoms to occupy the interspaces between the smectic layers rather than their interiors (the deviation from the uniform distribution of the atoms is of the order of 5%). The main result is that the isotropic part of σ increases as the density of the solvent in the immediate neighborhood of the solute decreases. In the case of the quadrupolar coupling, the effect of the redistribution of the atoms is small, as there is no isotropic contribution to B. The behavior of the quadrupolar coupling of 21Ne in the nematic phase indicates that there are two distinct contributions to B. One is due to the distortion of the electron cloud from the spherical symmetry and the other arises directly from the external electric field gradient produced by the neighboring molecules. The temperature dependence of the latter contribution is not determined by the variation of S alone.  
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76.60.-k Nuclear magnetic resonance and relaxation
76.20.+q General theory of resonances and relaxations

Fourier transform infrared‐matrix isolation study of gaseous cerium dicarbide species

A. Feltrin, M. Guido, and S. Nunziante Cesaro

J. Chem. Phys. 97, 8986 (1992); http://dx.doi.org/10.1063/1.463325 (4 pages) | Cited 5 times

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For the first time, experimental evidence was obtained for the presence of a C=C bond in the structure of a gaseous rare‐earth dicarbide species. A value of 1754.5 cm−1 could be unambiguously determined for the corresponding stretching frequency in the Ce12C2 molecule. Strong support was also obtained for the assignment of a weak broadband centered at 1720.5 cm−1 to analogous modes in 13C monosubstituted cerium dicarbide molecules. The possible assignment of a weak band at 789.1 cm−1 to stretching modes of Ce–C bonds in Ce12C2 molecules remains tentative. In order to fit the above assignments, the normal coordinate analysis was performed for each of the structures which preserve the C=C bond and the results were compared.
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33.20.Ea Infrared spectra
33.20.Tp Vibrational analysis

Photoelectron spectroscopy of small antimony cluster anions: Sb−, Sb2−, Sb3−, and Sb4−

Mark L. Polak, Gustav Gerber, Joe Ho, and W. C. Lineberger

J. Chem. Phys. 97, 8990 (1992); http://dx.doi.org/10.1063/1.463326 (11 pages) | Cited 16 times

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We report the 351 nm photoelectron spectra of Sb−, Sb2−, Sb3−, and Sb4−. The electron affinity of atomic Sb is measured to be 1.046(5) eV. The Sb2− photoelectron spectrum displays rich vibrational and electronic structure. Low‐lying electronically excited states are observed for both the anion and the neutral. Several features in both the 351 and 364 nm photoelectron spectra of Sb2− cannot be explained as Franck–Condon processes, indicating that we are accessing autodetaching resonances of the negative ion at these wavelengths. The adiabatic electron affinity of Sb2 is determined to be 1.282(8) eV. For the photoelectron spectra of Sb3− and Sb4−, the observed electronic structure is explained in terms of recently reported ab initio calculations. The adiabatic electron affinity of Sb3 is estimated to be 1.85(3) eV, and an upper bound on the electron affinity of Sb4 is reported, EA(Sb4)≤1.00(10) eV. The vertical detachment energies of Sb3− and Sb4− to the neutral ground states are determined to be 1.90(2) and 1.57(5) eV, respectively. We report photoelectron angular distributions for all the observed spectra, and find that the autodetaching resonance causes unusual angular distributions for Sb2− photodetachment. Finally, electron affinity trends for group V atoms, dimers, and small clusters are discussed in light of the present study.
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36.40.-c Atomic and molecular clusters
33.60.+q Photoelectron spectra

Multidimensional path integral calculations with quasiadiabatic propagators: Quantum dynamics of vibrational relaxation in linear hydrocarbon chains

Maria Topaler and Nancy Makri

J. Chem. Phys. 97, 9001 (1992); http://dx.doi.org/10.1063/1.463327 (15 pages) | Cited 56 times

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This paper presents the first application of a new method for multidimensional real time quantum dynamics described in a previous Letter [Chem. Phys. Lett. 193, 435 (1992)]. The key feature of the method is the use of an improved zeroth order representation in the Feynman propagator, which allows large time steps in the path integral. Use of the adiabatic approximation in the case of a system coupled to a harmonic bath leads to a path integral over the system coordinate with a one‐dimensional propagator which is constructed numerically and which corresponds to dynamics along the adiabatic path, and with a nonlocal influence functional that accounts for nonadiabatic effects. We have performed accurate quantum mechanical calculations on the dynamics of CH overtone relaxation in linear hydrocarbon chains by direct numerical evaluation of the path integral in the quasiadiabatic representation. Converged results for the survival probability of the υ=5 and υ=8 states of HC6 are reported up to five vibrational periods of the CH stretch and compared to those obtained from standard classical and semiclassical simulations.
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82.20.Rp State to state energy transfer
03.65.Db Functional analytical methods

Reactive scattering from oriented molecules: The three‐center reaction K+ICl→KI+Cl, KCl+I

H. J. Loesch and J. Möller

J. Chem. Phys. 97, 9016 (1992); http://dx.doi.org/10.1063/1.463328 (15 pages) | Cited 32 times

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In a crossed molecular beam experiment, we have measured the angular and time‐of‐flight (TOF) distributions of the products KCl and KI formed in the reaction K+ICl→KI+Cl, KCl+I at an elevated collision energy of Etr=1.64 eV. Employing the brute force method, we have prepared an oriented ICl beam and studied in addition also the orientation dependence of these distributions. The results are (i) KCl is the dominant product, but also KI is substantially formed with a branching ratio of 4:1; (ii) the double differential reaction cross section in the center‐of‐mass frame (contour maps) indicates that all products are preferentially forward scattered and constrained to the forward hemisphere; (iii) the KCl flux consists of two distinct components which differ markedly in kinetic energy and dependence on the ICl orientation; there are also indications of the existence of two components of KI; (iv) 65%, 84%, and 64% of the available energy is vested into the internal degrees of freedom for the fast, slow component of KCl and KI, respectively; (v) the existence of two components can be rationalized on the basis of the harpooning mechanism where the jumping electron accesses the ground state or one of the low excited states of the ICl− ion and triggers the subsequent explosion of the ion with more or less kinetic energy of the fragments depending on the initially populated state; (vi) the energies released during dissociation of ICl− in the 2Σ ground state and the first 2Π state are ≤0.19 and ≤1.2 eV, respectively; (vii) the fast KCl component features a negative steric effect suggesting favorable product formation for attacks of K to the I end of ICl, the steric effect of the slow KI component is positive, i.e., attacks to the Cl end form products favorably; the other components exhibit no significant steric effect; (viii) the steric effects can be quantitatively rationalized using the same model as mentioned above; (ix) the magnitude of the steric effect suggests a rotational temperature of the low‐lying states of the ICl beam of 14±1 K.  
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
34.50.Lf Chemical reactions

Effect of temperature on the attachment of slow (≤1 eV) electrons to CH3Br

P. G. Datskos, L. G. Christophorou, and J. G. Carter

J. Chem. Phys. 97, 9031 (1992); http://dx.doi.org/10.1063/1.463329 (5 pages) | Cited 19 times

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The electron attachment rate constant ka(〈ϵ〉, T) for CH3Br has been measured in a buffer gas of N2 using a high pressure electron swarm apparatus, within the mean electron energy range from thermal (∼0.046 eV) to 0.87 eV and over the temperature range, T, 300–700 K. At room temperature, CH3Br attaches low energy electrons weakly but as T is raised from 300 to 700 K the total electron attachment cross section increases by more than 2 orders of magnitude. At T=300 K the electron attachment cross section exhibits a peak at 0.38 eV which shifts progressively to lower electron energies as T is increased. The thermal value of ka is 1.08×10−11 cm3 s−1 at 300 K and 3.28×10−9 cm3 s−1 at 700 K.
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33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
07.77.-n Atomic, molecular, and charged-particle sources and detectors
37.20.+j Atomic and molecular beam sources and techniques

CO product distributions from the visible photodissociation of HCO

D. W. Neyer, S. H. Kable, J‐C. Loison, P. L. Houston, I. Burak, and E. M. Goldfield

J. Chem. Phys. 97, 9036 (1992); http://dx.doi.org/10.1063/1.463330 (10 pages) | Cited 18 times

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The final state distribution of carbon monoxide produced in the photodissociation of the formyl (HCO) radical has been studied both experimentally and theoretically. Renner–Teller coupling between the excited HCO math state and the ground state leads to dissociation and yields H and CO. Vibrational and rotational distributions have been measured for CO produced after excitation to specific vibrational levels on the math←math transition of HCO cooled in a supersonic expansion. The strongest transitions are for excitation to vibrational states with six to 16 quanta in the bending mode, and dissociation from these states produces inverted CO rotational distributions with average rotational quantum numbers <J≳ in the 22–33 range. The value of <J≳ increases monotonically with the vibrational quantum number describing the bend of the excited triatomic. Experiments involving excitation of one quantum of the C–H stretching motion have revealed that this vibration results in increased rotational excitation of the product CO with values of <J≳ as high as 41. In contrast, experiments indicate that the C–O stretching mode of HCO acts nearly as a spectator during the dissociation process. Excitation of HCO states with one quantum of C–O stretch yields vibrationally excited CO as the dominant dissociation product, but with a rotational distribution similar to that for CO(ν=0) produced following the excitation of HCO states without the quantum of C–O stretch. Classical trajectory calculations on an ab initio potential energy surface have modeled many of the experimental features and trends of the CO product distributions. There are, however, some discrepancies in the positions of rotational maxima and in the efficiency of the coupling of the C–O vibration of HCO to the dissociation coordinate. It is not clear whether these are due to approximations made in the modeling or inaccuracies in the potential energy surface.
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82.20.Hf Product distribution
82.20.Rp State to state energy transfer
82.20.Fd Collision theories; trajectory models
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light

Femtosecond real‐time probing of reactions. IX. Hydrogen‐atom transfer

J. L. Herek, S. Pedersen, L. Bañares, and A. H. Zewail

J. Chem. Phys. 97, 9046 (1992); http://dx.doi.org/10.1063/1.463331 (16 pages) | Cited 102 times

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The real‐time dynamics of hydrogen‐atom‐transfer processes under collisionless conditions are studied using femtosecond depletion techniques. The experiments focus on the methyl salicylate system, which exhibits ultrafast hydrogen motion between two oxygen atoms due to molecular tautomerization, loosely referred to as intramolecular ‘‘proton’’ transfer. To test for tunneling and mass effects on the excited potential surface, we also studied deuterium and methyl‐group substitutions. We observe that the motion of the hydrogen, under collisionless conditions, takes place within 60 fs. At longer times, on the picosecond time scale, the hydrogen‐transferred form decays with a threshold of 15.5 kJ/mol; this decay behavior was observed up to a total vibrational energy of ∼7200 cm−1. The observed dynamics provide the global nature of the motion, which takes into account bonding before and after the motion, and the evolution of the wave packet from the initial nonequilibrium state to the transferred form along the O–H—O reaction coordinate. The vibrational periods (2π/ω) of the relevant modes range from 13 fs (the OH stretch) to 190 fs (the low‐frequency distortion) and the motion involves (in part) these coordinates. The intramolecular vibrational‐energy redistribution dynamics at longer times are important to the hydrogen‐bond dissociation and to the nonradiative decay of the hydrogen‐transferred form.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.80.-d Chemical analysis and related physical methods of analysis

Multiconfigurational time‐dependent Hartree study of complex dynamics: Photodissociation of NO2

U. Manthe, H.‐D. Meyer, and L. S. Cederbaum

J. Chem. Phys. 97, 9062 (1992); http://dx.doi.org/10.1063/1.463332 (10 pages) | Cited 99 times

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The multiconfigurational time‐dependent Hartree (MCTDH) approach is applied to an example showing very complex dynamics: the wave‐packet dynamics on the three‐dimensional B2 potential‐energy surface of NO2. The ability of the MCTDH scheme to describe accurately the severe splitting of the wave packet on a saddle‐shaped surface is demonstrated. Internal checks of the MCTDH calculation enable us to assess the degree of convergence without the need to resort to a numerically exact wave‐packet calculation. As a representative observable the photodissociation spectrum is calculated and discussed. The A1/B2 vibronic coupling is neglected in our study, but the dynamics on the diabatic B2 surface is treated in its full three dimensionality.  
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31.15.V- Electron correlation calculations for atoms, ions and molecules
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions

Excimer formation dynamics in pure and mixed naphthalene clusters

Hiroyuki Saigusa, Sheng Sun, and Edward C. Lim

J. Chem. Phys. 97, 9072 (1992); http://dx.doi.org/10.1063/1.463333 (9 pages) | Cited 12 times

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The excited‐state dynamics of the pure naphthalene clusters and of the mixed clusters involving two naphthalenes solvated with durene have been investigated as a function of the excess vibrational energy of the initially excited clusters. All of the clusters studied exhibit excimer fluorescence, depending strongly on the excess vibrational energy and on the solvation number. The isomerization rate has been measured by detecting a buildup in the excimer fluorescence and used as a probe of the cluster geometry. The trimer requires an excess energy of 870 cm−1 to promote the excimer formation with a rise time of 32 ns, suggesting a structurally less favorable process. The dimer and tetramer undergo rapid isomerization at much lower energies. The solvent addition is found to impede excimer formation, due to geometry restriction. The singly solvated cluster exhibits an excimer evolution time as slow as 60 ns.
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36.40.-c Atomic and molecular clusters
31.50.Df Potential energy surfaces for excited electronic states
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