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8 Apr 1998

Volume 108, Issue 14, pp. 5653-6048

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An order–disorder phase transition in monolayer CO/LiF(001)

N.-T. Vu and D. B. Jack

J. Chem. Phys. 108, 5653 (1998); http://dx.doi.org/10.1063/1.475974 (4 pages) | Cited 2 times

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Monte Carlo simulations of CO physisorbed on a LiF(001) surface show that a monolayer of CO molecules forms an ordered p(2√×√)R45 herringbone structure which undergoes an order–disorder phase transition around 30 K. The CO molecules sit near the Li+ sites (C atom down) with a tilt of ∼ 40° from the surface normal. © 1998 American Institute of Physics.
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64.70.K- Solid-solid transitions
68.35.Rh Phase transitions and critical phenomena
68.08.-p Liquid-solid interfaces
68.43.-h Chemisorption/physisorption: adsorbates on surfaces

Systematic location of intersecting seams of conical intersection in triatomic molecules: The 1 2A′–2 2A′ conical intersections in BH2

Mark S. Gordon, Vassiliki-Alexandra Glezakou, and David R. Yarkony

J. Chem. Phys. 108, 5657 (1998); http://dx.doi.org/10.1063/1.476318 (3 pages) | Cited 16 times

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Points of conical intersection are continuously connected forming seams. Recently a quite unanticipated situation has been found in which two distinct seams of conical intersection—one symmetry-allowed and one same-symmetry—originating from the same two states intersect each other. The identification of these confluences, based on ab initio electronic wave functions has been somewhat serendipitous. A systematic approach for locating such confluences, based solely on information obtained on the symmetry-allowed portion of the seam, has been suggested. In this work that approach is applied to identify the point where a Cs seam of conical intersection intersects a symmetry-allowed C2v seam of conical intersection for the 1 2A′ and 2 2A′ states of BH2, states that correlate with B(1s22s22p,2P)+H2. It is suggested, based on this and previous work, that this unexpected situation, which has fundamental implications for our understanding of nonadiabatic processes, is not at all uncommon. © 1998 American Institute of Physics.
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31.15.A- Ab initio calculations
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Analytic energy gradients for multiconfigurational self-consistent field second-order quasidegenerate perturbation theory (MC-QDPT)

Haruyuki Nakano, Kimihiko Hirao, and Mark S. Gordon

J. Chem. Phys. 108, 5660 (1998); http://dx.doi.org/10.1063/1.475975 (10 pages) | Cited 15 times

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An analytic energy gradient method for second-order quasidegenerate perturbation theory with multiconfigurational self-consistent field reference functions (MC-QDPT) is derived along the lines of the response function formalism (RFF). According to the RFF, the gradients are calculated without solving coupled perturbed equations. Instead, it is necessary to solve seven sets of linear equations in order to determine Lagrangian multipliers, corresponding to four sets of parameter constraining conditions and three sets of additional parameter defining conditions in the Lagrangian. Just one of these linear equations is a large scale linear equation; the others are reducible to just partial differentiations or simple equations solvable by straightforward subroutines. © 1998 American Institute of Physics.
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31.15.xr Self-consistent-field methods
31.15.xp Perturbation theory
02.30.Jr Partial differential equations
02.60.Lj Ordinary and partial differential equations; boundary value problems

Discrete variational quantum reactive scattering method with optimal distorted waves. I. Theory

Gerrit C. Groenenboom

J. Chem. Phys. 108, 5670 (1998); http://dx.doi.org/10.1063/1.475976 (7 pages) | Cited 1 time

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The size of the interaction region grid in a discrete Kohn variational reactive scattering calculation may be minimized by using distorted waves (DWs) in the trial wave function. Fully converged state-to-state results may be obtained with a small grid if (1) closed channels are included in the coupled channels expansion of the DWs and (2) asymptotically vanishing DWs are included in the trial wave function. This may be done without spoiling the sparsity of the interaction region Hamiltonian, which allows the use of an iterative method for solving the linear equations. We define boundary conditions for the regular, irregular, and asymptotically closed DWs, that minimize the number of DWs needed for convergence. The application to the reaction H+O2→ OH+O, is given in part II. © 1998 American Institute of Physics.
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82.20.Fd Collision theories; trajectory models
02.60.-x Numerical approximation and analysis
02.30.Xx Calculus of variations
02.30.Yy Control theory

Discrete variational quantum reactive scattering method with optimal distorted waves. II. Application to the reaction H+O2→ OH+O

Gerrit C. Groenenboom

J. Chem. Phys. 108, 5677 (1998); http://dx.doi.org/10.1063/1.475977 (6 pages) | Cited 8 times

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The discrete Kohn variational reactive scattering method presented in the preceding paper is applied to the reaction H+O2→ OH+O. The essential features of the method are the use of an interaction region grid, fully coupled open and closed distorted waves (DWs) and the use of a singular value decomposition technique to construct the optimal boundary conditions for the open DWs. A convergence test is presented at a total energy of 0.817 eV above the bottom of the H+O2 well. It is shown that very well converged results may be obtained in a calculation with a relatively small interaction region grid, when at least a few asymptotically closed DWs are included in the trial wave function. Furthermore, the number of open distorted waves (DWs) may be considerably smaller than the number of open channels. Six additional points are computed in an energy range of 1.2 meV, scanning through a narrow resonance around 0.817 eV. The results are in very good agreement with the hyperspherical coordinate propagation calculations by R. T Pack, E. A. Butcher, and G. A. Parker [J. Chem. Phys. 102, 5998 (1995)]. © 1998 American Institute of Physics.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Fd Collision theories; trajectory models
82.20.Kh Potential energy surfaces for chemical reactions
02.30.Xx Calculus of variations
02.30.Yy Control theory
02.10.Ud Linear algebra
02.10.Xm Multilinear algebra

Time correlation functions for mixed quantum-classical systems

J. Liam McWhirter

J. Chem. Phys. 108, 5683 (1998); http://dx.doi.org/10.1063/1.475978 (12 pages) | Cited 8 times

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We consider the time correlation function of observables pertaining to a (quantum subsystem +bath), where the bath is coupled to a reservoir with many degrees of freedom. Integrating over the coordinates of this reservoir and assuming no initial correlations between the (quantum subsystem+bath) and the reservoir, we obtain an expression for the time correlation function that contains an influence functional. We then take the semiclassical and Fokker–Planck limits while modeling the reservoir with an Ohmic continuum of harmonic oscillators coupled bilinearily to the coordinates of the bath. The semiclassical limit is taken using a variant of Pechukas’ stationary phase analysis of the reduced propagator that yields a time correlation function written in terms of connected “classical” paths. These paths are got by solving the concatenation of several short-time interval Pechukas equations; as a result, the determination of these paths is more feasible than the determination of the “classical” path associated with a single long-time interval Pechukas equation. This concatenation includes the dissipative and stochastic forces associated with a classical Brownian particle. We then use decoherence arguments derived from an inspection of the influence functional to eliminate the phase interference structure of the bath. This elimination yields a mixed quantum-classical time correlation function that can be evaluated using nonadiabatic mixed quantum-classical dynamics schemes similar to those proposed recently by Webster and Tully. © 1998 American Institute of Physics.
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03.65.-w Quantum mechanics
03.65.Ge Solutions of wave equations: bound states

Nearside–farside analysis of differential cross sections: Ar+N2 rotationally inelastic scattering using associated Legendre functions of the first and second kinds

P. McCabe, J. N. L. Connor, and D. Sokolovski

J. Chem. Phys. 108, 5695 (1998); http://dx.doi.org/10.1063/1.475979 (9 pages) | Cited 14 times

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We have carried out the first nearside–farside (NF) analysis of angular scattering for molecular collisions in which the partial wave series for the scattering amplitude is expanded in a basis set of associated Legendre functions (of the first kind.) The practical implementation of the NF theory is described, which exploits in an essential way properties of associated Legendre functions of the second kind. The new concept of a restricted nearside–farside (resNF) decomposition of the scattering amplitude is introduced, which takes into account the caustic structure of the associated Legendre functions. The resNF theory is used to analyze polarization and degeneracy averaged differential cross sections for the Ar+N2 collision system, treated as an atom+rigid rotor. The resNF analysis always provides a clear physical interpretation of the scattering (except sometimes for scattering angles ≈ 0,180°) for phenomena such as diffraction oscillations, potential rainbows, and rotational rainbows, as well as more complicated interference effects. © 1998 American Institute of Physics.
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34.50.Ez Rotational and vibrational energy transfer
34.50.-s Scattering of atoms and molecules

Transition states for chemical reactions I. Geometry and classical barrier height

David K. Malick, G. A. Petersson, and John A. Montgomery

J. Chem. Phys. 108, 5704 (1998); http://dx.doi.org/10.1063/1.476317 (10 pages) | Cited 53 times

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A new computational procedure for the characterization of transition states for chemical reactions is proposed and tested. Previous calculations have frequently employed a single point high-level energy calculation at a transition state geometry obtained with a less expensive computational method, Energy[Method(1)]//Geom[Method(2)]. If we instead search the “inexpensive” intrinsic reaction coordinate (IRC) for the maximum of Energy[Method(1)] along this reaction path, the resulting “IRCMax method”, Max{Energy[Method(1)]}//IRC{Geom[Method(2)]}, reduces errors in transition state geometries by a factor of 4 to 5, and reduces errors in classical barrier heights by as much as a factor of 10. When applied to the CBS-4, G2(MP2), G2, CBS-Q, and CBS-QCI/APNO model chemistries, the IRCMax method reduces to the standard model for the reactants and products, and gives rms errors in the classical barrier heights for ten atom exchange reactions of 1.3, 1.2, 1.0, 0.6, and 0.3 kcal/mol, respectively. © 1998 American Institute of Physics.
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82.30.-b Specific chemical reactions; reaction mechanisms
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.90.+s Other topics in the theory of the electronic structure of atoms and molecules (restricted to new topics in section 31)

A stochastic technique for solving the Lorentz–Boltzmann equation for hard spheres: Application to the kinetics of gas absorption

C. P. Lowe and A. J. Masters

J. Chem. Phys. 108, 5714 (1998); http://dx.doi.org/10.1063/1.475980 (9 pages) | Cited 1 time

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The Lorentz–Boltzmann equation for tagged particle motion in a hard sphere fluid may be interpreted as describing the motion of a particle propagating via a series of binary uncorrelated collisions in a structureless bath of fluid particles with a Maxwellian distribution of velocities. We describe a very general stochastic technique for solving the equation. The method can also be extended to the Enskog level, valid up to somewhat higher densities, by a simple scaling of the time. Having reproduced several known results for the Lorentz–Boltzmann equation we extend the method to a simple reaction process where there is no analytic result—the kinetics of gas absorption for a gas confined between two plates. For this process there are two simple analytic limits—the Knudsen limit (in which there are no collisions between absorbing particles) and the diffusive limit (where there are a large number of collisions between absorbing particles). We show that regardless of the Knudsen number, Kn, the Knudsen limit describes the very short time kinetics and the diffusive limit describes the long time kinetics. However, at moderate values of the Knudsen number the rate constant characterizing the long time kinetics differs from the diffusive value. This discrepancy scales away slowly (as 1/Kn) with increasing Knudsen number. © 1998 American Institute of Physics.
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61.20.Gy Theory and models of liquid structure
05.20.Dd Kinetic theory
51.10.+y Kinetic and transport theory of gases
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
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Scattering state spectroscopy of the reaction Mg∗(3s3p 1P1)+CH4→MgH(v = 0,1;N)+CH3

T. H. Wong, C. Freel, P. D. Kleiber, and K. M. Sando

J. Chem. Phys. 108, 5723 (1998); http://dx.doi.org/10.1063/1.475981 (5 pages) | Cited 12 times

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We report scattering state spectroscopic studies of the chemical quenching dynamics of Mg∗(3p(1P)) by CH4. We have measured the final-state resolved action spectra for the MgH(v = 1,N) reactive product channels, following excitation of the Mg∗(3p)–CH4 transient bimolecular collision complex. As in earlier work on the ground vibrational state of the product, we have found a strong electronic orbital alignment effect: Reaction to the vibrationally excited product follows only on the attractive excited potential-energy surfaces in “Π-like” symmetry. For both MgH(v = 0 and 1) product channels we have found that the rotational quantum state distribution is independent of laser excitation wavelength, indicating that the rotational energy partitioning is determined by exit channel dynamics. However, our results show that the product vibrational energy disposal is a function of excitation laser wavelength, suggesting that the vibrational energy partitioning is correlated with the collisional impact parameter. We have also carried out a careful search for the MgCH3 reactive product in this system, finding no evidence for any observable branching to this product. We discuss the implications of these results for the chemical dynamics of this metal-alkane reaction system. © 1998 American Institute of Physics.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Rp State to state energy transfer

On the ground electronic states of copper silicide and its ions

Alexander I. Boldyrev, Jack Simons, J. J. Scherer, J. B. Paul, C. P. Collier, and R. J. Saykally

J. Chem. Phys. 108, 5728 (1998); http://dx.doi.org/10.1063/1.475982 (5 pages) | Cited 8 times

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The low-lying electronic states of SiCu, SiCu+, and SiCu− have been studied using a variety of high-level ab initio techniques. As expected on the basis of simple orbital occupancy and bond forming for Si(s2p2)+Cu(s1) species, 2Πr, 1Σ+, and 3Σ− states were found to be the ground electronic states for SiCu, SiCu+, and SiCu−, respectively; the 2Πr state is not that suggested in most recent experimental studies. All of these molecules were found to be quite strongly bound although the bond lengths, bond energies, and harmonic frequencies vary slightly among them, as a result of the nonbonding character of the 2π-MO (molecular orbital) [composed almost entirely of the Si 3p-AO (atomic orbital)], the occupation of which varies from 0 to 2 within the 1Σ+, 2Πr, and 3Σ− series. The neutral SiCu is found to have bound excited electronic states of 4Σ−, 2Δ, 2Σ+, and 2Πi symmetry lying 0.5, 1.2, 1.8, and 3.2 eV above the 2Πr ground state. It is possible but not yet certain that the 2Πi state is, in fact, the “B state” observed in the recent experimental studies by Scherer, Paul, Collier, and Saykally. © 1998 American Institute of Physics.
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31.15.A- Ab initio calculations
33.15.Dj Interatomic distances and angles

Photoionization spectroscopy of Ga-rare gas complexes

A. Stangassinger, A. M. Knight, and M. A. Duncan

J. Chem. Phys. 108, 5733 (1998); http://dx.doi.org/10.1063/1.475983 (9 pages) | Cited 8 times

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New electronic states, F 2Δ3/2, G 2Δ5/2, H 2Π1/2, and I 2Π3/2, are investigated for the complexes GaAr, GaKr, and GaXe with resonant two-photon photoionization spectroscopy. These excited states correlate to the 2D←2P (4d←4p) atomic transition of gallium. Vibronic structure in these spectra are used to obtain vibrational constants, and extrapolated progressions are used to determine dissociation energies. The upper 2Δ states are more than twice as strongly bound as the corresponding 2Π states. Excited state values of dissociation energies are used in energetic cycles to determine ground-state dissociation energies for GaAr, GaKr, and GaXe. In all three cases, the values obtained are significantly lower than previous estimates. The ground state of GaAr is extremely weakly bound, with D0″ = 20±20 cm−1, while the corresponding value for GaKr is only 35±20 cm−1. The B 2Σ+ excited states of the Ga–RG complexes are confirmed to have substantial barriers in their long-range potentials. © 1998 American Institute of Physics.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
31.50.Df Potential energy surfaces for excited electronic states
33.80.Eh Autoionization, photoionization, and photodetachment
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.15.Fm Bond strengths, dissociation energies
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Photodissociation study of CH3Br in the first continuum

Theodosia Gougousi, Peter C. Samartzis, and Theofanis N. Kitsopoulos

J. Chem. Phys. 108, 5742 (1998); http://dx.doi.org/10.1063/1.475984 (5 pages) | Cited 45 times

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The photolysis of CH3Br is studied in the energy region of the A band between 4.94 and 5.76 eV using ion imaging. Velocity distributions for both the bromine-atom and methyl-radical photofragments are determined. Our results indicate that transitions to the 3Q0 and 3Q1 states dominate the absorption cross section and the partial cross section to each state is determined. The [Br∗]/[Br] branching ratio is found to be strongly dependent on the excitation energy varying between 0.6 and 1.5. Both the bromine-atom and the methyl-radical translational energy distributions suggest that the vibrational distribution in the nascent CH3 is nonstatistical with appreciable excitation in the v2 umbrella mode. The lifetime of the A band is estimated at τ = 120±40 fs. © 1998 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light

193 nm photolysis of H2S in rare-gas matrices: Luminescence spectroscopy of the products

Leonid Khriachtchev, Mika Pettersson, Esa Isoniemi, and Markku Räsänen

J. Chem. Phys. 108, 5747 (1998); http://dx.doi.org/10.1063/1.475985 (8 pages) | Cited 19 times

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The 193 nm photolysis of hydrogen sulfide (H2S) in solid rare gases is studied at 7.5 K. In order to get the most reliable data of the photolysis process, Fourier transform (FT) infrared and time-resolved luminescence methods are used in the same experiment. The 193 nm photolysis of H2S in Ar and Kr matrices was found to be very similar to the gas phase. A kinetic scheme of H2S photolysis, which is consistent with all the experimental features, was constructed. The major channel is formation of (H+SH) pairs, which are stabilized in the matrix. Then SH radicals decompose to (S+H) pairs, providing the main source for S atoms. No experimental evidence of a cage-induced reaction H+SH→S+H2 was observed in our study, which can be connected with high probability for hydrogen-atom exit from the parent cage, and/or with high probability of the recombination reaction H+SH→H2S. The available spectroscopic information for S atoms and SH radicals in Ar and Kr matrices is further specified, and new spectroscopic data on the photolysis products in Ne and Xe matrices are reported. In particular, the luminescence data on SH radicals in solid rare-gas matrices (Ne, Ar, Kr, and Xe) were found to resemble the tendencies known for OH radicals. Also, the infrared absorptions of SH radicals in Ar and Kr matrices were identified to be at 2607 and 2594 cm−1, respectively, and a novel rare-gas molecule HXeSH with the Xe–H stretch at 1119 cm−1 was detected. © 1998 American Institute of Physics.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
33.50.Dq Fluorescence and phosphorescence spectra
33.20.Ea Infrared spectra
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment

Study of the total and partial fragmentation dynamics of Ar–HCl after uv photodissociation

A. García-Vela

J. Chem. Phys. 108, 5755 (1998); http://dx.doi.org/10.1063/1.475986 (12 pages) | Cited 23 times

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The uv photolysis of the Ar–HCl cluster is studied applying an exact time-dependent wave packet method in three dimensions, assuming zero-total angular momentum. The photodissociation process is found to occur via two different fragmentation mechanisms, depending on the initial excitation energy of the cluster. One mechanism leads to total dissociation of the complex, producing three fragments, Ar–HCl+hν→H+Ar+Cl. The fragmentation dynamics in this case is governed by resonance states at relatively low energies of the cluster, in which the H atom collides a number of times with Ar and Cl before dissociating. Manifestations of these collisions are found in the final kinetic energy distribution of the photofragments, which is redshifted in the case of the H fragment, and blueshifted in the Ar and Cl cases. The second type of mechanism consists of a fast and direct photodissociation of the hydrogen, leading to a partial fragmentation of Ar–HCl into hot H fragments and bound Ar–Cl radical molecules. This mechanism dominates at higher energies, which are those mostly populated by the wave packet initially prepared in the present calculations. The experimental implications of the results are discussed. © 1998 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
82.20.Hf Product distribution

Ultracold photoassociative spectroscopy of heteronuclear alkali-metal diatomic molecules

He Wang and William C. Stwalley

J. Chem. Phys. 108, 5767 (1998); http://dx.doi.org/10.1063/1.475987 (5 pages) | Cited 72 times

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We have examined and identified the excited long-range Hund’s case (c) molecular states of the ten heteronuclear alkali metal diatomic molecules which support bound states and can be probed by ultracold photoassociative spectroscopy. Analytical expressions for the heteronuclear long-range free-bound Franck–Condon factors as a function of the internuclear distance R and the vibrational quantum number v are derived and discussed. The KRb system has considerably stronger excited long-range interactions than any other heteronuclear alkali diatomic molecule and has very favorable Franck–Condon factors for the photoassociation process. The heteronuclear photoassociative spectroscopy will provide spectroscopic measurements of the binary elastic scattering lengths of heteronuclear cold collisions which are the crucial parameters for sympathetic cooling of mixed atomic gases. © 1998 American Institute of Physics.
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37.10.Mn Slowing and cooling of molecules
37.10.Pq Trapping of molecules
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Effect of geometrical conformation on the short-time photodissociation dynamics of 1-iodopropane in the A-band absorption

Xuming Zheng and David Lee Phillips

J. Chem. Phys. 108, 5772 (1998); http://dx.doi.org/10.1063/1.475988 (12 pages) | Cited 15 times

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We have taken resonance Raman spectra and made absolute Raman cross section measurements at six excitation wavelengths for 1-iodopropane. The resonance Raman spectra have most of their Raman intensity in features that may be assigned as fundamentals, overtones, and combination bands of three Franck–Condon active vibrational modes (the nominal C–I stretch, the nominal CCC bend, and the nominal CCI bend) for the trans and gauche conformations of 1-iodopropane. The resonance Raman and absorption cross sections of the trans and gauche conformations of 1-iodopropane were simulated using a simple model and time-dependent wave packet calculations. The results of the simulations were used in conjunction with the vibrational normal-mode coefficients to find the short-time photodissociation dynamics of trans and gauche conformers of 1-iodopropane in terms of internal coordinate changes. The trans and gauche conformers display significantly different Franck–Condon region photodissociation dynamics, which indicates that the C–I bond breaking is conformational dependent. In particular, there are large differences in the trans and gauche short-time photodissociation dynamics for the torsional motion (xGBx) about the GB carbon–carbon bond and the GBC angle (where C = α-carbon atom attached to the iodine atom, B = β-carbon atom attached to the α-carbon atom, G = methyl group carbon atom attached to the β-carbon atom). The major differences in the trans and gauche A-band short-time photodissociation dynamics can be mostly explained by the position of the C–I bond in the trans and gauche conformers relative to the plane of the three carbon atoms of the n-propyl group of 1-iodopropane. © 1998 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
33.20.Fb Raman and Rayleigh spectra (including optical scattering)
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.70.Fd Absolute and relative line and band intensities
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Photodissociation of acrylonitrile at 193 nm: A photofragment translational spectroscopy study using synchrotron radiation for product photoionization

David A. Blank, Arthur G. Suits, Yuan T. Lee, Simon W. North, and Gregory E. Hall

J. Chem. Phys. 108, 5784 (1998); http://dx.doi.org/10.1063/1.475989 (11 pages) | Cited 10 times

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We have investigated the photodissociation of acrylonitrile (H2CCHCN) at 193 nm using the technique of photofragment translational spectroscopy. The experiments were performed at the Chemical Dynamics Beamline at the Advanced Light Source and used tunable vacuum ultraviolet synchrotron radiation for product photoionization. We have identified four primary dissociation channels including atomic and molecular hydrogen elimination, HCN elimination, and CN elimination. There is significant evidence that all of the dissociation channels occur on the ground electronic surface following internal conversion from the initially optically prepared state. The product translational energy distributions reflect near statistical simple bond rupture for the radical dissociation channels, while substantial recombination barriers mediate the translational energy release for the two molecular elimination channels. Photoionization onsets have provided additional insight into the chemical identities of the products and their internal energy content. © 1998 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
41.60.Ap Synchrotron radiation
33.80.Eh Autoionization, photoionization, and photodetachment
31.90.+s Other topics in the theory of the electronic structure of atoms and molecules (restricted to new topics in section 31)
82.20.Kh Potential energy surfaces for chemical reactions

Calculated paramagnetic hyperfine structure of pentagonal bipyramid Ag7 cluster

Ramiro Arratia-Pérez, Lucía Hernández-Acevedo, and Luis Alvarez-Thon

J. Chem. Phys. 108, 5795 (1998); http://dx.doi.org/10.1063/1.475990 (4 pages) | Cited 15 times

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Symmetry-adapted angular momentum basis functions have been generated for the D5h∗ molecular double point group to obtain the self-consistent Dirac cluster wave function Φ and the Dirac cluster orbitals. Once Φ is obtained, we proceed throughout a relativistic first-order perturbation procedure to calculate the magnetic hyperfine tensors of the Ag7 cluster. The calculated spin distribution and magnetic hyperfine tensors fully support the ESR assignment made by Weltner et al. of a cluster composed of seven silver atoms with a pentagonal bipyramid structure. The single unpaired electron spin spend 40.3% of its time on each axial silver atom. © 1998 American Institute of Physics.
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36.40.-c Atomic and molecular clusters
31.30.Gs Hyperfine interactions and isotope effects
33.15.Pw Fine and hyperfine structure
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions

The ultrafast photodissociation of Fe(CO)5 in the gas phase

L. Bañares, T. Baumert, M. Bergt, B. Kiefer, and G. Gerber

J. Chem. Phys. 108, 5799 (1998); http://dx.doi.org/10.1063/1.475991 (13 pages) | Cited 30 times

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The photodissociation dynamics of Fe(CO)5 in a molecular beam have been investigated with femtosecond time resolution. In single pulse experiments, the parent ion Fe(CO)5+ and all the fragment ions Fe(CO)n+, n = 0–4 could be observed in linear and reflectron time-of-flight (TOF) spectrometers. Ladder switching is suppressed by the use of femtosecond laser pulses. The TOF spectra show that the fragmentation patterns strongly depend on the laser wavelength, the laser intensity, and the laser pulse duration. Femtosecond pump–probe experiments were performed for the parent and every fragment molecule. We present a photodissociation model for the neutral Fe(CO)5. After the absorption of two 400 nm photons, Fe(CO)5 looses four CO ligands in about 100 fs. The subsequent dissociation of the fragment Fe(CO) takes place on a longer time scale of about 230 fs. The measured transient ionization spectra of the Fe(CO)n, n = 2–4 fragments represent within the proposed model the fingerprints of the evolution of the [Fe(CO)5]‡ transition state on the way to dissociation. We also report on the observation of a metastable ionic fragmentation mechanism. © 1998 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment
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Multidimensional femtosecond spectroscopies of vibrational motions in liquids: Semiclassical expansion

V. Chernyak and S. Mukamel

J. Chem. Phys. 108, 5812 (1998); http://dx.doi.org/10.1063/1.475992 (14 pages) | Cited 59 times

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Fifth- (χ(5)) and seventh- (χ(7)) order electronically off-resonant Raman spectroscopies in molecular liquids are investigated using a new semiclassical expansion of the optical response which applies for weak anharmonicities and low temperatures. The leading contribution can be calculated using classical equations of motion for nuclear wave packets, even when the system itself may be highly nonclassical. Two sources of nonlinearities which generate the signals—the nonlinear dependence of the polarizability on nuclear coordinates and vibrational anharmonicities—are identified. Formal analogy between the present equations and the time-dependent Hartree–Fock equations used in electronic nonlinear spectroscopy suggests specific experimental signatures of the various nonlinearities. © 1998 American Institute of Physics.
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07.57.Ty Infrared spectrometers, auxiliary equipment, and techniques
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment

Phase diagram of argon clusters

A. Rytkönen, S. Valkealahti, and M. Manninen

J. Chem. Phys. 108, 5826 (1998); http://dx.doi.org/10.1063/1.475993 (8 pages) | Cited 18 times

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Systems containing both argon cluster with number of atoms N = 100, 200, or 400 and argon vapor were studied with constant energy molecular dynamics simulations. The vapor pressure versus temperature phase diagram including the melting temperatures was determined as a function of cluster particle number. The melting temperature of the cluster was determined with a new method based on the nearest-neighbor exchange of atoms, and it was found to approach the bulk melting temperature linearly as a function of N−1/3. The vapor pressure versus temperature curve approached the corresponding bulk curve as the particle number of the cluster increased. At temperatures lower than the melting temperatures the clusters were found to contain an icosahedral core. © 1998 American Institute of Physics.
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36.40.Ei Phase transitions in clusters
61.20.Ja Computer simulation of liquid structure
64.70.D- Solid-liquid transitions

Remarks on the information entropy maximization method and extended thermodynamics

Byung Chan Eu

J. Chem. Phys. 108, 5834 (1998); http://dx.doi.org/10.1063/1.475994 (11 pages) | Cited 10 times

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The information entropy maximization method was applied by Jou et al. [J. Phys. A 17, 2799 (1984)] to heat conduction in the past. Advancing this method one more step, Nettleton [J. Chem. Phys. 106, 10311 (1997)] combined the method with a projection operator technique to derive a set of evolution equations for macroscopic variables from the Liouville equation for a simple liquid, and a claim was made that the method provides a statistical mechanical theory basis of irreversible processes and, in particular, of extended thermodynamics which is consistent with the laws of thermodynamics. This line of information entropy maximization method is analyzed from the viewpoint of the laws of thermodynamics in this paper. © 1998 American Institute of Physics.
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05.70.Ce Thermodynamic functions and equations of state
05.70.Ln Nonequilibrium and irreversible thermodynamics

Reorientational tunneling of partially deuterated methyl groups: A single-crystal deuteron NMR study of aspirin-CH2D

A. Detken and H. Zimmermann

J. Chem. Phys. 108, 5845 (1998); http://dx.doi.org/10.1063/1.475995 (10 pages) | Cited 16 times

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Partially deuterated methyl groups in single crystals of aspirin are investigated by deuteron NMR at temperatures between 8 K and room temperature. The CH2D groups perform reorientations which are governed by a rotational potential with three wells, two of which are almost equally deep whereas the third is significantly deeper. At temperatures below 20 K, a so far unobserved type of incoherent tunneling process is identified. This process consists in reorientations between the two upper potential wells which are fast on the time scale of the quadrupolar interaction, whereas transitions into the deeper well are slow on this time scale. At temperatures above 35 K, the methyl groups perform thermally activated stochastic reorientations between all three potential wells. By determining the relative populations of the three wells as a function of temperature, the energy difference between the lower and the two upper wells is found to be 3.3 meV. This amounts to almost 8% of the average barrier height, which is determined from the temperature dependence of the spin-lattice relaxation time to be 43 meV. © 1998 American Institute of Physics.  
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76.60.Es Relaxation effects
61.50.Lt Crystal binding; cohesive energy

An in situ Raman spectroscopy study of subcritical and supercritical water: The peculiarity of hydrogen bonding near the critical point

Yutaka Ikushima, Kiyotaka Hatakeda, Norio Saito, and Masahiko Arai

J. Chem. Phys. 108, 5855 (1998); http://dx.doi.org/10.1063/1.475996 (6 pages) | Cited 63 times

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The Raman spectra of water are measured at temperatures up to 510 °C and at pressures up to 40 MPa. The peak frequency increases with temperature, indicating the break of hydrogen bonding, and it changes only slightly at higher temperatures above the critical point. The peak frequency has a maximum near the critical pressure, and the extent of hydrogen bonding significantly changes with pressure in this near-critical region. The deviation of the maximum frequency Δf relative to that of the monomer is in good agreement with the chemical shifts with the literature NMR data. The extent of the hydrogen bonding can be estimated from the Δf values. The Δf values at the near-critical points are significantly lower compared with those in super- or sub-critical conditions, and the strength of the hydrogen bonding weakens uniquely in the near-critical region. © 1998 American Institute of Physics.
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78.30.C- Liquids
33.20.Fb Raman and Rayleigh spectra (including optical scattering)
33.15.Fm Bond strengths, dissociation energies
64.70.-p Specific phase transitions
76.60.Cq Chemical and Knight shifts
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