bluuebunny/crossref_metadata_2025 huggingface dataset:
Comment on ‘‘Two isomers of SF•5 and SF+5: Structures and energetics’’ [J. Chem. Phys. 100, 1759 (1994)]
Authors: Cheung Yu-San , Li Wai-Kee , Chiu See-Wing , Ng C. Y.
Published: July 2002

Using the same level of ab initio quantum chemical theory as that applied by Becker et al. [J. Chem. Phys. 100, 1759 (1994)], we have reexamined the structures of SF5 and SF+5. Contrary to their report, we find that the SF5/(D3h) and SF+5(C4v) structures are not local minima. The adiabatic ionization energies (IE) for SF4 and SF5, and the heats of formation at 0 K (ΔHf00) for SF4, SF+4, SF5, SF+5, and SF6 have also been calculated using the G2(MP2) procedure [Curtiss et al., J. Chem. Phys. 98, 1293 (1993)]. Excellent agreement is found between the theoretical G2(MP2) and accepted experimental IEs for SF4 and SF5. The theoretical ΔHf00’s for SF4, SF+4 and SF6 are also in excellent accord with the literature values. However, the G2(MP2) calculation indicates that the current recommended experimental ΔHf00(SF5) may be too low by about 10 kcal/mol.
A combined experimental and theoretical study of the neutral, cationic, and anionic Si3N cluster molecule
Authors: Goldberg Norman , Iraqi Muhammad , Schwarz Helmut , Boldyrev Alexander , Simons Jack
Published: July 2002

Collision experiments and high-level ab initio calculations are used for the first characterization of the neutral, cationic, and anionic Si3N cluster molecules. The experimentally observed fragmentation processes are well described by calculated reaction energies of all dissociation channels. In addition, the geometries and harmonic frequencies of SinN (n=1–3) have been calculated for the cationic, anionic, and neutral species.
Vibrational distributions of KrF(B) and XeCl(B) produced from ionic-recombination reactions of Kr+(2P3/2)+SF−6 and Xe+(2P3/2)+Cl−+He
Authors: Tsuji Masaharu , Muraoka Toshihiko , Ide Masahiro , Ujita Hiroki , Nishimura Yukio , Obase Hiroshi
Published: July 2002

Nascent vibrational distributions of KrF(B) and XeCl(B) resulting from Kr+(2P3/2)+SF−6 and Xe+(2P3/2)+Cl−+He reactions have been determined by a spectral simulation of the bound–free B–X transition. The vibrational distribution of KrF(B) has a maximum at v′=0 and decreases exponentially with increasing v′. It is in reasonable agreement with the statistical prior distribution in which all vibrational and rotational degrees of freedom in the products are taken into consideration. It is therefore concluded that the Kr+(2P3/2)+SF−6 reaction proceeds through a long-lived [Kr+(2P3/2)SF−6] intermediate. The vibrational distribution of XeCl(B) decreases linearly with increasing v′. It is more vibrationally excited than the prior distribution, indicating that a (Xe+Cl−He) intermediate decomposes before a complete energy randomization. The average fractions of total available energy deposited into vibrational energy of the excimer 〈fv〉 are estimated to be 5.8% for the Kr+(2P3/2)+SF−6 reaction and 16% for the Xe+(2P3/2)+Cl−+He reaction. On the basis of the conservation of total angular momentum during the reactions, one reason for the low 〈fv〉 values is attributable to a very large impact parameter in the entrance channels, which requires a significant transfer of potential energy into rotational energy of the excimer and kinetic energy of the products. The other reason is a significant transfer of potential energy into kinetic energy because of the acceleration of the positive and negative ions due to a strong Coulombic attractive force.
Millimeter-wave rotational spectrum of H2SiO
Authors: Bailleux Stephane , Bogey Marcel , Demuynck Claire , Destombes Jean-Luc , Walters Adam
Published: July 2002

The first high resolution rotational spectrum of H2SiO in the gas phase has been obtained. This molecule was identified among several short-lived unknown species produced by a low power ‘‘abnormal’’ glow discharge in a mixture of SiH4, O2, and Ar. On/off modulation of the discharge allowed lines belonging to each species to be characterized by their lifetime measured using a digitizing oscilloscope. In total 107 lines of H2SiO were measured between 140 and 470 GHz. Evidence for a possible large amplitude motion has been found from anomalies in the fit of close lying Ka doublets.
Spectroscopy of a chromophore coupled to a lattice of dynamic two-level systems. II. Spectral diffusion kernel
Authors: Reilly Philip D. , Skinner J. L.
Published: July 2002

We consider the spectral diffusion of a chromophore coupled through dipolar interactions to a regular lattice of flipping two-level systems. In particular, we calculate the spectral diffusion kernel, P(ω,t‖ω0), which is the conditional probability density that the chromophore will have transition frequency ω at time t, given that it had frequency ω0 at time 0. At very short times we find that the spectral diffusion kernel is Lorentzian, for any value of the two-level system excitation probability, p. For longer times the form of the spectral diffusion kernel depends on the value of p. We derive several approximate expressions for the spectral diffusion kernel, all of which go to the correct equilibrium distribution of frequencies for long times. For p≂1/2, when the frequency distribution is nearly Gaussian, we find that the spectral diffusion kernel is not at all Gaussian for short times. We compare all of our approximate expressions with numerically exact results. Motivated by certain optical spectral diffusion experiments on individual molecules in solids, we also calculate P(Δ;t), the distribution of spectral jumps, which is the probability density that the chromophore’s frequency will change by an amount Δ in time t. In a subsequent paper we will analyze these spectral diffusion experiments with our results.
Neumann expansion of the interelectronic distance function for integer powers. I. General formula
Authors: Budziński J. , Prajsnar S.
Published: July 2002

In this paper the generalized Neumann expansion of the interelectronic distance function r12k with integer values of the exponent k, in the interval k≥−1, is presented. The problem is formulated in the prolate spherical coordinates and the uniform expression for the expansion in the form of the series of products of the powers of ξ1 and ξ2 variables and the certain auxiliary function Hl(m)(ξ<,ξ≳) is obtained. The function Hl(m)(ξ<,ξ≳) is expressed by means of the Legendre polynomials of the first and the second kind in the case of odd number k and is constant, when k is even.
Theoretical study of rotational-echo double-resonance and related experiments
Authors: Li Yan , Evans Jeremy N. S.
Published: July 2002

This paper discusses the theory of the rotational-echo double-resonance (REDOR) and related experiments by using a different approach than that used by previous researchers. The theory has been used to discuss the REDOR, rotational-echo double-resonance total sidebands suppression (REDORTOSS), and interrupted REDOR experiments. In the case of REDORTOSS experiment, an analytical solution has been derived which ensures that the quantification of the dipolar coupling constant and therefore the measurement of the distance between two anisotropic heteroatoms can be made. An alternative REDOR method for situations in which anisotropy is a problem is also proposed, called the interrupted REDOR.
Excitonic interaction in the fluorene dimer
Authors: Wessel John , Beck Steven , Highstrete Clark
Published: July 2002

The fluorene van der Waals dimer exhibits a complex origin spectrum. This region has been studied by resonance two-photon ionization and by fluorescence excitation spectroscopies. The spectra can be interpreted on the basis of intermediate strength exciton coupling, in which the electronic interaction is comparable to the van der Waals vibrational energies. The spectra are reasonably well described by two distorted adiabatic potential surfaces, which correspond to the two excitonic components of the origin system. A single Franck–Condon active intermolecular mode provides a reasonable description of the system, however the potentials have significant cubic and quartic contributions. Non-Born–Oppenheimer nuclear momentum coupling is present and intermodal (IVR) interactions are observed, even for intermolecular modes as low as v=1. The results are remarkably different from prior observations of excitonic structure in other systems, providing a detailed picture of coupling between electronic and intermolecular motion in a van der Waals dimer.
Determination of the spectroscopic constants of a dark vibrational state: Fermi and Coriolis perturbations in the ν2 band of formyl chloride
Authors: Joo D. L. , Clouthier Dennis J. , Merer A. J.
Published: July 2002

The ν2 infrared fundamental of the transient molecule formyl chloride, HCOCl, has been recorded at high resolution. Detailed assignments of the rotational structure have been made for both HCO 35Cl and HCO 37Cl. The ν2 band, whose origin is at 1784.1377 cm−1 in HCO 35Cl, is predominantly A type with a very weak B-type component. Numerous excited state perturbations occur throughout the band, caused mainly by ‖ΔKa‖=2 Fermi resonance and ‖ΔKa‖=1 Coriolis interactions with the 3151 vibrational state. Even though no ‘‘extra’’ lines resulting from the perturbations could be assigned, a complete analysis of the perturbed rotational structure has been possible. Accurate molecular constants for both the ‘‘bright’’ perturbed 21 state and the ‘‘dark’’ 3151 perturbing state have been obtained, using a Hamiltonian which includes both the Fermi and Coriolis interactions; only two interaction parameters are needed, one for each mechanism. Further perturbations at high Ka are assigned to Coriolis interactions with the 5261 level.
Theoretical investigation of the nonlinear optical properties of oligomers of polythienylenemethylidene, a low band-gap material
Authors: Adant C. , Beljonne D. , Brédas J. L.
Published: July 2002

We present a theoretical investigation by the sum-over-states formalism of the static first- and third-order polarizabilities of thienylenemethylidene oligomers; following the incorporation of a single conjugated carbon between the rings, these compounds present a partially quinoid geometry. We use a multireference determinant single–double configuration interaction approach to calculate excited-state energies, state dipole moments, and transition dipole moments which are introduced in the sum-over-states expressions. All the results are discussed in comparison to those obtained in thiophene oligomers of similar sizes; they indicate that the third-order polarizabilities are smaller for the methine-bridged compounds than for the corresponding thiophene oligomers. This feature is rationalized by analyzing the main optical (virtual excitation) channels that contribute to the nonlinear response.
The formulation of quantum statistical mechanics based on the Feynman path centroid density. V. Quantum instantaneous normal mode theory of liquids
Authors: Cao Jianshu , Voth Gregory A.
Published: July 2002

The concept of instantaneous normal modes in liquids is extended into the quantum regime using the Feynman path centroid perspective in quantum statistical mechanics. To accomplish this goal, the variational quadratic approximation for the effective centroid potential is recast in a general multidimensional phase space form. In the context of the effective quadratic approximation, the velocity autocorrelation functions of liquids can then be predicted based on a set of instantaneous quantum normal modes. Representative applications are presented for quantum Lennard-Jones liquids and a quantum particle solvated in a classical fluid. The quantum effective phonon spectrum leads to some revealing observations and interpretations for these systems.
Laser double-resonance study of the collisional removal of O2 (A 3Σ+u, ν=6, 7, and 9) with O2, N2, CO2, Ar, and He
Authors: Knutsen Karen , Dyer Mark J. , Copeland Richard A.
Published: July 2002

The collisional removal of O2 molecules prepared in selected vibrational levels of the A 3Σ+u state is studied using a two-laser double-resonance technique. The output of the first laser excites the O2 to A 3Σ+u, ν=6, 7, or 9, and the ultraviolet output of the second laser monitors these levels via resonance-enhanced ionization through either the ν=5 level of the C 3Πg Rydberg state, or the valence state or states tentatively associated with the 5 3Πg state. The temporal evolution of the A 3Σ+u state vibrational level is observed by scanning the time delay between the two pulsed lasers. Collisional removal rate constants are obtained for A 3Σ+u, ν=7 and 9 colliding with O2, N2, CO2, Ar, and He; and for ν=6 colliding with O2 and N2. We find the collisional removal of the A 3Σ+u state to be fast (k≥10−11 cm3 s−1) for all colliders studied. The rate constants vary by about an order of magnitude from the fastest collisional deactivator, CO2 to the slowest studied, the rare gases Ar and He. The rate constants for the atmospherically important colliders O2 and N2 are similar in magnitude and suggest that N2 collisions will dominate the removal rate in the Earth’s atmosphere.
Electronic excitation of silane (SiH4) by low-energy electron impact
Authors: Winstead Carl , Pritchard Howard P. , McKoy Vincent
Published: July 2002

We report results of a study of the electronic excitation of silane by low-energy electron impact, including cross sections from 10 to 40 eV impact energy for excitation of the first triplet and singlet excited states (2t2→4sa1)1,3T2. Our results are calculated using the Schwinger multichannel method as implemented for massively parallel computers and include a correction for high-impact-parameter excitation of the optically allowed 1T2 transition. We also report values of the total dissociation cross section derived from our calculations and total scattering measurements, and we compare these with direct measurements of the dissociation cross section.
Charge transfer induced local Frenkel exciton states in naphthalene crystals heavily doped with durene
Authors: Brovchenko Ivan V. , Eilmes Andrzej , Petelenz Piotr
Published: July 2002

Absorption and luminescence spectra of durene-doped naphthalene crystals are reported for different dopant concentrations. New impurity-induced absorption bands are interpreted as charge transfer induced local Frenkel exciton states. The results of theoretical calculations, performed (without invoking any adjustable parameters) for a series of finite model clusters are in very good agreement with experiment and support this interpretation.
A detailed study of conformations in the ground state of CH+4
Authors: Kella Dror , Vager Zeev
Published: July 2002

The results of Coulomb explosion imaging for cold CH+4 molecular ions are converted to the molecular conformation probability density. This is the first complete conversion of such data for a relatively complex molecule. The results are compared with the corresponding predicted potential energy surfaces which manifest a Jahn–Teller symmetry breaking. The density of conformations along a nuclear rearrangement path is deduced and the comparison with the theory is satisfactory in almost every respect, except for the density near the minimum of the theoretical potential
Polyexponential kinetics of chemical reactions in condensed media within the quasiclassical approximation
Authors: Basilevsky M. V. , Davidovitch G. V.
Published: July 2002

Polyexponential kinetical behavior typical for condensed phase reactions in highly viscous media is studied on a simple example of one-dimensional diffusion equation with a sink modeling a chemical conversion of reactants. The corresponding polyexponential regime is demonstrated to have a thorough analogy with the quasiclassical approximation of one-dimensional quantum mechanics and a relevant approximation for the Green’s function is developed. The asymptotic short- and long-time kinetics are examined at the analytical level. Contrary to the frozen medium approximation according to which the slow diffusion motion of the medium is entirely ignored, the present quasiclassical model is fit for a qualitative description of the total time interval covering the reaction events from the initial moment up to the ultimate steady-state monoexponential evolution. The range of validity of the quasiclassical approach is discussed. Numerical tests expose some peculiarities of the present treatment for equilibrium and nonequilibrium initial distributions. The work presents a qualitative development of the theory of nonexponential kinetics pioneered by papers of Agmon and Hopfield, Sumi and Marcus, and Nadler and Marcus.
Second order many-body perturbation approximations to the coupled cluster Green’s function
Authors: Nooijen Marcel , Snijders Jaap G.
Published: July 2002

The time-consuming step in coupled cluster Green’s function or equivalently equation of motion coupled cluster calculations of ionization potentials is the solution of the CCSD equations. We investigate here the accuracy that can be obtained if the CCSD coefficients are replaced by their MBPT(2) analogs. We discuss some additional diagonal approximations that might prove especially useful in polymer calculations, and compare with traditional Green’s function calculations based on a second order approximation to the irreducible self-energy.
The structure and internal dynamics of CO–CO–H2O determined by microwave spectroscopy
Authors: Peterson K. I. , Suenram R. D. , Lovas F. J.
Published: July 2002

The rotational spectra of CO–CO–H2O, CO–CO–HDO, 13CO–CO–H2O, and 13CO–13CO–H2O have been measured using a pulsed-molecular-beam Fabry–Perot Fourier-transform microwave spectrometer. The complex exhibits internal motion involving an exchange of the CO subunits as well as an hydrogen exchange. In the normal species this is indicated in the spectrum by transition doublets separated by a few hundred kHz and an effective shift of alternating transitions which prevents a good semirigid rotor fit. The other isotopically substituted complexes have spectra in which the transitions are either singlet, doublet or quartets depending on the appropriate spin weights or because of dampening of the internal motion. All the spectra are mutually consistent with a tunneling path with four isoenergetic states. By treating the tunneling frequency of the CO interchange as a vibrational frequency, the rotational constants of two internal rotor states and a tunneling frequency could be determined. The tunneling frequency in CO–CO–H2O is 372 kHz and the ground state rotational constants are A=4294.683(70) MHz, B=1685.399(35) MHz, C=1205.532(35) MHz. The tunneling frequency corresponding to the hydrogen exchange is not determined but the observed transition splittings are comparable to those found for other van der Waals complexes containing a water subunit. The dipole moments determined for CO–CO–HDO are μa=4.790(87)×10−30 C m [1.436(26) D], μb=1.79(12)×10−30 C m [0.533(35) D], and μc=1.10(37)×10−30 C m [0.33(11) D]. The general structure of the complex is found to be cyclic. The CO–CO configuration is approximately T-shaped with the carbon atom of one subunit directed toward the molecular axis of the other subunit. The H2O subunit has a hydrogen atom directed toward the CO subunits but not in the expected linear hydrogen bonded configuration. The uncertainties given in parentheses are one standard deviation.
Vibronic coupling in the X̃ 2Π and Ã 2Σ+ states of HCN+
Authors: Wiedmann Ralph T. , White Michael G.
Published: July 2002

The vibronic structure associated with the X̃ 2Π and Ã 2Σ+ states of HCN+ have been examined by high resolution threshold photoelectron spectroscopy using coherent vacuum ultraviolet radiation and pulsed field ionization techniques. Rotationally resolved spectra for 12 vibronic bands are presented from which vibronic symmetry assignments and cation spectroscopic constants are derived. The latter are compared to the ab initio calculations of Köppel et al. [Chem. Phys. 37, 303 (1979)] who calculated the vibronic structure of HCN+ based on strong X̃–Ã interstate coupling. Finally, the observed rotational branch structure is used to infer the ionization dynamics of strongly coupled vibronic levels for which the Born–Oppenheimer approximation is not valid.
Torsion-inversion coupling in the S1 state of acetaldehyde
Authors: Weersink Robert A. , Cramb David T. , Wallace Stephen C. , Gordon Robert D.
Published: July 2002

Coupling between the methyl torsion (υ15) and inversion (υ14) modes is used to model the singlet π*←n spectrum of jet-cooled acetaldehyde and its deuterated analogs. The anomalies observed in the two torsion progressions built onto the lowest inversion doublet are accurately reproduced by using a potential of the form 1/2V3[1−cos3(τ−kq)], where k indicates the proportionality of interaction between the torsion and inversion motions. The coupling also results in intensity to torsion–inversion combination levels that, in the absence of coupling, would not be observed. Consequently, many of the observed transitions are simultaneously Franck–Condon allowed and vibronically induced. The weak origin transition and long progression in the methyl torsion mode is also reproduced in the intensity calculation, indicating that the equilibrium position of the methyl rotor has changed from the S0 eclipsed position to a nearly staggered geometry in the S1 state. The calculated potential energy surface and Franck–Condon intensity calculations predict an S1 equilibrium geometry in which the methyl rotor is staggered with respect to the CO bond by 54° and where the acetyl hydrogen is bent out of the CCO plane by 35°.
Ab initio investigation of internal rotation in the ethylene–sulfur dioxide dimer
Authors: Resende Stella M. , De Almeida Wagner B.
Published: July 2002

The phenomenon of tunneling motion in the C2H4...SO2 dimer has been studied in the ab initio Hartree–Fock (HF) and Mo/ller–Plesset second-order perturbation theory (MP2) levels, employing the DZP basis set. Basis set superposition errors (BSSE) were accounted for using the counterpoise correction method. A simple model for treating the problem in one dimension was utilized, where coupling of intramolecular vibration and dimer internal rotation have been neglected. A comparative study between two potentials was carried out. The first potential is based on pointwise ab initio calculations of the intermolecular potential energy surface (PES) including BSSE correction, and the second is based on the expression VN=V0 [1−cos(Nα)]/2, where V0 was obtained from the ab initio fully optimized equilibrium and transition state structures. The resultant Hamiltonian was solved using the variational method, and the calculated splitting of transition frequencies compared with experimental data.
Quantum ‘‘melting’’ of orientationally ordered physisorbates
Authors: Marx D. , Nielaba P.
Published: July 2002

Based on path-integral Monte Carlo simulations we investigate systematically the influence of quantum fluctuations on a model for orientational ordering. The quantum anisotropic-planar-rotor Hamiltonian with its herringbone ground state serves as our model for strictly two-dimensional, orientationally ordered linear molecules. We find several distinct regimes as the strength of the quantum fluctuations is increased: (i) a regime with renormalized transition temperature and ground state order parameter which describes systems like N2 physisorbed on graphite; (ii) crossover behavior with residual ground state order but an increased order parameter at higher temperatures; (iii) a regime characterized by vanishing ground state order but significant order in some range of higher temperatures; and (iv) a regime with orientational disorder at all temperatures. This sequence is interpreted in terms of reentrant rotational quantum melting. In addition we probe the range of validity of simple approximation schemes such as the quasiharmonic and the quadratic Feynman–Hibbs effective potential approximations. Both methods fail to describe the full scenario for large quantum fluctuations even on a qualitative level.
Potential functions for carbon dioxide–hydrogen halide and hydrogen halide dimer van der Waals complexes
Authors: Muenter J. S.
Published: July 2002

A potential function model containing distributed repulsion, dispersion, and electrostatic interactions is applied to hydrogen halide dimers and carbon dioxide–hydrogen halide complexes. The (HX)2 functions are compatible with all experimental data and are compared with both empirical and large scale ab initio potential surfaces. The main focus of this paper is the CO2–HX group. We obtain excellent results for CO2–HF and CO2–HCl, describing essentially linear molecules undergoing large amplitude vibrations that closely resemble rigid bender behavior. The calculated CO2–HBr equilibrium structure is not in agreement with vibrationally averaged experimental results. This may arise from vibrational averaging effects. It is shown that different views of the same potential function can give very different impressions of structure and internal motions.
Inhibition of fluorescence emission in a donor molecule upon interaction with an excited acceptor
Authors: Sedarous S. S. , Kirk W. R.
Published: July 2002

We present results of a strong laser field interacting with a two-center system. The two centers consist of a donor and acceptor pair which are coupled by Fœrster–Dexter resonance energy transfer. The donor molecule is the indole chromophore in indolylethylenediaminetetraacetic acid (indolyl-EDTA) which chelates the lanthanide ion acceptor holmium (Ho3+). The changes in the donor lifetime consequent upon excitation of the acceptor were examined. Excitation of the lanthanide acceptor with a second laser field resulted in the appearance of a substantially increased lifetime component. This additional (new) lifetime component is on the order of a second, compared with the original nanosecond lifetime of the donor in its complex with the acceptor. This new component is not attributable to the spontaneous lifetime of the acceptor. Rather, we propose that the large lifetime is due to the interaction of the donor with the excited state acceptor.
State-specific unimolecular dissociation dynamics of HFCO. II. CO rotational distribution and Doppler widths
Authors: Choi Young S. , Moore C. Bradley
Published: July 2002

Rovibrational state distributions and Doppler widths of CO fragments formed from unimolecular dissociation of HFCO in its ground electronic state are measured by vacuum ultraviolet laser-induced fluorescence, following state-selective preparation of the molecule in a single quantum state in the energy region of 2000 to 3000 cm−1 above the dissociation threshold by stimulated emission pumping. CO fragments are rotationally hot and distributed over J≤15 to J=63 with distributions peaking at J=45 to 50 depending upon the initial HFCO dissociative state. Although CO rotational distributions are significantly different for different initial states, about 20% of the total available energy is released on average as rotational energy of CO for all three initial states studied. The yield of CO(v=1) fragments is determined to be about 10% and CO(v≥2) fragments are not observed. The average Doppler width of CO fragments is 0.85 cm−1, which indicates that ∼50% of the total available energy is released as translation. The CO product state distributions and Doppler widths may be rationalized using a modified impulsive model with the ab initio transition state geometry. The dependence of product state distributions on the initial HFCO quantum state may reflect incomplete intramolecular vibrational energy redistribution.
Classical description of nonadiabatic photoisomerization processes and their real-time detection via femtosecond spectroscopy
Authors: Stock Gerhard
Published: July 2002

A classical-path approach to the description of photoinduced isomerization dynamics as well as the interrelated electronic and vibrational relaxation processes is outlined. Adopting a three-mode model of photoisomerization that has been recently proposed by Seidner and Domcke (Chem. Phys. 186, 27 (1994)), we perform detailed numerical studies and compare the results of the classical simulations to available exact quantum-mechanical results. It is shown that the classical model reproduces semiquantitatively time-dependent diabatic and adiabatic electronic population probabilities, state-specific torsional wave functions, and energy contents of vibrational degrees of freedom. Furthermore it is demonstrated that the classical approach is able to simulate at least qualitatively time- and frequency-resolved pump-probe spectra of these processes. In accordance with exact quantum calculations, the classical simulations reveal the decay of the stimulated emission of the reactants and the delayed onset of the absorption of the photoproducts. To demonstrate the capability of the classical approach, the three-mode model of Seidner et al. is augmented by a hundred weakly-coupled harmonic modes. This allows to roughly simulate the relaxation dynamics of a chromophore interacting with a solvent. The simulations reveal that the time evolution of the full system within the first few hundred femtoseconds is quite similar to the case of the bare three-mode model. For later times, however, the dynamics of the three-mode model becomes quasistationary, whereas the calculations for the full system reflect the redistribution of the excess energy of the reaction mode into the bath nuclear degrees of freedom. It is found that the quantum yield of the cis-trans photoreaction depends to a large extent on the specific chromophore-solvent coupling employed, as it governs directly the competition of the various relaxation pathways. Simulations of the corresponding time- and frequency-resolved pump-probe spectra reveal that the cooling of the vibrationally hot photoproducts in the solvent is mainly reflected in a blue shift and a narrowing of the width of the absorption spectrum.
Response to ‘‘Comment on ‘Negative velocity correlation in hard sphere fluid’ ’’ [J. Chem. Phys. 103, 9512 (1995)]
Authors: Burshtein A. I.
Published: July 2002

Two different approaches to the velocity relaxation in dense fluid are compared in the limiting case common for both and the relationship between their major parameters is found.
Resonance Raman spectrum and excitation profile of mass-selected zirconium trimers
Authors: Haouari Hanae , Wang Huaiming , Craig Robert , Lombardi John R. , Lindsay D. M.
Published: July 2002

We present the resonance Raman and Raman excitation profile of mass-selected zirconium trimers in argon matrices. In the Raman spectra, two fundamentals and one overtone are observed. Average Raman shifts, along with standard deviations (in parentheses) are 176.7 (13) cm−1 (ν2), 258.0 (12) cm−1 (ν1), and 516.1 (8) cm−1 (2ν1). The ratio of the frequencies of the two lowest lines (ν1/ν2) is 1.46, which is very close to √2. This is indicative of a symmetrical equilateral geometry (D3h). In such a case we expect two normal frequencies, one for a totally symmetric stretch of symmetry a′1 (ν1) and a doubly degenerate bend of symmetry e′(ν2). The Raman excitation profiles of the ν1(a′1) line shows two broad maxima: one near 491 nm and the other near 614 nm. The ν2 (e′) profile shows a broad region of intensity only near 614 nm. With the aid of theory we assign the 614 nm band to be 1A′1−1E′ (x,y polarized) while the 491 nm band must be 1A′1−1A″2 (z polarized).
CO chemisorption on TiO2(110): Oxygen vacancy site influence on CO adsorption
Authors: Linsebigler Amy , Lu Guangquan , Yates John T.
Published: July 2002

CO chemisorption has been studied on TiO2(110) under surface conditions where oxygen anion vacancy sites are not present (oxidized surface), compared to conditions where the vacancy sites are present (annealed surface). The binding energy of CO on the nondefective TiO2(110) surface is 9.9 kcal/mole in the limit of zero coverage. CO...CO repulsive interactions have been observed at higher coverages. When anion vacancy sites are produced under controlled annealing conditions in vacuum at 900 K, a significant increase in the desorption temperature of a portion of the chemisorbed CO is observed. This observation, coupled with measurements showing that defective TiO2(110) does not have enhanced CO chemisorption capacity, suggests that CO adsorbs more strongly on lattice Ti sites in the vicinity of anion vacancy sites. It is postulated that enhanced CO bonding occurs via the interaction of the O moiety of CO with the anion vacancy site while primary adsorbate bonding occurs via the C moiety to Ti lattice sites. Neither CO2 production nor oxygen exchange in CO occurs when CO desorbs from defective TiO2(110).
Atom–atom structure factors of hydrogen halides: A molecular approach revisited
Authors: Alvarez M. , Lomba E. , Martín C. , Lombardero M.
Published: July 2002

The aim of this paper is to explore the possibilities of a novel molecular integral equation approach to produce information on the atom–atom microscopic structure of molecular fluids via multidimensional integration of the molecular pair distribution function. In particular, atom–atom structure factors for hydrogen halides (HCl and HI) are computed from the integral equation for heteronuclear fluids modeled by a two-center Lennard-Jones potential with and without multipole terms. Theoretical results are compared both with experimental partial structure factors and computer simulation results. Theory and simulation agree remarkably well both for thermodynamics and microscopic structure. The comparison with experimental partial structure factors is satisfactory within the limitations due to the rough modeling used for describing the real fluid.
A semiclassical approach to dissipation in quantum mechanics
Authors: Grossmann Frank
Published: July 2002

Employing the path integral formalism, we study a quantum (test) system coupled to an environment consisting of infinitely many harmonic oscillators. A simple semiclassical approxi- mation, in which only real classical trajectories are required, is used to derive the propagator of the reduced density matrix for Ohmic dissipation and high temperatures. The inclusion of a summation over trajectories with the correct Maslov phases in the final expression for the propagator permits investigation of nonharmonic test systems. In a numerical example for a Morse oscillator interacting with a high temperature heat bath, the semiclassical result, obtained by the method described here, correctly displays the suppression of quantum behavior as observed in the exact calculation.
Excitonic interactions and excimer formation in pure and mixed cluster isotopomers of naphthalene
Authors: Saigusa Hiroyuki , Lim E. C.
Published: July 2002

The nature of electronic excitation and subsequent excited-state dynamics in small naphthalene clusters has been elucidated using isotope labeling techniques. The S1←S0 spectra of tetramer isotopomers are characterized by structurally inequivalent site splitting which is induced by an exciton interaction in the S2 state through vibronic coupling. It is suggested that the tetramer involves a pair of nearly overlapped chromophores, which is responsible for the excitonic interaction, and other two chromophores acting as solvents. An excitonic state originating in this pair is proposed to correlate with the lowest excimer state of the cluster as the interplanar separation and dihedral angle are reduced. In contrast, the spectra of trimer isotopomers are analyzed by invoking a symmetric geometry in which three chromophores are less overlapped and thus give rise to weak excitonic effects. The isomerization dynamics of these clusters is discussed in terms of a vibrationally activated process which makes the cluster framework wobble, allowing for stronger exciton interaction.
High resolution infrared molecular beam spectroscopy of cyanoacetylene clusters
Authors: Yang X. , Kerstel E. R. Th. , Scoles G. , Bemish R. J. , Miller R. E.
Published: July 2002

High resolution infrared optothermal spectroscopy has been used to study small clusters of cyanoacetylene, (HCCCN)n, containing three or more monomer units. For the linear HCCCN trimer the fundamental of the free C–H stretch vibration has been rotationally resolved and analyzed, yielding a ground vibrational state rotational constant (B0) of 94.031(13) MHz. In addition, an inner C–H stretch fundamental band of the linear trimer (rotationally unresolved) has been identified by observing the effect of large electric fields on the band shape. This assignment is also consistent with spectral intensity measurements as a function of molecular beam stagnation pressure. Predissociation lifetimes of the upper states of these two bands were determined from the observed homogeneous linewidths. Several other rotationally unresolved vibrational bands have also been observed. The stagnation pressure and electric field dependence of these spectra are used to estimate the cluster sizes and to make tentative structural assignments.
Theoretical study of the effect of reagent rotation on the reaction of F+H2(ν=0,J)
Authors: Song Ju-Beom , Gislason Eric A.
Published: July 2002

Quasiclassical calculations on the Muckerman V potential energy surface were carried out on the reaction F+H2(ν=0, J)→FH+H at a relative energy of 2 kcal/mol for J=0 to 10. This surface is, to use the classification of Levine and co-workers, very oblate; for a given distance Rc.m. from F to the center of mass of H2 the potential energy is much lower for the collinear configuration (χ=0) than for the perpendicular configuration (χ=90 deg). The goal of the work was to understand the effect of molecular rotation on such an extremely oblate surface. It proved useful to decompose the reactive cross section QR(J) into the product of a hitting cross section Q≠hit(J) for F hitting H2 times the probability PR(J) of reaction occurring once F hits H2. Both Q≠hit(J) and PR(J) go through minima at J≊4–5. We determined that Q≠hit(J=0) is increased by about a factor of 2 by ‘‘reorientation’’ of the H2 molecule towards a linear configuration by the F atom as it approaches. For J≳0 Q≠hit(J) declines due both to loss of this reorientation effect as well as to the more oblique approach of the trajectory to the reactant valley. Many trajectories bounce off the repulsive wall near χ=90 deg before the F atom can hit H2; this effect has been discussed by other authors. The initial decline of PR(J) with J is due to a relatively unusual feature of the potential surface, whereby rotation of the H2 molecule away from a linear F–H–H configuration can easily switch the system from the product region back to the reactant region of the system. Both Q≠hit(J) and PR(J) increase above J=5 because the H2 molecule now has enough rotational energy to rotate through the barrier at χ=90 deg rather than bounce off it.
Comparison of quantum and semiclassical variational transition state models for the HO2→H+O2 microcanonical rate constant
Authors: Song Kihyung , Peslherbe Gilles H. , Hase William L. , Dobbyn Abigail J. , Stumpf Michael , Schinke Reinhard
Published: July 2002

Comparisons, for J=0, are made between HO2→H+O2 reaction path anharmonic energy levels, variational transition states, and unimolecular rate constants determined by three different semiclassical models and a quantum mechanical model. The semiclassical models are based on the reaction path Hamiltonian. However, to determine anharmonic energy levels, the harmonic potential of this Hamiltonian is replaced by the actual anharmonic DMBE IV potential for the HO2 system. Two of the semiclassical models use Einstein–Brillouin–Keller (EBK) quantization to determine energy levels for motion orthogonal to the reaction path; i.e., one model neglects anharmonic coupling between modes, while the other retains all the coupling. The third semiclassical model is based on a quartic expansion of the potential and second-order perturbation theory to determine the energy levels. A comparison of the results of these three semiclassical models shows that anharmonic coupling between modes orthogonal to the reaction path is unimportant for HO2 dissociation. The separable EBK model gives a RRKM rate constant versus energy in very good agreement with that obtained from a quantum mechanical calculation which retains full coupling between modes in determining the reaction path energy levels. If anharmonicity is treated, the reaction path Hamiltonian and its vibrator transition state give accurate RRKM rate constants for HO2 dissociation. Rate constants calculated with the flexible transition states model are in very good agreement with those of the semiclassical and quantum vibrator transition state models, if the O2 stretch conserved mode is treated as an anharmonic oscillator in the flexible model. However, in contrast to the vibrator transition state models, ‘‘steps’’ are not observed in the rate constants for the flexible model, since the transitional mode is treated classically. Harmonic and anharmonic rate constants are compared for both the vibrator and flexible transition state models.
Reaction dynamics of O(1D) with HCN(0 0 v3)
Authors: Kreher Christoph , Theinl Robert , Gericke K.-H.
Published: July 2002

The quantum state resolved reaction dynamics of HCN(0 00 v3) with O(1D) atoms were investigated by analyzing the complete product state distributions of OH(X 2ΠΩ,v,J) and CN(X 2Σ+,v,J) using laser induced fluorescence (LIF). The influence of the CH-stretching mode on the reaction dynamics and different branching ratios was inspected by exciting HCN to its first overtone band of the ν3 CH stretch in the 1.5 μm region. The oxygen atom in the 1D state was generated in a laser photolysis of ozone at a wavelength of 266 nm. The measured rotational and vibrational distributions of the products were compared with statistical results from phase space theory (PST). Nearly statistical rotational and vibrational distributions are obtained for CN(X 2Σ+) in v=0–3. The rotational and vibrational distributions of OH(X 2ΠΩ) are colder than statistically expected. Insertion of O into the CN bond with subsequent hydrogen migration seems to be a better characterization of the the reaction mechanism than an insertion of the oxygen atom into the CH bond. Direct abstraction of hydrogen to form OH is improbable to describe the molecular process.
Anharmonic contributions to the inversion vibration in 2-aminopyrimidine
Authors: McCarthy W. J. , Lapinski L. , Nowak M. J. , Adamowicz L.
Published: July 2002

The out-of-plane vibrations of the amino group in 2-aminopyrimidine have large amplitudes, and cannot be properly described within the harmonic approximation. The normal mode analysis carried out at this level of approximation at the restricted Hartree–Fock level and at the second-order Mo/ller–Plesset perturbation theory level failed to match the experimental transition frequency of ν≊200 cm−1 of the inversion vibration in this compound. In an effort to better understand this vibration motion, we went beyond the harmonic approximation. The inversion vibration was treated as being uncoupled from all other nuclear degrees of freedom. An internal coordinate (ω) was chosen whose displacement mimicked the out-of-plane distortion of the amino group during the inversion vibration. Electronic energy was calculated at the second-order Mo/ller–Plesset perturbation theory level at selected values of ω to form a double-well curve describing a model potential within which the nuclei move during the vibration. This potential was incorporated into a one-dimensional Hamiltonian, and vibrational energy expectation values were variationally determined by utilizing the harmonic wavefunctions as the basis set. Two sets of calculations were performed: one in which the mirror plane of symmetry was preserved throughout the vibrational deformation limiting the internal coordinates to 17, and another in which the symmetry was unconstrained permitting description by 3N−6=30 internal coordinates. These calculations resulted in prediction of the v=0→v=1 transition energy of ν=130.1 cm−1 and ν=206.7 cm−1, respectively, reasonably matching the experimental value of ≊200 cm−1.
Inhomogeneous rotational dynamics of a rodlike probe in 1-propanol
Authors: Imeshev G. , Khundkar L. R.
Published: July 2002

The rotational correlation time of the dipolar probe p,p′-cyano thiomethyl diphenylacetylene varies linearly with the emission Stokes’ shift in 1-propanol. This effect is not seen in nonpolar (pentane, hexadecane) or ‘‘fast’’ polar (methanol, acetonitrile) solvents. We ascribe these observations to dielectric friction in the different local environments experienced by the probe. These inhomogeneities persist on time scales much longer than the solvation time.
Quantum decoherence in mixed quantum-classical systems: Nonadiabatic processes
Authors: Bittner Eric R. , Rossky Peter J.
Published: July 2002

We address the issue of quantum decoherence in mixed quantum-classical simulations. We demonstrate that restricting the classical paths to a single path among all the quantum paths affects a coarse graining of the quantum paths. Such coarse graining causes the quantum paths to lose coherence as the various possible classical paths associated with each quantum state diverge. This defines a reduction mapping of the quantum density matrix, and we derive a quantum master equation suitable for mixed quantum-classical systems. The equation includes two terms: first, the ordinary quantum Liouvillian which is parametrized by a single classical path, and second, a quantum decoherence term that includes both a coherence time and length scale which are determined by the dynamics of the classical paths. Model calculations for electronic coherence loss in nonadiabatic mixed quantum-classical dynamics are presented as examples. For a model charge transfer chemical reaction with nonadiabatic transitions, application of the present formulation reveals that nonadiabaticity is diminished as the decoherence timescale becomes shorter and adiabatic dynamics are recovered in the limit of rapid decoherence.
Structure and bonding in the formamide crystal: A complete fourth-order many-body perturbation theoretical study
Authors: Suhai Sándor
Published: July 2002

Structural and energetic properties of hydrogen bonded infinite chains and of two-dimensional infinite periodic networks of formamide molecules were investigated by the ab initio crystal orbital method using several, partly highly polarized, atomic basis sets of increasing size at the Hartree–Fock (HF) level and by including electron correlation effects in different orders of Mo/ller–Plesset (MP) perturbation theory, up to the complete MP4 level. For comparison, calculations at the same theoretical levels were also performed for the monomer and for dimers in three different configurations (linear, zig–zag, and cyclic). Besides full structural optimizations, the intermolecular interaction energies were corrected for basis set superposition errors taking into account monomer relaxation effects as well. The results show that hydrogen bonding in the formamide crystal is a highly cooperative phenomenon, both from the structural and energetic points of view. The lengths of the hydrogen bonds, RHB, will be reduced by 0.12–0.18 Å in the crystalline environment, as compared with dimers, both for HF and MP theories and the binding energies will be increased typically by 50%–60%. Electron correlation effects substantially influence the structural features (reducing, e.g., RHB by about 0.08–0.10 Å) and contribute 15%–20% to the cohesion energy. The theoretical model explains why the RHB values for open-chain dimers become shorter in the crystal than those obtained for the cyclic ones (as opposed to free dimers), correctly predicts changes of bond lengths in going from the monomer to the crystal, and provides N–H...O bond lengths and lattice constants very close to experiments.