bluuebunny/crossref_metadata_2025 huggingface dataset:
Symmetry-adapted perturbation theory of three-body nonadditivity in the Ar2HF trimer
Authors: Lotrich Victor F. , Jankowski Piotr , Szalewicz Krzysztof
Published: July 2002

Symmetry-adapted perturbation theory (SAPT) has been used to analyze the radial and angular dependence of the nonadditivity of the Ar2HF trimer interaction energy through fourth order. This represents the first application of the high-order SAPT to a nonadditive interaction including a polar molecule. The magnitude and anisotropy of the Hartree-Fock nonadditivity is well reproduced (to within 20%) by the sum of the first-order exchange and exchange-quenched third-order induction nonadditivities. The second-order induction effects play a smaller role. The computed SAPT corrections which contribute to the second-order supermolecular many-body perturbation theory (MBPT2) nonadditivity, Eexch-disp(2;0)[3,3] and Eind-disp(3;0)[3,3], reproduce MBPT2 values rather poorly. Using the pseudo-dimer approach it was found that the exchange quenching of the third-order induction-dispersion energy is strong. Inclusion of this quenching led to good agreement with the MBPT2 nonadditivity. The third-order MBPT nonadditivity was very well reproduced by the third-order dispersion energy. The fourth-order MBPT nonadditivity was only moderately well reproduced by the SAPT components Edisp(3;1)[3,3] and Edisp(4;0)[3,3], indicating that these terms are most likely appreciably quenched by exchange counterparts. The total nonadditivities computed using SAPT and the supermolecular method through fourth order agree remarkably well. The total SAPT nonadditivity is expressed in terms of physically interpretable components which can be easily modeled.
Total differential cross sections for Ar–CH4 from an ab initio potential
Authors: Heijmen Tino G. A. , Moszynski Robert , Wormer Paul E. S. , van der Avoird Ad , Buck Udo , Steinbach Christof , Hutson Jeremy M.
Published: July 2002

Total differential cross sections for the Ar–CH4 scattering complex at ECM=90.1 meV were obtained from converged close-coupling calculations based on a recent ab initio potential computed by symmetry-adapted perturbation theory (SAPT). Agreement with experiment is good, which demonstrates the accuracy of the SAPT potential.
Nonequilibrium intramolecular transport in isomerization dynamics of Ar7-like molecules in liquidlike phase
Authors: Seko Chihiro , Takatsuka Kazuo
Published: July 2002

In isomerization dynamics of Ar7-like molecules of high energy, which is in the so-called liquidlike phase, a peculiar characteristic has been observed [K. Takatsuka and C. Seko, J. Chem. Phys. 105, 10356 (1996)], that is, the occurrence of a given geometric isomerization in a short lifetime is less frequent than expected by an exponential distribution based on the mixing in dynamics. This behavior is exactly the reverse to those observed in the simpler system such as dissociation reaction of H3+ [M. Berblinger and C. Schlier, J. Chem. Phys. 101, 4750 (1994)], in which many of the so-called direct paths are ejected before the mixing takes effect and thereby the short lifetime isomerization (or dissociation) occurs more frequently than the exponential distribution. The former fact implies that the classical trajectories take somewhat longer time (induction time) to find their ways out to the other isomers in phase space, and therefore it can be a prototype of the so-called slow dynamics that is frequently observed in large and complicated molecular systems. The present paper discusses a possible mechanism to describe the present induction phenomenon. We first show a numerical fact that an ensemble of trajectories turns into a stage very quickly that can be regarded as a diffusion process getting out of a potential basin, if projected onto a one-dimensional configuration space. Thus, a natural idea arises that the induction time should be a consequence for the group of trajectories to be transported to the reaction regions, or transition regions, with a limited speed. In contrast, the standard statistical theories assume that the population in a transition region that is lost to the product side is to be supplied instantaneously from the reactant region. We present a simple diffusion model to examine the above idea. It has been found that the frequencies of isomerization can be reproduced in a good quantitative level by the estimate in terms of the first passage time based on the calculated diffusion coefficients and related quantities. The remarkable uniformity of the average passage-times (lifetimes) that was previously found by us is also described well in this simple model.
Quasiclassical trajectory simulations of collisional vibrationally excited HgBr(B 2Σ). II. Dependence on rotational excitation
Authors: Velardez G. F. , Bollati R. A. , Ferrero J. C.
Published: July 2002

The collisional deactivation of HgBr(B 2Σ) by different inert gases has been studied using quasiclassical trajectory calculations, with initial vibrational energy Evib=6452 cm−1, at different initial rotational energies in the range 0–6452 cm−1 and a temperature of 415 K for the translational energy. The effect of rotational energy on vibrational, rotational, and translational energy transfer was examined in terms of 〈ΔE〉 and 〈ΔE2〉1/2 for the inert gases (He, Ne, Ar, Kr, and Xe). The influence of mass of the collider and the interaction potential was analyzed computing trajectories using pseudo-isotopes of He and Xe. Collisional transition probabilities for vibrational, rotational, and translational degrees of freedom were obtained as a function on rotational energy. The computed transition probabilities became broader as the mass of collider and rotational energy increases and show a double exponential behavior for all gases.
Equilibrium order-to-disorder transition temperature in block copolymers
Authors: Floudas G. , Pakula T. , Velis G. , Sioula S. , Hadjichristidis N.
Published: July 2002

The process of order-to-disorder transition (ODT) has been studied in a symmetric diblock copolymer AB and in an asymmetric miktoarm star block copolymer of the AB3 type. We find that the TODT depends on the ordering temperature just as the melting temperature in semicrystalline homopolymers depends on the crystallization temperature. In this work we show, for the first time, the existence of an equilibrium transition temperature (TODT0) and we demonstrate how this can be obtained. The dependence of the apparent TODT′ on the supercooling is attributed to grains of different sizes.
Spontaneous emission spectra from microdroplets
Authors: Holler Stephen , Goddard Noël L. , Arnold Stephen
Published: July 2002

The emission spectrum from a fluorescent microdroplet is compared with theory for the first time. The spectrum of oriented surfactant molecules on a levitated microdroplet is found to be in good agreement with semiclassical theory of the interaction of whispering gallery modes with excited molecules. The orientation of these surface entities is estimated through this comparison.
Computer simulations of self-avoiding polymerized membranes
Authors: Drovetsky B. Y. , Chu J. C. , Mak C. H.
Published: July 2002

In this letter we propose a hybrid molecular dynamics/Monte Carlo (MD/MC) method to simulate the statistical behavior of tethered and fluid membranes which does not suffer from severe slowing-down problems as in conventional MC and MD methods. A parallel version of the algorithm allows for reliable determination of the scaling behavior of open and closed membranes.
Excitation energy transport and conformational-librational motion in chains
Authors: Pálszegi T. , Mollay B. , Kauffmann H. F.
Published: July 2002

Incoherent electronic excitation transport (EET) along chromophore-sites has been studied in the presence of conformational and librational motion by using a rotor chain model as the mobile vehicle. The time-evolution of the electronic site excitation probability vector Pexc(t) averaged by conformational-librational dynamic disorder has been calculated by a second-order cumulant expansion (CE) approach. A master equation for conformer distributions in torsional space has been used [G. J. Moro, J. Chem. Phys. 94, 8577 (1991) and J. Chem. Phys. 97, 5749 (1992)] to describe the coupled, motional stochastic dynamics of a rotor chain. Based upon this model, the correlation functions of excitation transfer rates, important for the calculation of the second cumulants of the stochastic excitation transfer rate matrices, have been constructed. A simplified procedure for evaluating the correlation functions for the L steep minima of torsional angle-dependent symmetrical potentials of nearest-neighbor rotor-pairs and for a Gaussian distribution of torsional angles displacement has been carried out. By using analytical fits to the second cumulants, the evolution of excitation energy transfer has been calculated in terms of the averaged site probabilities 〈Pexc(t)〉 for chromophores, substituted to the L-fold symmetric rotors. The calculated profiles show a pronounced dependence on the curvature ratio in the saddle point of the rotor pair potential, on the number of the pair potential minima as well as on the average of the mean-square deviation of torsional angles from the stable conformations.
Shear deformation of two interpenetrating polymer brushes: Stochastic dynamics simulation
Authors: Neelov I. M. , Borisov O. V. , Binder K.
Published: July 2002

The interaction between two polymer brushes (layers of polymer chains grafted at one end onto an impermeable surface) brought into a close contact and moving with respect to each other in the lateral direction is studied by the method of stochastic dynamics simulation. The structural properties of the brushes such as the monomer density profile, the distribution of the free ends of the chains, as well as the disjoining pressure have been studied as functions of the separation between grafting planes (the degree of static overlapping of brushes) and of the lateral displacement rate. We have found that at zero shear rate the extension of grafted chains perpendicular to the grafting planes monotonically decreases with decreasing distance between the planes, but not as rapidly so that significant overlapping and interpenetration of brushes occurs. At strong compression the free ends of the chains are distributed all over the gap between grafting planes. An increase in the shear rate is accompanied by inclination and extension of chains in the direction of shear and by a simultaneous decrease in the chain dimensions in the direction perpendicular to the grafting planes: the free ends are localized near the middle plane and progressive decoupling of brushes occurs. At large shear rate the width of the interpenetration zone almost vanishes. The inclination of chains at given shear rate increases with decreasing grafting density and decreasing width of the gap between the grafting planes. The disjoining pressure exhibits a weak maximum as a function of the shear rate that is in qualitative agreement with the predictions of scaling theories.
Quantum Monte Carlo with model potentials for molecules
Authors: Yoshida Takashi , Miyako Gensho
Published: July 2002

The diffusion quantum Monte Carlo (QMC) method simulating the Green’s function to quadratic accuracy combined with ab initio core model potentials (MP) is applied to molecular calculations. The dissociation energies of the CO, HCl, Na2, and K2 molecules are estimated. The results are found to be in very good agreement with the corresponding experimental values.
Raman and IR spectroscopy of (GeO)n, with n=1,2,3,4, isolated in solid argon
Authors: Zumbusch Andreas , Schnöckel Hansgeorg
Published: July 2002

Raman and IR spectroscopy of GeOn, with n=1,2,3,4, isolated in solid Ar were used for the elucidation of the corresponding molecular structures. The experiments were complemented by extensive ab initio calculations at different theoretical levels, in the case of Ge2O2 additionally by normal-coordinate analysis. The assignment of a planar, cyclic D2h symmetric structure to Ge2O2, made formerly on the basis of IR spectroscopic investigations, was confirmed. It was possible to determine all vibrational frequencies of the molecule. The IR and Raman spectra of Ge3O3 allow for the unambiguous assignment of a highly symmetric D3h ring structure for this molecule, as it has been proposed by other authors. In contrast we find that the previously assumed ring structure for Ge4O4 is incompatible with the measured IR and Raman spectra, as well as with the results of ab initio calculations. Instead a heterocubanelike structure, that is found for the heavier homologes molecules of Sn and Pb, is adopted in this case.
Vibrational overtones in the electronic ground state of the benzene-Ar complex: A combined experimental and theoretical analysis
Authors: Neuhauser R. , Braun J. , Neusser H. J. , van der Avoird A.
Published: July 2002

Using the recently developed method of coherent ion dip spectroscopy (CIS), we were able to measure rotationally resolved spectra of high lying overtones of the intermolecular van der Waals vibrations of the benzene-Ar complex. Optimized pulse sequences for the CIS experiment were obtained by numerical calculations of the population dynamics in the coherently excited few-level system. Due to the high resolution achieved in this experiment, the small isotope shifts upon deuteration of the benzene molecule could be measured and compared with the simple classical harmonic oscillator and with anharmonic three-dimensional quantum calculations. By comparing the latter calculations with the results of the CIS experiments, we tested the quality of several benzene-Ar interaction potentials.
Two-pulse photon echoes from zinc-meso-tetraphenylporphine/polymethylmethacrylate are not consistent with the tunneling two-level system model
Authors: Geva Eitan , Skinner J. L.
Published: July 2002

We argue that the results of a recent two-pulse photon echo study below 2 K on the system zinc-meso-tetraphenylporphine/polymethylmethacrylate [S. J. Zilker and D. Haarer, J. Chem. Phys. 105, 10648 (1996)] are inconsistent with the tunneling two-level system model of low temperature glasses. More specifically, the experimental echo decay is exponential in time, and the decay rates are linear in temperature, while the theoretical predictions from the model are nonexponential in time and nonlinear in temperature, for the parameters that appear to be appropriate for this system.
An ab initio path integral Monte Carlo simulation method for molecules and clusters: Application to Li4 and Li5+
Authors: Weht Ruben O. , Kohanoff Jorge , Estrin Darı́o A. , Chakravarty Charusita
Published: July 2002

A novel method for simulating the statistical mechanics of molecular systems in which both nuclear and electronic degrees of freedom are treated quantum mechanically is presented. The scheme combines a path integral description of the nuclear variables with a first-principles adiabatic description of the electronic structure. The electronic problem is solved for the ground state within a density functional approach, with the electronic orbitals expanded in a localized (Gaussian) basis set. The discretized path integral is computed by a METROPOLIS Monte Carlo sampling technique on the normal modes of the isomorphic ring polymer. An effective short-time action correct to order τ4 is used. The validity and performance of the method are tested by studying two small lithium clusters, namely Li4 and Li5+. Structural and electronic properties computed within this fully quantum-mechanical scheme are presented and compared to those obtained within the classical nuclei approximation. Quantum delocalization effects turn out to be significant as shown by the fact that quantum simulation results at 50 K approximately correspond to those of classical simulations carried out at 150 K. The scaling factor depends, however, on the specific physical property, thus evidencing the different character of quantum and thermal correlations. Tunneling turns out to be irrelevant in the temperature range investigated (50–200 K).
Electron correlation and relativistic contributions to dipole moments of heavy oxides and sulfides: SnO, PbO, SnS, and PbS
Authors: Kellö Vladimir , Sadlej Andrzej J. , Fægri Knut
Published: July 2002

In order to resolve some puzzling discrepancies between theoretical and experimental dipole moment data for oxides of the Group IVa metals a series of nonrelativistic and relativistic high-level-correlated calculations have been performed for SnO and PbO. The same methods have been used to calculate the dipole moments of SnS and PbS for which the earlier results were found to be in reasonable agreement with experiment. The main attention of the present study has been focused on the treatment of the electron correlation contribution to dipole moment of highly ionic oxides. This contribution has been found to be strongly method-dependent. A comparison of different single and multiconfiguration methods shows that the coupled cluster approach with iterative treatment of single and double excitations and perturbative correction for the contribution of triple excitations, CCSD(T), offers presumably the most reliable scheme for the evaluation of the electron correlation contribution to dipole moments of the studied molecules. We have also verified the reliability of the earlier approximate (mass-velocity and Darwin terms) treatment of relativistic contributions by using the scalar Douglas–Kroll approximation. The PbO molecule has also been studied at the level of the Dirac–Hartree–Fock approximation with the results revealing a large spin-orbit contribution to its dipole moment. The present most accurate results, calculated by using the scalar Douglas–Kroll scheme and the CCSD(T) approximation for the electron correlation contribution, are found to give the oxide dipole moments systematically too low by about 0.1 a.u. In the case of the PbO molecule they are also compared with recent results obtained in the framework of the pseudopotential formalism.
“Supercollision” energy dependence: State-resolved energy transfer in collisions between highly vibrationally excited pyrazine (Evib=37 900 cm−1 and 40 900 cm−1) and CO2
Authors: Wall Mark C. , Mullin Amy S.
Published: July 2002

The quenching of highly vibrationally excited pyrazine through collisions with CO2 is investigated as a function of initial pyrazine internal energy using a high-resolution laser transient absorption spectrometer. Experiments focus on energy exchanging collisions that result in excitation of rotations and translations in the ground vibrationless (0000) state of CO2. Highly vibrationally excited pyrazine (Evib=37 900 cm−1 or Evib=41 000 cm−1) is prepared via pulsed excitation at 266 nm or 246 nm, followed by rapid radiationless decay to the ground electronic state. The nascent CO2 rotational populations are measured by collecting the transient absorption of individual rovibrational lines at short times following the pyrazine excitation. The translational energies of CO2 recoiling from hot pyrazine are measured for numerous individual rotational levels. Energy dependent rate constants and probabilities are reported for both donor energies and results are compared with earlier studies using 248 nm excitation. These experiments reveal that for both donor energies, significant rotational and translational excitation of CO2 results from collisions with highly vibrationally excited pyrazine, as evidenced by the similarity in the observed rotational and translational distributions. Remarkably, however, the probabilities for the individual energy transfer pathways increase by as much as a factor of 3 for a 7% change in the pyrazine internal energy. The magnitudes and probabilities of energy transfer are described in terms of an energy transfer distribution function for the different donor molecule energies and implications for sequential quenching collisions are discussed.
Harmonic inversion of time cross-correlation functions: The optimal way to perform quantum or semiclassical dynamics calculations
Authors: Mandelshtam Vladimir A.
Published: July 2002

We explore two new applications of the filter-diagonalization method (FDM) for harmonic inversion of time cross-correlation functions arising in various contexts in molecular dynamics calculations. We show that the Chebyshev cross-correlation functions ciα(n)=(Φα|Tn(Ĥ)Φi) obtained by propagation of a single initial wave packet Φi correlated with a set of final states Φα, can be harmonically inverted to yield a complete description of the system dynamics in terms of the spectral parameters. In particular, all S-matrix elements can be obtained in such a way. Compared to the conventional way of spectral analysis, when only a column of the S-matrix is extracted from a single wave packet propagation, this approach leads to a significant numerical saving especially for resonance dominated multichannel scattering. The second application of FDM is based on the harmonic inversion of semiclassically computed time cross-correlation matrices. The main assumption is that for a not-too-long time semiclassical propagator can be approximated by an effective quantum one, exp[−itĤeff]. The adequate dynamical information can be extracted from an L×L short-time cross-correlation matrix whose informational content is by about a factor of L larger than that of a single time correlation function.
A new treatment of nonadiabatic dynamics: Application to the determination of the He++H2→He+H+H+ differential cross section
Authors: Aguillon François
Published: July 2002

The dynamics of the dissociative charge exchange reaction He++H2(v⩽4)→He+H+H+ is investigated theoretically at a relative collision energy ranging from 2 eV to 10 eV. Because of the weakness of the nonadiabatic couplings involved in this process, it has been possible to use a new method, which consists in dividing the theoretical treatment in two parts; a semiclassical coupled wave packet method describes the charge exchange between He+ and H2, while the dynamics of the dissociation of H2+ in the presence of the He atom is handled classically. The time dependent description of the reaction provides a simple interpretation of the strong dependence of the cross section with the collision energy and the initial vibrational excitation of H2. It is shown that the reaction mechanism is qualitatively different for the different initial vibrational quantum states of H2. An interesting dynamical effect is observed in the v=0 case; the H+ ions are scattered primarily in the forward direction relative to the initial H2 direction, while the neutral H atoms are backward scattered. Moreover, the H+ ions are on average faster than the neutral H atoms. The results are successfully compared with the available integral and differential experimental data.
Structure of the liquid–vapor interface of a metal from a simple model potential: Corresponding states of the alkali metals
Authors: Chekmarev Dmitriy , Zhao Meishan , Rice Stuart A.
Published: July 2002

We present the results of self-consistent quantum Monte Carlo simulations of the structures of the liquid-vapor interfaces of alkali metals (Na, K, Rb, Cs) using a modified semiempirical empty-core model potential. The purpose of this investigation is to simplify the analysis of inhomogeneous metals sufficiently to permit qualitative inferences to be drawn about the properties of families of metals. Both electronic and ion density profiles along the normal to the surface show oscillations in the liquid–vapor transition zone. These oscillations closely resemble those found in previous simulation studies of the liquid–vapor interfaces of alkalis, based on sophisticated nonlocal model potentials. Because of its semianalytical representation, the model potential used in this paper allows considerable simplification in the computational scheme relative to the effort involved in the previously published simulations. We find liquid Na, K, Rb, and Cs to exhibit similar surface layering. Moreover, our results suggest the existence of a corresponding states representation of the properties of this class of metals. We expect this new analysis will be useful in predicting the qualitative properties of the surface structures of a broad range of pure liquid metals.
The effect of spin-orbit coupling on the magnetic properties of H2Ti(μ–H)2TiH2
Authors: Webb Simon P. , Gordon Mark S.
Published: July 2002

Excited states of singlet and triplet H2Ti(μ−H)2TiH2 have been calculated using multiconfigurational wave functions. The effects of orbital relaxation are determined by optimizing orbitals for all states separately and comparing to state-averaged calculations, and are found to be small. Dynamic electron correlation included through second-order perturbation theory is found to have a considerable effect on excited state relative energies, but not on the ordering of states. Spin-orbit coupling effects are introduced by a one-electron operator which uses an effective nuclear charge to replace two-electron effects. The resulting splittings of the lowest energy triplet state components are 0.027 and 0.199 cm−1, respectively. The former is due to the angular momentum operator which acts along the Ti–Ti axis; the latter is due almost entirely to the angular momentum operator which acts in the direction perpendicular to the plane of the Ti–H–Ti bridge. An overall ferromagnetic effect of 0.660 cm−1 on the ground state singlet-first excited triplet energy gap is predicted. Orbital interactions responsible for spin-orbit coupling effects are identified.
Investigations of pure rotational transitions of H2 self-perturbed and perturbed by He. I. Measurement, modeling, and quantum calculations
Authors: Michaut X. , Saint-Loup R. , Berger H. , Dubernet M. L. , Joubert P. , Bonamy J.
Published: July 2002

High resolution stimulated gain Raman spectroscopy is used to investigate the collisional parameters of pure rotational S0(j=0–4) lines of H2 in pure H2 and H2–He mixture. Measurements are performed between 300 and 1000 K in a density regime where the lines are essentially collisionally broadened (typically 10 amagat). For the first time, these highly accurate measurements of the frequencies of pure rotational lines allow one to correct previously measured values that did not take into account the collisional frequency shift. For both collisional systems, the shifting coefficients exhibit a linear behavior with the square root of temperature, similar to the behavior already observed in the Q branch. The broadening coefficients of the S0 branch increase nonlinearly with temperature contrary to the Q branch. For the H2–He system, both these new S0(j) data and previously measured Q(j) data are analyzed using a modeling of the broadening coefficients in terms of elastic and inelastic contributions. These different contributions are analyzed as a function of temperature and of the rotational quantum number j. Preliminary quantum calculations are used to assess the validity of the model. Further calculations will be presented in paper II.
Absorption and fluorescence of OClO Ã 2A2–X̃ 2B1 in solid Ne, Ar, and Kr. I. Vibrationally unrelaxed Ã→X̃ emission
Authors: Liu Chin-Ping , Lai Lih-Huey , Lee Ying-Yu , Hung Shao-Ching , Lee Yuan-Pern
Published: July 2002

Dispersed laser-induced fluorescence of the Ã 2A2→X̃ 2B1 transition of OClO in solid Ne in the spectral range 500–770 nm was recorded when the origin at 20 991 cm−1 was excited. Progressions with spacings near 939 and 446 cm−1 are associated with vibrational modes ν1 and ν2 of the X̃ state. A simultaneous fit of both modes yields ω1″=957.1±1.4, ω2″=452.6±0.4, x11″=4.47±0.04, x22″=0.54±0.05, and x12″=4.00±0.05 cm−1. When the 101 line of the Ã–X̃ system at 21 699 cm−1 was excited, vibrationally unrelaxed emission was observed in the spectral region 480–600 nm. Excitation of the 201 line at 21 284 cm−1 generated weak vibrationally unrelaxed progressions. The visible absorption spectrum of OClO in solid Ne in the region 415–488 nm was recorded with a Fourier-transform spectrometer, yielding ν00=20 991.3, ν1′=707.9, ν2′=292.5, and 2ν3′=887.6 cm−1 for the Ã state. Simultaneous fits considering either only ν1 and ν2 modes or all three modes yield corresponding spectral parameters. Similar experiments were performed with OClO in solid Ar and Kr. Pronounced increases in ν1′ (716.0 cm−1 in Ar and 712.5 cm−1 in Kr) and ν2′ (302.3 cm−1 in Ar and 303.0 cm−1 in Kr) and a decrease in ν00 (188 cm−1 and 331 cm−1 red-shifted, respectively) from that in the gas phase indicate substantial perturbation of the Ã state in solid Ar and Kr. An absorption continuum underlying the Ã–X̃ system is attributed to absorption to the 1̃ 2A1 state above the predissociation barrier. The phonon interaction increases and the threshold of the continuum decreases as the matrix host is altered from Ne to Ar to Kr.
The application of a VUV Fourier transform spectrometer and synchrotron radiation source to measurements of: I. The β(9,0) band of NO
Authors: Yoshino K. , Esmond J. R. , Parkinson W. H. , Thorne A. P. , Murray J. E. , Learner R. C. M. , Cox G. , Cheung A. S.-C. , Leung K. W.-S. , Ito K. , Matsui T. , Imajo T.
Published: July 2002

State-of-the-art models of the vacuum ultraviolet (VUV) absorbing properties of the atmosphere call for absorption cross sections with detail on the scale of the Doppler widths. As a consequence, spectroscopic data at resolving powers of the order of 106 are needed. To meet these requirements in the vacuum ultraviolet region, we have used the VUV Fourier transform spectrometer from Imperial College, London, at the synchrotron radiation facility at Photon Factory, KEK, Japan, to measure photoabsorption cross sections of NO from 195 to 160 nm, and of O2 from 185 to 175 nm. The analysis of the β(9,0) band (B 2Πr–X 2Πr) of NO provides accurate rotational line positions and term values. Molecular constants of the B(9) 2Π level are T0=54 205.097±0.012 cm−1, A=45.320±0.021 cm−1, Bυ=1.016 72±0.000 16 cm−1, Dυ=(10.61±0.32)×10−6 cm−1, and AD=0.001 22±0.000 11 cm−1. The rotational line strengths and the branching ratios are also presented. The band oscillator strength is obtained as f=2.65×10−4.
Statistical analysis of the computed X̃ 2A1/Ã 2B2 spectrum of NO2: Some insights into the causes of its irregularity
Authors: Santoro Fabrizio
Published: July 2002

We present a statistical analysis of the B2 symmetry lines of the nonadiabatic X̃ 2A1/Ã 2B2 spectrum of NO2 in the energy range 10 000–22 000 cm−1, obtained by ab initio diabatic potentials refined with respect to the experimental lines until 10 000 cm−1. The two most common statistical measures, the nearest neighbor spacing distribution and the Δ̄3 are taken into account. Both the statistics show a high degree of irregularity of the spectrum. The analysis is repeated in restricted ranges for which analogous investigations on experimental data are reported in the literature. The comparison is always satisfactory. Comparison is also discussed with respect to previous theoretical results obtained by other authors. We investigate the causes of the spectrum irregularity by removing systematically some selected couplings from the full nonadiabatic Hamiltonian. We find that the irregularity is due to two main causes; the sensible vibronic coupling between the two diabatic states X̃ 2A1 and Ã 2B2 and the stretch–bend couplings on the diabatic ground surface X̃ 2A1.
Resonance Raman study of solvent dynamics on the spectral broadening and intramolecular charge transfer of a hemicyanine dye in aqueous solution
Authors: Cao Xuan , McHale Jeanne L.
Published: July 2002

The spectroscopic properties of 4-[2-(4-dimethylaminophenyl)ethenyl]-1-methyl-pyridinium iodide (HR) in different solvents reveal the important effects of solvent dynamics on the spectral broadening and the intramolecular charge transfer of HR. In this article, Raman excitation profiles for 18 vibrational modes of HR are reported in aqueous solution at wavelengths that span the S0→S1 charge transfer transition. The absorption spectra, fluorescence spectra and resonance Raman profiles of HR are modeled using time-dependent wave packet theory and the Brownian oscillator solvent dephasing model. The solvent reorganization energy in the absorption process is much greater than that due to internal vibrational modes, and the solvent reorganization energy for the emission process is considerably smaller than that for the absorption process. The fluorescence spectrum is mainly broadened by the inhomogeneous Gaussian distribution of the electronic energy, perhaps due to internal rotations in the molecule. The results suggest similar polarity of the emission state and the ground state, and strong coupling between the torsional motion and solvent relaxation. The different dependence of the torsional potential on solvent polarity in the S0and S1 state is the cause of different absorption and fluorescence spectral width. In D2O, the absorption cross section of HR is slightly lower, and the absorption and fluorescence spectra are slightly narrower, than in H2O. The smaller absorption spectral linewidth and generally increased Raman cross sections in D2O are accounted for by smaller amplitude of solvent dephasing, perhaps due to the larger inertial moment and stronger hydrogen bonding in D2O compared to H2O. The magnitude and direction of the solvent isotope effect on Raman intensity varies with normal mode, suggesting that the solvent-induced dephasing is mode dependent. Vibrational modes which are strongly coupled to the electronic transition are most sensitive to the solvent isotope.
Comparison of the structures of the liquid-vapor interfaces of Al, Ga, In, and Tl
Authors: Zhao Meishan , Chekmarev Dmitriy , Rice Stuart A.
Published: July 2002

We report the results of self-consistent quantum Monte Carlo simulations of the liquid-vapor interfaces of the members of the family of trivalent metals Al, Ga, In, and Tl. In each case the single particle density distribution along the normal to the liquid-vapor interface is stratified and the pair correlation function in the plane of the interface is essentially the same as that in the bulk liquid. We have examined the results of the calculations for structural similarities that hint at the existence of a corresponding states representation valid for a family of metals, such as has been shown for the family of alkali metals. The effective pair potentials of Al, Ga, In, and Tl are rather different from each other. Consequently, characterization of the interface structure using a corresponding states representation in two variables (reduced density and reduced diameter) is not accurate, unlike the case of the alkali metals for which such a characterization is accurate. Experimental data concerning the structure of the liquid-vapor interface exist for only one member of this family of metals, namely Ga. Good agreement between theory and experiment in that case argues for the validity of the results reported for all of the metals of this family, and the inferences drawn therefrom.
Molecular-beam infrared–infrared double-resonance spectroscopy study of the vibrational dynamics of the acetylenic C–H stretch of propargyl amine
Authors: Andrews Anne M. , Fraser Gerald T. , Pate Brooks H.
Published: July 2002

The acetylenic C–H stretch spectrum of propargyl amine near 3330 cm−1 has been measured at 0.0002 cm−1 (6 MHz) resolution with a tunable color-center laser in an electric-resonance optothermal spectrometer. The spectrum has been fully assigned through IR–IR double resonance measurements employing a tunable, microwave sideband-CO2 laser. The 10 μm spectrum of propargyl amine displays splittings in the two nuclear spin symmetry states arising from amino-proton interchange, allowing double-resonance assignment of the –NH2 group resultant proton nuclear spin quantum number in the highly fragmented 3 μm spectrum. The experimental state density is consistent with a (2J+1) increase that is expected if all near-resonant states are coupled. From this J-dependent growth in the state density we determine the density of states at J=0 to be 22 states/cm−1. This value is in reasonable agreement with the direct state count result of 16 states/cm−1. The unperturbed transition frequencies for the two different nuclear spin species at a given rotational level do not coincide, differing on average by about 50 MHz. The nonresonant coupling effects which produce effective splittings in the 10 μm spectrum appear to survive into the high state density regime. The measured IVR lifetimes are on the order of 500 ps for the low Ka values studied here (Ka<4) and show a Ka-dependence with the IVR rate increasing as Ka increases. The statistical properties of the spectrum have been compared to predictions from random matrix theory. The level spacings are not well represented by Wigner statistics as would be expected for underlying chaotic classical dynamics. However, the intensity fluctuations are consistent with a χ2-distribution, expected for classically chaotic systems, as measured by Heller’s F-statistic.
Semi-flexible polymers at a liquid–liquid interface: Self-consistent field calculations
Authors: van Eijk Marcel C. P. , Leermakers Frans A. M.
Published: July 2002

The adsorption of semi-flexible polymers at a liquid–liquid interface largely differs from that at a solid surface. The width of the interface is an additional length scale in the problem, making the system behavior particularly rich. We consider two phase-separating monomeric liquids, C and D, and a polymer AN which dissolves equally well in both liquids. We study this system in a self-consistent field model in the dilute regime. The stiffness of the polymer is controlled by the use of a rotational isomeric state approach. We show that the interfacial width ξ, the persistence length q, and the chain length N are relevant parameters in the adsorption behavior. A key observation is that, while keeping N1/2/ξ constant, the adsorbed amount goes through a minimum with increasing q/ξ. An initial increase of q/ξ (q/ξ≲1) effectively leads to a larger coil size, leading to a decrease of the adsorbed amount. However, when q/ξ≫1, alignment of parts of the polymer within the interfacial region occurs due to the lack of entropic penalties. This alignment process induces an increase of the adsorbed amount. These observations also have implications for the ongoing discussion, which species shows preferential adsorption in a mixture of flexible and stiff polymers. In this discussion one should consider the effects of the finite size of the interfacial region.
Quantum mechanical simulations of inelastic scattering in collisions of large clusters: Ar+(H2O)11
Authors: Fredj E. , Gerber R. B. , Ratner M. A.
Published: July 2002

An approach based on the Time-Dependent Self-Consistent Field (TDSCF) is used to carry out quantum calculations of inelastic atom scattering from large, highly anharmonic clusters. The computation is carried out for low-energy collisions of Ar with (H2O)11, and all the vibrational modes of the cluster are included. The method treats the collider atom classically, but the dynamics of the interacting anharmonic modes of (H2O)11 is handled quantum mechanically. The results provide insight into the collision physics of large systems having soft anharmonic modes, and into the role of quantum effects in such cases. The main findings are the following: (a) Large differences are found between quantum and classical results with regard to energy transfer into specific cluster modes. (b) Classical calculations wrongly predict efficient excitation of many stiff modes, including processes that are quantum-mechanically forbidden. (c) Single quantum excitations are the most important transitions at the collision energy used. (d) Atom–atom pair distribution functions of (H2O)11 after the collision show insignificant differences from the corresponding precollision distribution functions. The results show that quantum calculations of collision dynamics of low-temperature anharmonic clusters are feasible, and also necessary in view of the prediction of significant quantum effects.
Polarized librational spectra of proton-ordered ice XI by molecular dynamics simulations
Authors: Itoh Hidenosuke , Kawamura Katsuyuki , Hondoh Takeo , Mae Shinji
Published: July 2002

A complete set of vibrational spectra of ice XI, proton-ordered phase of ice Ih, has been obtained by molecular dynamics simulations using the Kumagai, Kawamura, and Yokokawa (KKY) potential model which allows unconstrained atomic motions. A remarkable difference between the power spectra of ice Ih and of ice XI was found in the librational band. In ice XI four major librational peaks were observed and assigned to three librational modes: wag, twist, and rock, whereas ice Ih is well known to have a broad librational band. Moreover, characteristic spectral properties, due to the ferroelectric structure of ice XI, were observed in the polarized power spectra of the atom velocities along the crystal axes.
Optical line shapes of single molecules in glasses: Temperature and scan-time dependence
Authors: Geva Eitan , Skinner J. L.
Published: July 2002

The dependence on temperature and scan time of the optical line shapes of individual chromophore molecules in low-temperature glasses is discussed within the framework of the two-level system model. It is shown that the line shapes of different individual molecules depend differently on the temperature and the scan time, reflecting the unique configuration of two-level systems in the environment of each chromophore. The experimental measurement of the distribution of temperature and scan-time dependencies would provide a sensitive test of the two-level system model.
Rationalization of the dielectric properties of common three-site water models in terms of their force field parameters
Authors: Höchtl Peter , Boresch Stefan , Bitomsky Wolfgang , Steinhauser Othmar
Published: July 2002

The dielectric properties (static dielectric constant ε0, Debye relaxation time τD and distance dependent Kirkwood G-factor Gk(R)) of commonly used three-site water models (the “simple point charge” models SPC and SPC/E, as well as the “transferable intermolecular potentials with three point charges” TIP3P and its CHARMM modified variant TIP3Pmod) were evaluated from 2 ns molecular dynamics simulations using the Ewald summation method to compute the electrostatic interactions. The results for SPC (ε0=65±5, τD=7.6±0.8 ps) and SPC/E (ε0=68±6, τD=12.1±1.3 ps) are in good agreement with the literature; for TIP3P (TIP3Pmod) we obtained a dielectric constant ε0=97±7 (97±6) and a Debye relaxation time τD=7.3±0.7 ps (6.9±0.6 ps). The surprisingly large differences in ε0 were rationalized by an investigation of the relationship between the force field parameters and the dielectric properties. Based on simulations of hybrid SPC/TIP3P models, the HOH bond angle was identified as the determining factor of the dielectric constant. By contrast, τD was shown to depend mainly on the OH bond length and the partial charges; the HOH angle is of secondary importance. The role of the HOH angle is clarified by a comparison of the orientational correlation functions of the water models.
Collision energy dependence and product recoil velocity analysis of O+(4S)+C2H2 charge-transfer and chemical reaction channels
Authors: Chiu Yu-hui , Dressler Rainer A. , Levandier Dale J. , Williams Skip , Murad Edmond
Published: July 2002

Guided-ion beam cross section and product ion time-of-flight (TOF) measurements are presented for the O+(4S)+C2H2 reaction over the center-of-mass collision energy range of 0.05–18 eV. Despite a large number of exothermic channels, the total reaction cross section at low energies is more than two orders of magnitude smaller than the capture cross section. A common energy onset for charge-transfer, CH+, and COH+/HCO+ products is observed at 1.70±0.10 eV, above which the total cross section for these channels rapidly rises with energy, eventually exceeding 5 Å2. Above 4 eV, the C2H+ and CO+ products also become significant, and weaker channels producing C+, C2+, and CH2O+ are also identified. The C2H+ fragment is interpreted as being primarily a dissociative charge transfer (CT) channel at collision energies above ∼3.79 eV, while the threshold for forming CO+ can be associated with a CO++H+CH channel. The TOF measurements demonstrate that efficient C2H2+ Ã state formation occurs above ∼2.7 eV. The onset at 1.70±0.10 eV is attributed to a transition state associated with an excited quartet hypersurface.
Bond-selective photofragmentation of jet-cooled HOD at 193 nm: Vibrationally mediated photochemistry with zero-point excitation
Authors: Plusquellic David F. , Votava Ondrej , Nesbitt David J.
Published: July 2002

Photofragment yields are reported for supersonically cooled H2O, D2O, and HOD via one photon, 193 nm photolysis in a slit jet expansion, with OH and OD fragments monitored by laser induced fluorescence methods. Detailed analysis of the dependence of OH vs OD photofragment signals on isotopic composition is used to extract relative photolysis cross sections and branching ratios for bond-selective cleavage in HOD, H2O, and D2O samples. Specified relative to the 193 nm cross sections for H2O→H+OH, the ratios are 0.392(20), 0.032(20), and 0.0157(19) for (i) HOD→H+OD, (ii) HOD→D+OH and D2O→D+OD channels, respectively. Specifically, these results indicate a propensity for H–OD vs D–OH bond cleavage in HOD of 12(8):1. This strong H/D isotopic selectivity reflects extreme non-Franck–Condon photolysis out of classically of forbidden regions of the ground-state wave function, i.e., bond-selective photochemistry mediated solely by zero-point vibrational excitation. However, when compared with theoretical predictions from full three-dimensional quantum scattering calculations on the ground (X̃ 1A1) and excited (Ã 1B1) potential-energy surfaces (PES) of water, the observed HOD branching ratio is found to be too low by an order of magnitude. These results provide additional evidence that photodissociation of water in the extreme non-Franck–Condon region is not adequately explained by current theoretical models and suggest that contributions from other electronic surfaces may be important.
Exact calculation of the solvation energy of a pair of ions in polar media within the framework of the dielectric continuum model
Authors: Miyazaki Kunimasa , Tachiya M.
Published: July 2002

The solvation energy of a pair of ions in polar media is exactly calculated within the framework of the dielectric continuum model. The product ions are modeled as either conducting spheres with charges on their surfaces or spherical vacuum cavities with point charges at their centers. The results show that, if the two ions are close to each other, the solvation energy is different from the results known in the literature.
Efficient truncation strategies for multi-reference configuration interaction molecular energies and properties
Authors: Palmieri Paolo , Tarroni Riccardo , Mitrushenkov Alexander O. , Rettrup Sten
Published: July 2002

Modified virtual orbitals are proposed for multi-reference configuration interaction (MRCI) treatments and a modified Fock operator is defined for the orbital transformation. The main property of the modified orbitals is to improve the convergence properties of the configuration interaction (CI) expansion, which can be exploited to truncate, partially, the expansion in the external space. Simple tests are presented to show that the orbital transformation may be useful to perform FullCI type of treatments for subsets of orbitals and electrons, and to improve the MRCI second-order corrections and energies. Compared to other well-established techniques for accurate MRCI treatments, it is believed that this method offers advantages for electronic structures with many active orbitals and electrons using large orbital basis sets.