bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational predissociation of an inert gas cluster containing an active molecule: The vHF=3 spectrum of Ar3HF
Authors: Chuang Cheng-Chi , Tsang Susy N. , Klemperer William , Chang Huan-Cheng
Published: July 2002

The vHF=3←0 HF valence excitation spectrum of Ar3HF is obtained by intracavity laser induced fluorescence. The spectroscopic constants determined for the vHF=3 level are band origin ν0=11 310.4520(4) cm−1 (corresponding to the vibrational redshift of Δν=−62.355 cm−1), the rotational constants B=0.039 743(5) cm−1 and DJ=2.04(12)×10−7 cm−1. The changes in rotational constants upon HF valence excitation to v=3 are ΔB=1.08(5)×10−4 cm−1 and ΔC=1.01(6)×10−4 cm−1. The increase in C is interpreted as the occurrence of a 0.2% decrease in the Ar–Ar separation. Appreciable line broadening is observed in the Ar3HF (v=3) spectrum. Twenty-three lines in P and R branches are fitted by the Voigt profile with 60 (10) MHz Lorentzian component establishing the occurrence of significant vibrational predissociation for Ar3HF at vHF=3. This rate is consistent with the empirical scaling of the vibrational predissociation rate with the frequency redshift, observed for stronger hydrogen bonded complexes of HF. The scaling of the vibrational predissociation rate for ArnHF (n=1–3) with the argon number, n, is poorly defined.
Centroid path integral molecular-dynamics studies of a para-hydrogen slab containing a lithium impurity
Authors: Kinugawa Kenichi , Moore Preston B. , Klein Michael L.
Published: July 2002

Six-layer para-hydrogen (p-H2) semi-infinite slabs, containing a Li impurity atom in different locations, have been investigated at 2.5–14.7 K by means of centroid path integral molecular-dynamics simulations. On the time scale of the simulations (15 ps), lithium atoms initially doped inside the slab appear to remain trapped in the lattice and show no tendency to diffuse toward the surface. The slab surface layers are observed to melt more readily than the inner ones, even when the dopant Li atom is initially placed in the interior. In some cases, before melting occurs, p-H2 molecules are ejected onto the slab surface and diffuse two dimensionally. Lithium is promoted to the surface only when it is initially buried in the surface layer or when the whole slab melts and a diffusion pathway is opened toward the surface.
Crystallization of dipolar hard spheres: Density functional results
Authors: Klapp Sabine , Forstmann Frank
Published: July 2002

We investigate the thermodynamic conditions under which an isotropic dipolar hard sphere fluid freezes into an unpolarized or ferroelectric crystalline state by minimizing a free-energy functional. This functional is based on the direct correlation function of the isotropic fluid, calculated by reference hypernetted chain integral equations. In the framework of our approach, the ferroelectric fluid phase, found in simulations, is surpassed by a ferroelectric body-centered tetragonal crystal.
Resonance enhanced multiphoton ionization time-of-flight study of CF2I2 photodissociation
Authors: Bergmann Klaus , Carter Robert T. , Hall Gregory E. , Huber J. Robert
Published: July 2002

The resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) technique was applied to study the complex dissociation of CF2I2 in a supersonic expansion. Using nanosecond excitation at 248, 266, and 304 nm and probing the I3/2 and/or I1/2 photoproducts, we determined the photofragment speed distribution, the recoil anisotropy parameter β and the branching ratio for each of the decay channels. At 248 nm the dissociation proceeds according to a concerted three-body decay, CF2I2→hvCF2+I1/2+I3/2 (90%) or CF2I2→hvCF2+I3/2+I3/2 (10%) consistent with our previous results obtained from photofragment translational spectroscopy. At 304 nm three competing decay channels were found to be operative. In decreasing order of branching, these are: two-body decay to CF2I +I1/2, sequential three-body decay CF2I2→ lim hvCF2I + I3/2→CF2 + I3/2+I3/2, and a concerted three-body decay yielding CF2 +I3/2+I3/2. The positive, almost maximum β values observed for all the primary steps in the 248, 266 and 304 nm photodissociation imply that the excited states which subsequently lead to dissociation are exclusively of B1 symmetry and that the primary bond-breaking processes occur on a subpicosecond time scale.
Photodissociation of O2 at 157 nm: Experimental observation of anisotropy mixing in the O2+hν→O(3P)+O(3P) channel
Authors: Lin Jim J. , Hwang Dennis W. , Lee Yuan T. , Yang Xueming
Published: July 2002

In this work, photodissociation of O2 at 157 nm has been studied using the photofragmentation translational spectroscopic technique. Two product channels O2+hν→O(1D)+O(3P), O2+hν→O(3P)+O(3P) have been observed. The relative yields and anisotropy parameters of both channels are determined. Anisotropy mixing of dissociation resulting from a perpendicular excitation and a parallel-type excitation has been observed in the dissociation channel O2+hν→O(3P)+O(3P). The observed results can be used to look at the detailed dynamical processes of the O2 dissociation through the Schumann–Runge band.
Rotational spectra of Ne20H+, Ne20D+, Ne22H+, and Ne22D+
Authors: Matsushima Fusakazu , Ohtaki Yuichiro , Torige Osamu , Takagi Kojiro
Published: July 2002

Rotational transitions of Ne20H+, Ne20D+, Ne22H+, and Ne22D+ were observed in the 1–5 THz frequency region with a high-precision far-infrared spectrometer using a tunable radiation source. Observed frequencies were incorporated with previous infrared measurement to redetermine Dunham coefficients Ykl and isotopically independent parameters Ukl, ΔklH, and ΔklNe, where Δ02H and Δ01Ne were determined for the first time.
Energetics of silicon hydrides on the Si(100)-(2×1) surface
Authors: Robinson Brown A. , Doren D. J.
Published: July 2002

Density functional theory methods are used to calculate the structures and energies of silicon trihydride (SiH3) and dihydride (SiH2) species on the Si(100)-(2×1) surface. These species are intermediates in the growth of silicon films by chemical vapor deposition of silane and disilane. The lowest-energy trihydride species is metastable with respect to the lowest-energy dihydride species, but two surface dangling bonds must be available to affect the transformation to the dihydride. In the lowest-energy configurations, dimers either have both dangling bonds occupied or both unoccupied. While the energy difference between isomers with fully occupied and partially occupied dimers will strongly favor fully occupied dimers at low temperatures, there will be a distribution of dimer occupations at high temperatures. The structures and energies of some other local minima corresponding to tri- and dihydrides are also described. While these species are energetically unfavorable and should only exist transiently, they illustrate the relative energetics of some alternative bonding behavior of the silicon surface.
Stability and structure of Ni+Arn and Pt+Arn clusters
Authors: Velegrakis Michalis , Froudakis George E. , Farantos Stavros C.
Published: July 2002

The mass spectra of argon clusters doped with d9 transition metal ions, Ni+Arn and Pt+Arn, show pronounced intensity at n=4 and n=6. Density functional theory geometry optimization calculations in Ni+Arn support a square planar geometry for Ni+Ar4 and an elongated octahedron for Ni+Ar6.
Correlation function formulation for the state selected total reaction probability
Authors: Garashchuk Sophya , Tannor David J.
Published: July 2002

A correlation function formulation for the state-selected total reaction probability, Nα(E), is suggested. A wave packet, correlating with a specific set of internal reactant quantum numbers, α, is propagated forward in time until bifurcation is complete at which time the nonreactive portion of the amplitude is discarded. The autocorrelation function of the remaining amplitude is then computed and Fourier transformed to obtain a reactivity spectrum. Dividing by the corresponding spectrum of the original, unfiltered, wave packet normalizes the reactivity spectrum, yielding the total reaction probability from the internal state, α. The procedure requires negligible storage and just one time-energy Fourier transform for each initial reactant state, independent of the number of open channels of products. The method is illustrated numerically for the one-dimensional Eckart barrier, using both quantum-mechanical and semiclassical propagation methods. Summing over internal states of reactants gives the cumulative reaction probability, N(E). The relation to the trace formula [W. H. Miller, S. D. Schwartz, J. W. Tromp, J. Chem. Phys. 79, 4889 (1983)], N(E)=12(2πℏ)2 tr(F̄δ(H−E)F̄δ(H−E)), is established, and a new variant of the trace formula is presented.
Thermodynamic investigation of small germanium–tin clusters with a mass spectrometer
Authors: Schmude Richard W. , Gingerich Karl A.
Published: July 2002

The partial pressures of GeSn, Ge2Sn, GeSn2, and Ge3Sn have been measured above a mixture of tin, silicon, and germanium using the Knudsen effusion mass spectrometric method. Partial pressures and estimated thermal functions yield the atomization enthalpies, ΔHa,0, in kJ mol−1, of: 230±13 (GeSn), 490±22 (GeSn2), and 559±27 (Ge2Sn). For the molecules Ge3Sn, Ge4Sn, and SnC upper limits (for the atomization enthalpy) have been obtained as: 996±32, 1389±42, and 452±14, respectively.
Mechanisms of solvation dynamics of polyatomic solutes in polar and nondipolar solvents: A simulation study
Authors: Ladanyi Branka M. , Maroncelli Mark
Published: July 2002

Molecular dynamics (MD) simulations of a benzenelike solute in acetonitrile and CO2 (298 K and 52.18 cm3/mol) are used to investigate the molecular basis of solvation dynamics in polar and nondipolar solvents. The solvation response to various charge rearrangements within the benzene solute are simulated in order to mimic the type of electrostatic solvation observed in typical experimental systems. From equilibrium MD simulations the solvation time correlation function [TCF; C(t)] and the corresponding solvation velocity TCF [G(t)] are used to study the mechanisms underlying time-dependent solvation within the linear response limit. Decomposition of G(t) into contributions from rotational and translational solvent velocities reveals that the relative mix of these two types of motion is quite similar in the two solvents but is strongly dependent on the multipolar order (m) of the solute perturbation. The contribution of translational solvent motions to both the short and long time dynamics of C(t) increases from about 10% for a monopolar perturbation (m=0; i.e., a change in net charge) to about 40% for a perturbation of octopolar (m=3) symmetry. Decomposition of both C(t) and G(t) into single-molecule and molecular-pair contributions shows that the collective nature of the solvation response depends markedly on the charge symmetry of both the solvent molecule’s charge distribution and the solute perturbation. In the nondipolar solvent CO2 neither C(t) nor G(t) differ significantly from their single-molecule counterparts—collective effects are therefore of little consequence to solvation in this solvent. However, in the highly dipolar solvent acetonitrile pair contributions to C(t) greatly suppress the magnitude of the solvation response and as a consequence greatly increase the speed of the response over what it would be in their absence. The importance of these intermolecular correlations in acetonitrile decreases substantially with m, such that the “suppression factors” (αs) vary from ∼9 for m=0 to ∼2 for m=3. The intermolecular correlations of primary importance in acetonitrile are of a static rather than a dynamic nature (i.e., pair effects on G(t) are of only secondary importance). This feature makes it possible to employ several approximate relationships to relate the collective dynamics of solvation in polar fluids to simpler single-solvent molecule dynamics.
Comparison of conjugate gradient density matrix search and Chebyshev expansion methods for avoiding diagonalization in large-scale electronic structure calculations
Authors: Bates Kevin R. , Daniels Andrew D. , Scuseria Gustavo E.
Published: July 2002

We report a comparison of two linear-scaling methods which avoid the diagonalization bottleneck of traditional electronic structure algorithms. The Chebyshev expansion method (CEM) is implemented for carbon tight-binding calculations of large systems and its memory and timing requirements compared to those of our previously implemented conjugate gradient density matrix search (CG-DMS). Benchmark calculations are carried out on icosahedral fullerenes from C60 to C8640 and the linear scaling memory and CPU requirements of the CEM demonstrated. We show that the CPU requisites of the CEM and CG-DMS are similar for calculations with comparable accuracy.
Intermolecular V–V energy transfer in the photodissociation of CO2–HF(v=1)
Authors: Oudejans L. , Miller R. E.
Published: July 2002

Photofragment final state distributions have been measured for the vibrational predissociation of CO2–HF corresponding to excitation of the H–F stretching vibration. The method used in these studies combines photofragment translational spectroscopy, pendular state orientation methods, and laser probing to provide distributions that include the interfragment state correlations. The results clearly show that the dominant dissociation channel involves intermolecular V–V energy transfer corresponding primarily to excitation of the asymmetric stretching vibration of the CO2 fragment. The dissociation energy of the complex has also been determined to be 672±4 cm−1.
An influence functional theory of multiphonon processes in molecular vibrational energy relaxation
Authors: Shiga Motoyuki , Okazaki Susumu
Published: July 2002

Influence functional theory has been applied to describe vibrational energy relaxation of molecules in the solution based upon harmonic oscillators bath approximation. An algebraic formula of the perturbative influence functional is presented for a number of quantum bath oscillators {qk} nonlinearly coupled to the system x as ∑kfk(x)qk+∑k∑lgkl(x)qkql+∑k∑l∑mhklm(x)qkqlqm. The approach opens a way to molecular based analysis of multiphonon processes making usage of a number of techniques and concepts in the field of path integral and quantum field theory. Based upon the functional, we also derive a computationally tractable expression for the relaxation time by executing the path integral exactly. The theory is of much higher approximation than Fermi’s golden rule including perturbations up to the infinite order. A recipe for the numerical work based upon classical molecular dynamics calculation followed by the normal mode analysis is also presented.
Zero-order 1B1(n,0,0) vibrational levels of sulfur dioxide
Authors: Hegazi E. , Al-Adel F. , Dastageer A. , Hamdan A.
Published: July 2002

More than 100 single vibronic level fluorescence spectra of SO2 throughout the 30 400–34 230 cm−1 spectral region were selectively excited and recorded in crossed supersonic jet and laser beams under conditions of cold rotational temperatures. It was found that the vibronic origins can be grouped into 5 overlapping regions based on identical Franck–Condon patterns of the X̃ 1A1(n,0,0) progressions. The spacing between the centers of these regions was found to range between 572 cm−1 and 647 cm−1 as opposed to the previously estimated value of 690 cm−1. The onset of the 1B1 state in the vibronic coupling was found to be at the 30 553 cm−1 band, which is below the previously reported threshold energy by 220 cm−1. The results provide qualitative verification of the multilevel coupling mechanism proposed by Kullmer and Demtröder [Chem. Phys. 92, 423 (1985)].
Resolving vibrational and structural contributions to isothermal compressibility
Authors: Stillinger Frank H. , Debenedetti Pablo G. , Sastry Srikanth
Published: July 2002

The well-known and general “compressibility theorem” for pure substances relates κT =−(∂ ln V/∂p)N,T to a spatial integral involving the pair correlation function g(2). The isochoric inherent structure formalism for condensed phases separates g(2) into two fundamentally distinct contributions: a generally anharmonic vibrational part, and a structural relaxation part. Only the former determines κT for low-temperature crystals, but both operate in the liquid phase. As a supercooled liquid passes downward in temperature through a glass transition, the structural contribution to κT switches off to produce the experimentally familiar drop in this quantity. The Kirkwood–Buff solution theory forms the starting point for extension to mixtures, with electroneutrality conditions creating simplifications in the case of ionic systems.
The C̃←X̃ transition in Ar⋅NO, Kr⋅NO, and Xe⋅NO studied using resonance-enhanced multiphoton ionization spectroscopy
Authors: Mack Paul , Dyke John M. , Smith David M. , Wright Timothy G. , Meyer Henning
Published: July 2002

The C̃ 2Π(vNO′=0)←X̃ 2Π(v″=0) transitions in Ar⋅NO and Kr⋅NO are investigated using (2+1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The spectra consist of vibrational progressions, each member of which is seen to have a reproducible pattern of much closer spaced features; the latter are assigned as rotational structure associated with the a inertial axis of a near T-shaped complex. The spectra are successfully simulated using this model, which yields information on the geometry of the molecular complex in these vibronic states. Comparison is made with previously reported spectra for the C̃ 2Π(vNO′=1 and 2)←X̃ 2Π(vNO″=0) transition. The corresponding spectrum for Xe⋅NO is also discussed.
Comment on a critique of the instantaneous normal mode (INM) approach to diffusion [J. Chem. Phys. 107, 4618 (1997)]
Authors: Keyes T. , Li Wu-Xiong , Zurcher U.
Published: July 2002

A critique of the instantaneous normal mode (INM) theory of diffusion by Gezelter, Rabani, and Berne (GRB) [J. Chem. Phys. 107, 4618 (1997)] is analyzed. GRB assert that imaginary-frequency INM are corrupted with modes unrelated to barrier crossing, that proposals for removing such nondiffusive (ND) modes are inadequate, and thus that INM cannot be used to predict the self-diffusion constant, D. In rebuttal it is argued that Lennard-Jones, the system studied by GRB, is anamolously rich in ND modes. INM in molecular liquids are shown to behave as excellent indicators of barrier crossing. Even in LJ ND-INM, while plentiful, do not dominate D except in supercooled liquids near Tg and in the crystal; hence the many successes of the theories already reported in LJ. Agreement of simulated Im-ω densities of states with calculations modeling the INM as excitations in the soft potential model also indicate that the INM reflect the potential energy landscape in liquids.
Polarizabilities of CO, N2, HF, Ne, BH, and CH+ fromab initiocalculations: Systematic studies of electron correlation, basis set errors, and vibrational contributions
Authors: Christiansen Ove , Hättig Christof , Gauss Jürgen
Published: July 2002

The dipole-polarizabilities of CO, N2, HF, Ne, BH, and CH+ have been investigated using coupled-cluster methods. An extensive basis set study has been carried out and electron correlation effects have been studied employing a hierarchy of coupled-cluster models. Vibrational-averaged electronic polarizabilities as well as the pure vibrational polarizabilities have been calculated based on ab initio energy and property curves. The frequency-dependency of the vibrationally-averaged electronic polarizabilities and vibrational polarizabilities are discussed. The final results are compared with available experimental data.
Four-dimensional model calculation of torsional levels of cyclic water tetramer
Authors: Sabo Dubravko , Bačić Zlatko , Graf Stephan , Leutwyler Samuel
Published: July 2002

Quantum four-dimensional model calculations of the coupled intermolecular torsional vibrations of the cyclic homodromic water tetramers (H2O)4 and (D2O)4 are presented, based on the analytical modEPEN4B potential energy surface [S. Graf and S. Leutwyler, J. Chem. Phys. 109, 5393 (1998), preceding paper] and a four-dimensional discrete variable representation approach. The lowest 50 torsional levels were calculated up to 420 and 500 cm−1 for (D2O)4 and (H2O)4, respectively. For both clusters, the torsional ground state is split by a synchronous O–H torsional inversion process, similar to inversion tunneling in ammonia, with calculated tunnel splittings of 21.8 and 0.000 12 MHz for (H2O)4 and (D2O)4, respectively. As for the cyclic water trimer and pentamer, the four torsional fundamentals of the tetramer lie above the torsional interconversion barriers, between 185–200 cm−1 for (D2O)4 and 229–242 cm−1 for (H2O)4, but also lie below the one-dimensional torsionally adiabatic barriers. The anharmonic fundamental frequencies lie both above and below the normal-mode frequencies, by up to 33%. Slightly above the fundamental torsional excitations, at 257–260 and 280–281 cm−1 for (H2O)4 and (D2O)4, respectively, lie four states corresponding to four versions of the {uudd} isomer, which form a pseudorotational manifold; the torsional interconversion occurs by a sequence of double O–H flips. Higher excited pseudorotational states are calculated up to a vibrational angular momentum of k=3. At ≈295 and ≈300 cm−1, a further group of eight states is found, corresponding to the eight permutationally equivalent versions of yet another isomer, the {uuud} structure. The four {uudd} and eight {uuud} states of (H2O)4 exhibit inverse isotope effects, and lie at lower energy than their (D2O)4 counterparts.
Towards a complete description of multiple dissociation events: A photoion–photoneutral (pi-pn3) coincidence study of Ar4+
Authors: Jukes P. , Buxey A. , Jones A. B. , Stace A. J.
Published: July 2002

By combining the techniques of ion and fast neutral detection into a single coincidence experiment, it is demonstrated that the scattering patterns of all the fragments from a multiple dissociation event can be correlated. The power of this new technique is demonstrated using Ar4+ as an example, where photoexcitation generates a total of 7 possible reaction products, each of which is, in theory, distinguishable by charge and/or kinetic energy. The two ionic products are Ar+ and Ar2+, and whilst the latter ion emerges with a low kinetic energy and is isotropically scattered, Ar+ has associated with it both a significant spread in laboratory-frame kinetic energy and features indicative of anisotropic scattering. By selecting an appropriate laboratory-frame kinetic energy for either ion, coincident neutrals can be identified and energy-analyzed using a time-of-flight technique. The method reveals the presence of two separate fragmentation channels, one of which is concerted and the other sequential.
(2+1)
REMPI spectra of Ω=0 states of the hydrogen halides: Spectroscopy, perturbations and excitation mechanisms
Authors: Kvaran Ágúst , Logadóttir Áshildur , Wang Huasheng
Published: July 2002

(2+1) REMPI spectra of HX (X=Cl, Br and I) have been recorded and analyzed by simulation calculations to derive rotational constants, band origins and isotope shift values for a number of vibrational bands of Ω=0 states. Our data for HCl compare nicely with those derived by Green et al. by conventional analysis methods [D. S. Green et al., J. Mol. Spectrosc. 150, 303, 354, 388 (1991); D. S. Green and S. C. Wallace, J. Chem. Phys. 96, 5857 (1992)]. New spectroscopic parameters were derived for eight vibrational bands which are assigned to the V(1Σ+) state, for v′=4 of the E(1Σ+) state, as well as for five new bands in HBr. New spectroscopic parameters were derived for four vibrational bands which are assigned to the V state and for v′=1 of the E state in HI. Anomalies observed in energy level spacings, rotational parameters and isotope shift values are interpreted as being largely due to homogeneous interactions between the V and the E states. It is argued that the interaction causes a compression of rovibrational levels in the E state manifold but an expansion of levels in the V state manifold, something which might be expected for a Rydberg to ion-pair interaction. Variations observed in the intensity ratio of O and S line series to Q line series in vibrational bands of the E and V states for HCl and HBr are discussed and mechanisms of two-photon excitation processes are proposed.
Reduced dimensionality quantum scattering calculations on the Cl+CH4→HCl+CH3 reaction
Authors: Nyman Gunnar , Yu Hua-Gen , Walker Robert B.
Published: July 2002

Reduced dimensionality quantum scattering calculations using the Rotating Line Approximation, RLA, are performed on the Cl+CH4→HCl+CH3 reaction, treating CH4 as a pseudo-diatom QH. A LEPS potential energy surface is used, where the zero-point energy of the modes not explicitly treated in the RLA calculations are included. The calculations are performed using hyperspherical coordinates and the improved log-derivative method of Manolopoulos. Boundary conditions have been applied using a hyperspherical projection method and an approximate method where the boundary conditions are applied directly in the hyperspherical coordinates. It is explicitly shown that the agreement between the methods is good. Scattering resonances are observed and related to the shape of the hyperspherical adiabats. Cumulative and state-to-state reaction probabilities are also presented. Vibrational adiabaticity is found to dominate without being exclusive. For Cl+CH4(v=1)→HCl(n=1)+CH3 and a translational energy of 0.159 eV, measured differential cross sections give predominantly forward scattering, while for Cl+CH4(v=1)→HCl(n=0)+CH3 there is predominantly backward and sideways scattering. Our calculated differential cross sections are in agreement with these general observations. Thermal rate constants have been calculated for the temperature range 200–800 K. The calculated rate constants are smaller than the experimental ones by less than 40% over the entire temperature range.
Shape of star-branched polymers at various solvent conditions. A computer simulation study
Authors: Sikorski Andrzej , Romiszowski Piotr
Published: July 2002

Monte Carlo simulations of a simple cubic lattice model of star-branched polymers were made. The solvent conditions (temperature) were varied in a wide range and the total chain length was up to 800 beads. The analysis of a shape of branched macromolecules was performed by means of the gyration tensor principal moments and asphericity factor. The changes of shape parameters during the transition from a random coil to a collapsed globule have been determined. In high temperatures, the instantaneous shape of the branched chain is more symmetric than that for linear chains. The shape of one star arm is very similar to that of the entire polymer. In lower temperatures, the collapsed model chain forms an isotropic globule. The shape of inner and outer parts of the star polymers is discussed.
A projection operator and self-consistent field equations for reduced nonequilibrium distribution functions
Authors: Eu Byung Chan
Published: July 2002

A projection operator is constructed which, when applied to the Liouville equation, yields self-consistent field equations for products of reduced nonequilibrium distribution functions for interacting particles. The self-consistent field equations superficially appear as the evolution equations for fictitious independent subunits making up the system of interest. They, in fact, represent closures of the Bogoliubov–Born–Green–Kirkwood–Yvon hierarchy at various levels of reduced description. When the integral kernel is suitably approximated, they yield the well-known Boltzmann equation and kinetic equations for reduced distribution functions for uncorrelated subsystems comprising the system. On the basis of the self-consistent field equations, some deductions are made for kinetic equations that may be used for constructing thermodynamic theories of irreversible processes consistent with the laws of thermodynamics.
Characterizations of the hydrogen-bond structures of 2-naphthol-(H2O)n (n=0–3 and 5) clusters by infrared-ultraviolet double-resonance spectroscopy
Authors: Matsumoto Yoshiteru , Ebata Takayuki , Mikami Naohiko
Published: July 2002

OH stretching vibrations of 2-naphthol-(H2O)n (n=0–3 and 5) hydrogen-bonded clusters in the S0 state have been observed by infrared-ultraviolet (IR-UV) double-resonance spectroscopy. In bare 2-naphthol, cis- and trans-isomers were identified by the comparison of the observed OH frequencies with those obtained by ab initio calculations with the HF/6-31G basis set. The OH stretching vibrations (νOH) of hydrogen-bonded 2-naphthol-(H2O)n show characteristic shifts depending on the cluster size. They are classified into hydrogen-bonded νOH, and νOH free from the hydrogen bond. The cluster structures were also examined by comparing the observed IR spectra with simulated ones. It was found that the clusters with n=2 and 3 form ring structures, while the cluster with n=5 exhibits an ice (I) structure.
Stochastic resonance in catalytic reduction of NO with CO on Pt(100)
Authors: Yang Lingfa , Hou Zhonghuai , Zhou Baojing , Xin Houwen
Published: July 2002

This paper presents a stochastic resonance occurring in a chemical reaction Pt(100)/NO+CO. The results were from numerical simulation of the nonlinear kinetic behavior of a three-variable reaction model obtained from the law of mass actions. The model exhibits a special region in the bifurcation scheme, where a stable node coexists with a stable limit cycle. When one of the control parameters is perturbed by a weak, low frequency periodic signal riding on a suitable external noisy background, transitions between the steady state and oscillatory state may become regular unexpectedly, and signal to noise ratio is thus enhanced at the signal frequency in the Fourier transform power spectrum of the time series output. That refers to stochastic resonance, in which the noise may play a constructive role in the detection of weak signals. The findings may suggest a new method to develop chemical sensitive devices in the field of applications. The paper also discusses the conditions of occurrence of stochastic resonance, and studies the laws it follows.
Theory of vibrational relaxation processes in resonant collisions of low-energy electrons with large molecules
Authors: Thoss Michael , Domcke Wolfgang
Published: July 2002

The effect of intramolecular vibrational relaxation in resonant collisions of low-energy electrons with large molecules is investigated. The vibrational relaxation results from the coupling of a few active modes, which are strongly coupled to the electronic degrees of freedom and therefore coherently excited during the formation of the collision complex, to a large number of inactive bath modes. The theory is formulated within the framework of the projection-operator formalism. Starting from a model Hamiltonian which includes all vibrational modes, the bath degrees of freedom are eliminated, using perturbation theory in the system-bath coupling and projection techniques within the bath Hilbert space. The result is an effective-Hamiltonian description of inelastic electron scattering and electron attachment which incorporates vibrational dissipation. In addition, the time-dependent description of resonant electron-molecule scattering is extended to include the possibility of vibrational relaxation in the resonance state. This time-dependent formulation, which is based on the Markovian master equation for the reduced density operator, provides more direct insight into the dynamics of the collision complex. Calculations for simple model systems are performed which yield insight into the characteristic effects of vibrational relaxation on electron scattering and attachment cross sections, as well as on the time-dependent dynamics of shape resonances. In particular, the competition between autodetachment and vibrational relaxation in shape resonances close to threshold is studied in some detail. The inclusion of vibrational relaxation allows us to develop a microscopic dynamical description of the nondissociative capture of low-energy electrons by large molecules.
Symmetry-adapted filter diagonalization: Calculation of the vibrational spectrum of planar acetylene from correlation functions
Authors: Chen Rongqing , Guo Hua , Liu Li , Muckerman James T.
Published: July 2002

A symmetry-adapted filter-diagonalization method is used to calculate the vibrational spectrum of planar acetylene. In this method, vibrational eigenvalues in a given symmetry are obtained by solving a generalized eigenproblem in which the Hamiltonian and overlap matrices are assembled from symmetry-adapted correlation functions. Since no filtered state is explicitly needed, the calculation requires a relatively small memory. The numerical efficiency is further improved as the correlation functions belonging to various symmetry species are generated from a single wave packet. Comparison with existing data for the acetylene system confirms its accuracy and efficiency.
Coupled cluster investigation of the electric-field-gradient-induced birefringence of H2, N2, C2H2, and CH4
Authors: Coriani Sonia , Hättig Christof , Jo/rgensen Poul , Rizzo Antonio , Ruud Kenneth
Published: July 2002

An ab initio investigation of the electric-field-gradient-induced birefringence of H2, N2, C2H2, and CH4 is presented. All linear and nonlinear optical properties contributing to the induced anisotropy of the refractive index are computed by means of coupled cluster singles and doubles response theory. For systems of cubic or icosahedral symmetry, the only nonvanishing contribution to the induced birefringence within the semiclassical approach is due to the frequency-dependent dipole–dipole–quadrupole and dipole–dipole–magnetic dipole hyperpolarizabilities. These hyperpolarizabilities are important for accurate experimental determinations of the molecular quadrupole moment from electric-field-gradient-induced birefringence measurements.
Electronic states of the C6H6/Cu{111} system: Energetics, femtosecond dynamics, and adsorption morphology
Authors: Velic D. , Hotzel A. , Wolf M. , Ertl G.
Published: July 2002

Two-photon-photoemission (2PPE) spectroscopy is employed to characterize electronic states of a bilayer C6H6/Cu{111} system at 85 K. The unoccupied benzene π* e2u state is observed with a binding energy of 4.6 eV above the Fermi level. This result agrees with inverse-photoemission (IPE) data and provides a case where the determination of the binding energy is identical for 2PPE and IPE. The π* e2u state is assigned in the 2PPE scheme as a final state which is the first observed final state in 2PPE of adsorbate-surface systems. The dependence of the electron dynamics on the morphology of an incomplete adsorption layer is also investigated. Two (n=1)-like image potential states A and B are observed which presumably originate from two different C6H6 adsorption geometries in the bilayer regime. The two image states A and B are characterized by electron effective masses of 1.1 and 1.9 me, binding energies of 3.30 and 3.45 eV above the Fermi level, and lifetimes of 40 and 20 fs, respectively. The dielectric continuum model and the Kronig–Penney model are employed to simulate the origin of (n=1)-like image states. The work function decreases from 4.9 eV at clean Cu{111} to 4.0 eV at bilayer coverage. The change of the work function and the observation of two image states suggest the redefining of the ratio of the numbers of benzene molecules in the first and the second layers of the bilayer regime to approximately 1:1 instead of 1:2, as previously reported. 2PPE is shown to be sensitive to the changes of morphologies, local work functions, and adsorbate-surface potentials during the layer formation.
Two-photon spectroscopy of the NO–Ne complex
Authors: Fleniken J. , Kim Y. , Meyer H.
Published: July 2002

Electronic states of the NO–Ne complex correlating with several low lying Rydberg states of the NO molecule are investigated using (2+1) resonance enhanced multiphoton ionization. The observed band systems for the states F 2Δ, E 2Σ, and C 2Π consist of at least two vibrational bands assigned to the stretching mode. For the H 2Π state five members of the stretch progression are observed. Binding energies range from 120 to 280 cm−1. The rotational structure of the observed bands is analyzed using a rigid rotor Hamiltonian. Jacobi bond angles between 50° and 85° are found for the vibrationally averaged structures. Spectra for the NO–X complexes (X=Ar, Ne) correlating with the vibrational levels C 2Π v′(NO)=0, 1, and 4 exhibit a constant red shift. The anomalous red shift found for the levels v′(NO)=2, 3 indicates a change in the degree of mixing with the state B 2Π upon complexation.
Critical embryo phase transitions in the nucleated binary glycerin–carbon dioxide system
Authors: Anisimov M. P. , Koropchak J. A. , Nasibulin A. G. , Timoshina L. V.
Published: July 2002

In order to develop a consistent nucleation theory, the main assumptions of the theory should be revised. One of the questionable problems is the role of the carrier gas in nucleation and the surface tension for the critical embryo as a function of cluster size. Using a flow diffusion chamber, the vapor nucleation rates were measured with high precision and phase transitions in critical embryos containing two and more dozen molecules were detected. Phase transitions in critical embryos were used as markers to detect that the new phase critical embryos contain two components. Phase transitions of the first order related with critical point second-order phase transitions in the pure CO2 carrier-gas were used as markers to demonstrate the presence of CO2 in critical embryos of condensate. Results of this research, in our opinion, very clearly demonstrate that vapor nucleation in a gaseous atmosphere is a binary process and must be interpreted from the point of view of nucleation theory within a binary system. “Supercritical” nucleation is a virtual term born by interpretation of binary vapor–gas nucleation by using the nucleation model of a single component. A critical condition for the binary system could be a higher level for the single component critical pressure and/or temperature, which can produce the illusion of supercritical nucleation. One component interpretation can be used far from the critical condition. On the other hand, the Laplace pressure practically always is able to approach the nucleation condition to the critical pressure. This level of detail is a problem for future studies. The traditional application of classical nucleation theory for vapor–gas nucleation should be modified to consider the nucleation conditions in pressure-temperature-composition space.
Ab initio potential energy surface and rovibrational spectra of He–CO2
Authors: Yan Guosen , Yang Minghui , Xie Daiqian
Published: July 2002

The potential energy surface for the He–CO2 van der Waals complex is calculated using the fourth-order Moller–Plesset (MP4) perturbation theory with a large basis set containing bond functions. The interaction energies are obtained by the supermolecular approach with the full counterpoise correction for the basis set superposition error. The MP4 interaction energies are then fitted to an exponential-spline-Morse–Morse-spline-van der Waals potential form. The ab initio potential energy surface contains two local minima corresponding to the T-shaped and linear structures. The T-shaped minimum is the global minimum with the well depth of 44.41 cm−1 at R(He–C)=3.10 Å. The linear minimum has a well depth of 27.69 cm−1 at R(He–C)=4.26 Å. A potential barrier that separates the two minima is located at R(He–C)=4.10 Å and θ=39° with a height of 19.81 cm−1. The rovibrational energy levels of He–CO2 with CO2 at its ground state and υ3 excited state are calculated using the vibrational self-consistent field configuration-interaction approach. Our calculated results show that the potential supports five vibrational bound states. The zero-point energy is 28.60 cm−1. The first excited bend frequency is 8.67 cm−1, which is very close to the experimental estimated value of about 9±2 cm−1. The calculated transition frequencies and spectroscopic constants are in good agreement with the experimental values.
Correlation effects in dilute particle-polymer mixtures
Authors: Chatterjee Avik P. , Schweizer Kenneth S.
Published: July 2002

The influence of chain connectivity and polymer excluded volume correlations on macromolecule-induced depletion interactions between spherical particles in the athermal limit is analyzed based on integral equation methods. Results for the sphere–sphere second virial coefficient (B2cc) and polymer-induced potential of mean force derived within the Percus–Yevick (PY) and hypernetted chain (HNC) closure approximations for the polymer reference interaction site model (PRISM) are compared with those from prior theories for the depletion interaction which employ simplified models for macromolecules and/or ignore intermolecular packing effects. Distinctive differences are often found depending on system parameters, which are interpreted in terms of many-body correlation effects or nonathermal solvent quality. Comparisons with scaling and field-theoretic approaches in the dilute polymer limit, and limitations of the PY closure, are briefly addressed.
State-specific vibrational anharmonicities in cyclobutadiene and evidence for fast automerization by C412H4
Authors: Redington Richard L.
Published: July 2002

MO computations of vibrational spectra for 1,3-cyclobutadiene (CB) and CB:CO2 van der Waals molecule isotopomers at the MP2/6-311++G(2d,2p) level of MO theory are analyzed together with the extensive Ar/CB matrix-isolation spectra of the Michl group, the low 3.2 kcal/mol (ZPE-corrected) MO-computed energy of the square-planar saddle-point for CB reported by Nakamura et al., and opportune one-dimensional analysis of the automerization energy levels of C412H4. The composite interpretation results in the assignment of the strongly anharmonic Raman transitions of Ar/CB observed at 723 and 1678 cm−1 to the E2-E0 and E4-E0 transitions, respectively, in the automerization coordinate. The C13 and D isotope dependencies of the nominal C=C stretching and δCH wagging vibrational normal modes of the CB valence isomers depend markedly on whether the labeled HC=CH moieties of CB are mass equivalent or not, and the pronounced normal mode differences are viewed as providing different gateways into the global anharmonic PES domains. The automerization coordinate Qa originates as the nominal C=C stretching normal coordinate of ag symmetry, followed by a curving to more efficiently interchange the C=C and C–C bond lengths as the square-planar D4h configuration is approached. The latter is crossed at a geometry dilated with respect to the saddle-point, thereby providing a barrier configuration about 5 kcal/mol above the PES minima (i.e., a dilation energy of about 2 kcal/mol plus the 3.2 kcal/mol saddle-point energy). The computed zero-point levels of C412H4 (1-D modeling) are separated by E1-E0=11 cm−1 to suggest ZP tunneling rates reaching the picosecond time scale. The analysis exposes new aspects of the intramolecular dynamics of CB, and the proposed automerization mechanism accounts for all of the presently known information pertaining to this interesting property of the cyclobutadiene molecule.
Molecular radiative transport. III. Experimental intensity decays
Authors: Nunes Pereira Eduardo J. , Berberan-Santos Mário N. , Fedorov Aleksandre , Vincent Michel , Gallay Jacques , Martinho José M. G.
Published: July 2002

A critical experimental test of a previously developed theory of molecular radiative transport is described. It is concluded that the theory gives an accurate description of the effect of radiative transport on fluorescence observables. The numerical coefficients of the fluorescence decay are computed from a Monte Carlo integration procedure that mimics the photon trajectories inside a realistic sample cell, and is carried out only using known molecular and geometrical parameters. The predicted parameters are confronted with the experimental observables accessible in a typical single-photon timing experiment, rhodamine 101 in ethanol being the system studied. The theoretical predictions quantitatively describe the effects of concentration and excitation and emission wavelengths experimentally observed in optical dense nondiffusing media for the two most common geometric arrangements: front-face and right-angle detection. It is shown that radiative transport leads to spatially heterogeneous fluorescence kinetics, as a direct consequence of the existence of a spatial distribution function of electronic excitation inside the sample cell. The agreement between theory and experimental results is good, with the average decay times predicted within ≃3% accuracy for front-face detection.
A simple weighted-density-functional method: Test and its application to hard sphere fluid in spherical cavity
Authors: Zhou Shiqi
Published: July 2002

A simple weighted-density approximation (SWDA) based on both local and bulk densities at the level of the first functional derivative of the system free energy is proposed. Its applicability to highly inhomogeneous hard-sphere fluid is verified by using Percus’ idea to investigate the equilibrium structure of a uniform hard-sphere fluid. Then, the SWDA is used to investigate the density profiles of hard-sphere fluid in spherical cavity, its prediction is compared favorably with simulation data.
Excitation and decay of N2(B 3Πg,v) states in a pulsed discharge: Kinetics of electrons and long-lived species
Authors: De Benedictis S. , Dilecce G. , Simek M.
Published: July 2002

The vibrational excitation and decay of N2(B 3Πg) state has been investigated in a N2 pulsed rf discharge. The effect of the pulsing frequency and the duty cycle on the N2 (B,v=1–12) vibrational distribution, obtained from the 1PG spectra taken at different times in discharge and afterglow, has been examined in the N2 pressure range 2.4 mTorr–1.5 Torr. The measured (B,v) distributions have been analyzed by a steady-state kinetic model taking into account the main excitation processes, like the electron impact from N2(X 1Σg+,v) and N2(A 3Σu+,v) states, the associative excitation of N2(X,v) with N2(A,v), the pooling by N2(A,v) molecules, the atomic recombination of N(4S), as well as the quenching processes. For the various processes, experimental state-to-state rate coefficients from the literature and/or calculated data sets have been used. Measured N2(A,v) and electron energy distribution functions, and estimated N2(X,v) distributions have been used as input data for the model. The model satisfactorily reproduces almost all the measured distributions. The kinetic analysis evidences that the shape of the (B,v=1–12) distribution depends on the competition between the different processes and in turn on the time variation of both density and the degree of internal excitation of the reaction species with discharge repetition rate, duty cycle, and pressure. In general, in the discharge the electron impact is always the predominant excitation mechanism, while the processes involving long-lived species: N2(A,v) and N2(X,v) are important in the postdischarge regime. Under conditions of high metastable density, the associative collision processes contribute to (B,v) excitation in the discharge, and the electron impact process from the A and X states gives a non-negligible contribution to the low v levels in the millisecond time scale afterglow. An analysis of the B quenching is carried out.