bluuebunny/crossref_metadata_2025 huggingface dataset:
Production and unimolecular decay rate of rotationally selected polyatomic molecular ions
Authors: Kiermeier A. , Kühlewind H. , Neusser H. J. , Schlag E. W. , Lin S. H.
Published: July 2002

We present the first example of unimolecular decay rates of a polyatomic system in which reactions are rotational state selected. Internal energy specification, including the selection of the J-rotational angular momentum quantum number, is achieved via a two laser pump–pump experiment. The first laser pumps selected rotational features in the 601 transition of the neutral molecule and creates vibrationally and rotationally state selected ions following 1+1 two-photon absorption. A second laser further excites the state selected ions to an energy of 5.3 eV which is above the dissociation threshold for four competing decay channels of the benzene cation. Slow reactive decay of these ions is observed in a reflectron time-of-flight mass spectrometer and the total decay rate constant k(E,J) is measured as a function of J while keeping the energy of the second laser constant. The results are compared with calculations based on a modified RRKM model that includes consideration of the J dependence of the decay rate.
Correlated van der Waals coefficients for dimers consisting of He, Ne, H2, and N2
Authors: Rijks W. , Wormer P. E. S.
Published: July 2002

Time-dependent coupled Hartree–Fock frequency-dependent polarizabilities have been corrected for true correlation effects by means of many-body perturbation theory. Polarizabilities have been computed for the monomers He, Ne, H2, and N2 through second order in the correlation potential. With these polarizabilities as input the van der Waals coefficients of all possible dimers have been obtained by the use of the Casimir–Polder relation.
Optogalvanic spectrum of CO. II. The rotational structure of the L 1Π state
Authors: Sekine Shigeyuki , Masaki Takashi , Adachi Yukio , Hirose Chiaki
Published: July 2002

The Q branch and P, R branches of the 4pπL 1Π–3sσB 1Σ+ transition of CO have been newly observed by high resolution and low resolution optogalvanic spectroscopy, respectively. The value of the Λ-type doubling constant in the L 1Π state has been determined as q=0.0213±0.0012 cm−1. An accidental perturbation was discovered in the Π− component of the L 1Π state. The Angstrom (0–3) and (0–4) bands were also observed.
Porosity for the penetrable-concentric-shell model of two-phase disordered media: Computer simulation results
Authors: Lee Sang Bub , Torquato S.
Published: July 2002

Computer-simulation results are reported for the porosity of a model of two-phase random media composed of identical D-dimensional spheres (D=2 or 3) distributed with an arbitrary degree of impenetrability λ, 0≤λ≤1; λ=0 corresponding to randomly centered or ‘‘fully penetrable’’ particles and λ=1 corresponding to totally impenetrable particles. We specifically consider the D-dimensional penetrable-concentric-shell model in which each sphere of diameter σ is composed of a mutually impenetrable core of diameter λσ, encompassed by a perfectly penetrable concentric shell of thickness (1−λ)σ/2. We develop two independent techniques to sample for the porosity. Simulation results agree with known exact results for the extreme limits of λ=0 and λ=1 up to three significant figures. The results for intermediate λ are new and compare favorably with approximate analytical expressions obtained by Rikvold and Stell.
The laser excitation spectrum of CH2Se and CD2Se in the near infrared
Authors: Judge R. H. , Clouthier D. J. , Moule D. C.
Published: July 2002

The polymers of CH2Se and CD2Se were pyrolyzed at 700 °C to yield monomeric selenoformaldehyde. Laser excitation spectra of these molecules were recorded. The origin of the weak singlet–singlet Ã 1A2←X̃ 1A1 system in CD2Se was observed at 13 631.5 cm−1 while that of CH2Se has been calculated to be at 13 555 cm−1. The much more extensive and intense singlet–triplet ã 3A2←X̃ 1A1 systems of CH2Se/CD2Se have origins at 12 171.0/12 262.7 cm−1. A double minimum potential energy function was used to fit the triplet state ν′4 inversion levels and was found to be similar to that observed for thioformaldehyde.
A systematic theoretical study of harmonic vibrational frequencies: The single and double excitation coupled cluster (CCSD) method
Authors: Besler Brent H. , Scuseria Gustavo E. , Scheiner Andrew C. , Schaefer Henry F.
Published: July 2002

Recently developed analytic CCSD gradient methods have been used to predict the harmonic vibrational frequencies of six molecules: CH4, NH+4, HCN, C2H2, HNC, and CO2. In every case a double zeta plus polarization (DZ+P) basis set of size C,N,O(9s5p1d/4s2p1d), H(4s1p/2s1p) was used. Previous analogous studies of H2O, H2CO, and NH3 are extended to form a statistical base of nine molecules. For these molecules 28 harmonic vibrational frequencies (out of total of 35 fundamentals) are thought to be known from experiment. The average errors with respect to experiment were found to be 9.1% (DZ+P self-consistent field), 3.7% (DZ+P configuration interaction including single and double excitations), and 2.2% (DZ+PCCSD). These statistics should provide guidance for the use of the CCSD method in situations where experimental vibrational frequencies are not available. Infrared intensities are also compared with available experimental data.
New equations of motion for molecular dynamics systems that change shape
Authors: Cleveland Charles L.
Published: July 2002

In the methods of molecular dynamics a large system is frequently subdivided into smaller regions which are periodic replications of each other. This subdivision defines the ‘‘computational cell’’ and may be performed in a number of equivalent ways, all producing cells of different shapes but of the same size and containing equivalent sets of particles. We consider the requirement that the form of the equations of motion be invariant under transformations connecting such equivalent computational cells. Since none of the equations of motion of the Parrinello–Rahman family proposed so far satisfy this condition, we construct some which do. These new equations are more practically useful than their antecedents for studies of systems whose computational cells may undergo extensive changes in shape, especially for studies of yield and flow under an applied stress.
The 458 nm diffuse band of the lithium dimer
Authors: Bahns John T. , Stwalley William C. , Pichler Goran
Published: July 2002

Laser induced fluorescence studies of the lithium diffuse band at 458 nm were performed using single-and double-photon excitations. Single-photon excitation was performed using UV lines of Ar++ and Kr++ lasers, and the diffuse band fluorescence was induced by collisions in 7Li2 and 6Li2. The double-photon excitation process populated relevant upper states through several accidental coincidences for two-step excitation in 7Li2. In two cases, in addition to structured continuum emission, an adjacent line spectrum was observed. In five other cases, two-photon accidental coincidences produced collision induced fluorescence of the 7Li2 diffuse band. We also found (in a few cases) that the (two-photon) excitation energy of the 2 3Πg state (with respect to the 1 3Σ+u state) was collisionally transferred to simultaneously excited lithium 2p atoms, thus creating a lithium atom in the 4d state.
Microscopic observation of gas-like, liquid-like, and crystal-like distributions of deionized colloidal spheres and the importance of the Debye-screening length
Authors: Okubo Tsuneo
Published: July 2002

A metallurgical microscope is used to observe directly the gas-like, liquid-like, and crystal-like structures of deionized polystyrene spheres (605 and 780 nm in diameter, specific gravity=1.05) both in sedimentation equilibrium and in mixed homogeneous suspension. The two-dimensional distance distribution function is obtained for these three phases. The center-to-center intersphere distances (D) in the liquid-like and crystal-like structures, which are estimated after deionization is complete, are explained by the effective hard-sphere model; a colloidal sphere is coated with an electrical double layer. D continues to decrease as the initial concentration of the spheres increases and from these data rigidities for the liquid-like (1.1 and 0.67 Pa) and crystal-like structures (8.3 and 5.8 Pa) are estimated. The way D is influenced by concentration shows that electrostatic interparticle repulsion and the elongated Debye-screening length around the spheres are both essential for the appearance of the gas-like, liquid-like, and crystal-like structures.
Infrared-microwave double resonance and diode laser spectroscopy of the ν1/ν3 dyad of SnH4
Authors: Jörissen Ludwig , Ohshima Yasuhiro , Matsumoto Yoshiyasu , Takami Michio , Kuchitsu Kozo
Published: July 2002

The interacting ν1 and ν3 bands of SnH4 have been studied by infrared absorption and infrared-microwave double resonance spectroscopy. The infrared spectrum has been taken with Doppler-limited resolution using a tunable diode laser. Pure rotational transitions within the ν3 state and rovibrational transitions between the ν1 and ν3 state have been observed by infrared-microwave double resonance. The infrared and microwave data have been fitted simultaneously to the ν1 and ν3 Hamiltonian coupled by one vibration–rotation interaction term. Seventeen spectroscopic constants have been determined for each of the five most abundant isotopic species.
Determination of the interatomic potential from elastic differential cross sections at fixed energy: Functional sensitivity analysis approach
Authors: Ho Taksan , Rabitz Herschel
Published: July 2002

Elastic differential cross sections in atomic crossed beam experiments contain detailed information about the underlying interatomic potentials. The functional sensitivity density of the cross sections with respect to the potential δσ(θ)/δV(R) reveals such information and has been implemented in an iterative inversion procedure, analogous to that of the Newton–Raphson technique. The stability of the inversion is achieved with the use of the regularization method of Tikhonov and Miller. It is shown that given a set of well resolved and noise-free differential cross section data within a limited angular range and given a reasonable starting reference potential, the recovered potential accurately resembles the desired one in the important region, i.e., the region to which the scattering data are sensitive. The region of importance depends upon the collision energy relative to the well depth of the potential under study; usually a higher collision energy penetrates deeper into the repulsive part of the potential and thus accordingly yields a more accurate potential in that part. The inversion procedure produces also a quality function indicating the well determined radial region. Moreover, the extracted potential is quite independent of the functional form of the reference potential in contrast to curve fitting approaches. As illustrations, the model inert gas systems He–Ne and Ne–Ar have been considered. For collision energies within an order of magnitude of the associated potential well depth, the attractive part of the potential can be determined to high precision provided that scattering data at small enough angles are available. On the other hand, the repulsive part of the potential must be scrutinized by high collision energy data.
Quenching and energy transfer processes of single rotational levels of Br2 B 3Π(0+u) v′=24 with Ar under single collision conditions
Authors: Yamasaki Katsuyoshi , Leone Stephen R.
Published: July 2002

State-specific total quenching rate constants are measured for selected rotational levels of Br2 under single collision conditions with argon at 296 K. A strict criterion is used to obtain single collision conditions in a cell experiment. A 0.04 cm−1 bandwidth, etalon-narrowed pulsed dye laser excites single rovibronic transitions of the B 3Π(0+u) state, and fluorescence decay traces with and without the argon collision partner are analyzed at early times to extract total quenching rate constants. The rotational levels that are initially prepared are J′=26, 32, 37, 41, 46, and 58. The total quenching rate constants dramatically decrease with increasing J′: for J′=26/32/37/41, respectively, they are =8.7×10−11/4.4×10−11/3.3×10−11/7.6×10−12 cm3 molecule−1 s−1. The effect of rotational excitation on the quenching is interpreted to be due to the decreased probability of forming collision complexes. In contrast to the levels J′<41, negative quenching rate constants are observed for J′>41, i.e., the decay rates with Ar are slower than those without. This result can be explained by taking into account the possibility of R↔V energy transfer processes in which a high rotational level is transferred to a lower rotational state in the next higher vibration and the lower rotational state has a much slower spontaneous predissociation rate.
Diffusion theory of multidimensional activated rate processes: The role of anisotropy
Authors: Kl/osek-Dygas M. M. , Hoffman B. M. , Matkowsky B. J. , Nitzan A. , Ratner M. A. , Schuss Z.
Published: July 2002

We consider an anisotropic multidimensional barrier crossing problem, in the Smoluchowski (diffusion) limit. The anisotropy arises from either or both the shape of the potential energy surface and anisotropic diffusion. In such situations, the separatrix, which separates reactant and product regions of attraction, does not coincide with the ridge of the potential surface, which separates reactant and product wells, thus giving rise to a complicated time evolution. In the asymptotically long time limit, the time evolution is governed by crossing the separatrix and is exponential with a rate which may be obtained as a generalization of Kramers’ theory to the anisotropic situation. In contrast, in long, though not asymptotically long times, the time evolution is dominated by repeated crossings of the ridge, and is nonexponential. Such nonexponential time evolution has been observed in many biochemical reactions, where many degrees of freedom and anisotropic diffusion processes lead to complicated dynamical behavior. Our model provides a simple prototype of such situations.
Converged quantum dynamics calculations for the F+H2 reaction on the well-studied M5 potential-energy surface
Authors: Yu Chin-hui , Sun Yan , Kouri Donald J. , Halvick Philippe , Truhlar Donald G. , Schwenke David W.
Published: July 2002

We present converged quantum dynamics calculations for the reaction F+H2(vi=0)→HF(vf=0,1,2,3)+H, where vi and vf denote initial and final vibrational quantum numbers, on potential-energy surface no. 5 of Muckerman.
Rotational analysis of Ã 2A1↔X̃ 2E electronic transition of the jet-cooled methylthio radical
Authors: Hsu Yen-Chu , Liu Xianming , Miller Terry A.
Published: July 2002

A set of molecular parameters describing both the X̃ and Ã states of CH3S, has been obtained by a joint fitting of the rotationally resolved electronic transitions observed in a free-jet-cooled laser-induced fluorescence study of CH3S and an earlier microwave study of its X̃ state. The present work shows that because of incomplete information, nearly all of the previously reported molecular parameters for CH3S must be significantly revised. The present observations show an unusual electronic structure for the radical, characterized by a short C–S bond distance and peculiar methyl group geometry in the ground state. The C–S bond is observed to lengthen markedly in the excited Ã state.
The intensity of the 105–000 transition of HCN
Authors: Smith Alice M. , Klemperer William , Lehmann Kevin K.
Published: July 2002

We have remeasured the intensity of the anomalously strong 105–000 band in HCN using long-path length absorption and a cw dye laser. Our initial measurement of this intensity using calibrated intracavity photoacoustic spectroscopy was challenged by a low-resolution photoacoustic spectrum which gave a ratio of intensity of the 105–000 to the 006–000 band that was close to the ratio of 1.5 predicted by theory. Our present result of 16.7 cm/mol, is 20% larger than our earlier result, yielding an intensity ratio of the 105–000 to the 006–000 of 7, which increases the disagreement with theory.
A finite temperature theory of rotationally inelastic diffraction: H2, HD, and D2 on Cu(100)
Authors: Cruz Astrid J. , Jackson Bret
Published: July 2002

The rotationally inelastic diffraction probabilities for H2, HD, and D2 from Cu(100) were computed as a function of surface temperature. The surface is treated in a quantum mechanical fashion using a recently developed formalism. The center of mass molecular translational motion is treated semiclassically, using Gaussian wave packets (GWPs), and the rotations are described quantum mechanically. Strong attenuation of the phonon elastic diffraction peaks with temperature is observed. This Debye–Waller-like attenuation increases with increasing molecular mass and kinetic energy, and decreases as the peaks become more off-specular. The phonon summed rotation–diffraction probabilities show a moderate temperature dependence for the most part. The 0→2 rotational excitation of D2 appears to be strongly phonon assisted above 300 K. At low temperatures our method reproduces the selection rules predicted by previous studies. As the temperature is increased these selection rules become less restrictive. The probability distribution for a scattering molecule exchanging an amount of energy ΔE with the surface was also computed. Rayleigh phonons were found to dominate the energy transfer, with bulk vibrations becoming more important for larger molecular masses, beam energies, and surface temperatures.
Observation of collision-induced-dipole absorption bands in strontium–rare-gas mixtures. I. The 5s–4d bands
Authors: Ueda K. , Komatsu T. , Sato Y.
Published: July 2002

Reduced absorption coefficients have been measured for the collision-induced-dipole (CID) absorption bands associated with the 5s–4d transition of atomic strontium perturbed by He, Ne, Ar, Kr, Xe, and Sr. The main CID absorption band, assigned to 5sΣ–4dΣ, extends from the position of the atomic 5s–4d transition towards the shorter-wavelength side for every perturber. For Xe and Kr, an additional small absorption band is seen at the foot of the atomic 5s–4d transition. The former 5sΣ–4dΣ band is interpreted to result from collisional mixing of the 5p state of atomic Sr into the molecular 4dΣ state for small internuclear separations, while the latter band is interpreted to result from the collisional quadrupole (strontium)–dipole (Xe or Kr) interaction.
X-ray diffraction study of liquid n-butane at 140 and 267 K
Authors: Habenschuss A. , Narten A. H.
Published: July 2002

X-ray diffraction data for liquid n-butane near the triple and boiling points are presented. The data are analyzed using scattering factors for −CH3 and −CH2 groups which permits extraction of structure and distribution functions for carbon sites. An intramolecular carbon–carbon distance at approximately 3.1 Å is assigned to a short end-to-end distance in a near-gauche conformation, which is in agreement with the electron diffraction results for gaseous n-butane. The intermolecular pair distribution functions show a large difference in the liquid structure for the low and high temperature states, similar to that found in liquid propane.
Outer-sphere electron transfer reactions in non-Debye solvents. Theory and numerical results
Authors: Fonseca Teresa
Published: July 2002

In this paper we present a theory that predicts, for a wide range of reaction adiabaticity, the time evolution of the reactants’ population, σR(t), for outer-sphere electron transfer reactions in polar solvents exhibiting a non-Debye relaxation behavior. A number of stochastic trajectory studies have been performed to test the validity of the theory in the limit where solvent relaxation constitutes the rate-limiting step. The agreement between the numerical and theoretical results is excellent. The theory is then used to study a number of different reactive regimes ranging from nonadiabatic to solvent-controlled adiabatic limits. The studies performed show that the character of the decay of σR(t) for low activation barrier reactions constitutes a useful qualitative way of identifying a priori the dielectric relaxation properties of the solvent: For a non-Debye solvent the decay of σR(t) is strongly nonexponential in contrast to the results for Debye solvents where σR(t) decays mainly exponentially. The solvent relaxation time scales that are important for the reaction kinetics are identified. Our results are also expressed in terms of rate kernels. For high energy barriers the Stable States Picture results for the forward and reverse rate constants are recovered.
Layer resolved spectroscopy of potassium adsorbed on a Ru(001) surface: Photoemission and thermal desorption study
Authors: Hrbek J. , Shek M.-L. , Sham T. K. , Xu G.-Q.
Published: July 2002

High-resolution photoemission spectroscopy and thermal desorption were used to study the coverage dependence of the K 3p3/2,1/2 core levels of K overlayers on Ru(001). Three doublets were observed to evolve sequentially and to shift to higher binding energy with increasing coverage of potassium. The doublets were assigned to photoemission from the interface (first layer), ‘‘bulk’’ (second layer), and surface (third layer). Spin–orbit splitting was observed for the first time in the condensed potassium phase by photoemission. The results are discussed in terms of the equivalent core approximation using a Born–Haber cycle. In the thermal desorption data three coverage regimes can be distinguished: a first layer desorbing with first-order kinetics and a strongly decreasing heat of adsorption (EA =2.80–0.87 eV); a second layer with zero-order desorption kinetics and EA =0.78 eV; a third layer and multilayers with the same kinetic order but with EA increasing from 0.78 to 0.93 eV, which is close to the heat of sublimation of potassium.
Changes in electronic polarizability densities due to shifts in nuclear positions, and a new interpretation for integrated intensities of vibrational Raman bands
Authors: Hunt K. L. C. , Liang Y. Q. , Nimalakirthi R. , Harris R. A.
Published: July 2002

The nonlocal polarizability density α(r;r′,ω) is a linear-response tensor that determines the electronic polarization induced at point r in a molecule, by an external electric field of frequency ω, acting at r′. This work focuses on the change in α(r;r′,ω) when a nuclear position shifts infinitesimally. We prove directly that the electronic charge distribution responds to the change in Coulomb field due to the nucleus via the same hyperpolarizability density that describes its response to external fields. This generalizes a result found previously for the static (ω=0) polarizability density. The work also provides a new interpretation for the integrated intensities of vibrational Raman bands: it proves that the intensities depend on the hyperpolarizability densities and the dipole propagator.
Observation of the ν3 fundamental bands of HOSi+ and DOSi+
Authors: Moazzen-Ahmadi N. , McKellar A. R. W. , Warner H. E. , Amano T.
Published: July 2002

The ν3 (O–Si stretch) fundamental bands of the HOSi+ and DOSi+ molecular ions in the 9 μm region have been detected for the first time, using a tunable infrared diode laser spectrometer and a hollow cathode discharge cell. Analysis of the results yielded accurate values for the molecular rotational and centrifugal distortion parameters, as well as for the band origins, which are 1127.009 cm−1 for HOSi+ and 1103.112 cm−1 for DOSi+ . The ground vibrational state parameters are in excellent agreement with those determined from the ν1 bands of the two isotopes.
Diode laser spectroscopy of the hydrogen bond vibration ν2 OC---HF in a continuous wave supersonic jet
Authors: Wang Zhongcheng , Bevan John W.
Published: July 2002

The high resolution spectrum of the ν2 (C≡O) stretching vibration in the hydrogen bonded dimer, OC---HF, has been recorded in a continuous wave (cw) supersonic molecular jet using a diode laser spectrometer. Spectroscopic analysis gives the following rovibrational parameters (in cm−1): ν0=2167.69 9 04(11); B0=0.102 200 647(13); D0J =3.244(18)×10−7; B2=0.101 552 5(15); D2J =3.449(36)×10−7. Investigation of observed line profiles allows a lower limit of 0.68 ns to be made for the excited state vibrational predissociative lifetime.
Application of excluded volume map sampling to phase equilibrium calculations in the Gibbs ensemble
Authors: Stapleton M. R. , Panagiotopoulos A. Z.
Published: July 2002

An improved method for studying liquid–liquid phase equilibria in the Gibbs ensemble is described. The improvement is based on the excluded volume map sampling (EVMS) technique, recently introduced by Deitrick et al., which has been shown to increase the liquid density range over which the Widom particle insertion method can be applied for the calculation of the chemical potential. Application of EVMS in the Gibbs ensemble greatly improves the efficiency of the particle interchange moves in a Gibbs simulation. We first demonstrate the improved method with Gibbs simulations of liquid-liquid phase equilibria in a two-component Lennard-Jones fluid. The map method results in a significant reduction in the computing time required for these simulations when the number of attempted interchanges is large. A detailed simulation study of fluid–fluid phase equilibria in a model nitrogen-helium mixture was performed using the EVMS Gibbs technique. Typically, a Gibbs simulation of liquid–liquid equilibrium in this fluid, using the map method, requires only 40% of the time used by a conventional Gibbs ensemble simulation. The results demonstrate that it is possible to determine the form of complex phase diagrams using computer simulation. In this work we calculate the pressure-composition sections of the phase diagram close to the temperature minimum of gas–gas immiscibility. The potential model we used was a Lennard-Jones plus quadrupole interaction, with parameters the same as in a recent theoretical study of this fluid mixture. Comparison of our simulation results with the theoretical work shows that the perturbation theory consistently overestimates the mole fraction of nitrogen in the coexisting phases. At the temperatures and pressures studied the simple potential model predicts thermodynamic properties in surprisingly good agreement with experimental data, even though the potential parameters were not adjusted in any way.
Temperature dependence of self-diffusion in compressed monohydric alcohols
Authors: Karger N. , Vardag T. , Lüdemann H.-D.
Published: July 2002

The p,T-dependence of the self-diffusion coefficient D for methanol, methan(2H)ol and ethanol has been studied between 150 and 450 K at pressures up to 250 MPa. The experiments were performed in strengthened high-pressure glass cells by the application of the nuclear magnetic resonance (NMR) spin-echo technique with pulsed magnetic-field gradients. Upon cooling, molecular mobility is strongly reduced, leading to a pronounced non-Arrhenius temperature dependence of D. Applying the rough hard-sphere model (Chandler) to our data, a dramatic decrease of the A-parameter with falling temperature is observed. This behavior indicates that attractive intermolecular interactions dominate translational mobility. The best description of the data is given by the empirical Vogel–Tammann–Fulcher (VTF) equation, with ideal glass transition temperatures T0, that are in excellent agreement with those obtained from calorimetric studies. The isotope effect for self-diffusion in methanol and methan(2H)ol increases from ∼5% at high temperatures to about 40% in the supercooled region. This drastic increase is assumed to originate from a difference in hydrogen bond strength of the isotopes, as has already been found for light and heavy water.
Periodic orbits and diffuse structures in the photodissociation of symmetric triatomic molecules
Authors: Schinke Reinhard , Engel Volker
Published: July 2002

We reinvestigate the collinear photodissociation of a symmetric triatomic molecule studied a decade ago by Kulander and Light [J. Chem. Phys. 73, 4437 (1980)], within the time-dependent formalism. The diffuse structures in the absorption spectrum are uniquely related to three, well separated recurrences which, in turn, are explained by three generic periodic orbits of the corresponding classical Hamiltonian. One of these orbits describes pure symmetric stretch motion while the other two orbits combine symmetric stretch and hyperspherical motion. The applicability of simple one-dimensional, zeroth-order pictures is reviewed in the light of these new results.
Gas phase acidities and molecular geometries of H3SiOH, H3COH, and H2O
Authors: Sauer Joachim , Ahlrichs Reinhart
Published: July 2002

Theoretical predictions are made by the coupled pair functional method employing large basis sets, [6s,5p,2d,1f/5s,3p,2d,1f/3s,2p] for geometries and [6s,5p,2d,1f/8s,5p,3d,1f/4s,2p] for deprotonation energies. The geometries obtained for H2O and CH3OH agree with the observed ones within the experimental uncertainty limits. The predicted equilibrium geometry (picometers, degrees) for silanol is r(SiO)=165.0, r(OH)=95.76, r(SiH)=146.8 (in plane), r(SiH)=147.6 (out of plane), angle (SiOH)=117.7. Nuclear motion corrections to geometries and vibrational contributions to deprotonation energies are considered. The predicted gas phase acidities (ΔH0298) are 392.3 (H2O), 384.5 (CH3OH), and 359.3 (SiH3OH) kcal/mol. The differences are discussed in terms of the potential on the acidic proton as well as of contributions from rearrangement of electrons and relaxation of nuclei. The remaining basis set error is assessed on calculations for H2O which employ a [10s,8p,4d,2f,1g/7s,3p,1d] basis set. The best theoretical estimate of ΔH0298 for H2O is 391.0±2.5 kcal/mol.
Rotational spectra and structures of small clusters containing the HCN dimer: X–(HCN)2 with X=CO, N2, NH3, and H2O
Authors: Ruoff R. S. , Emilsson T. , Chuang C. , Klots T. D. , Gutowsky H. S.
Published: July 2002

This work is the counterpart of a previous report on the (HCN)2–Y trimers with Y=HF, HCl, HCF3, and CO2 [J. Chem. Phys. 90, 4069 (1989)]. Rotational spectra have been observed for several isotopic species of the OC–, N2–, H3N–, and H2O–(HCN)2 trimers, using a pulsed nozzle, Fourier transform Balle/Flygare microwave spectrometer. The structures are basically composites of those reported for the (HCN)2 and X–HCN dimers. The trimers are effectively axially symmetric, but have some shrinkage of dimensions. Rotational constants found for the main isotopic species of each trimer are: For X=OC, a B0 of 421.142 MHz and DJ of 110 Hz; for X=N2, 435.573 MHz and 155 Hz; for X=H3N, a symmetric top, a B0 of 675.777 MHz, DJ of 180 Hz, and DJK of 41.1 kHz; and for X=H2O, with C2v symmetry, a (B0+C0)/2 of 667.028 MHz, (B0−C0)/2 of 0.617 MHz, DJ of 173 Hz, and a DJK of 62.9 kHz. The rotational constants for the isotopic species of each trimer were used to determine the distances r1 and r2 between the centers of mass (c.m.) of adjacent monomers, r1 being that for X–HCN and r2 that for (HCN)2. For X=OC, N2, H3N, and H2O the shrinkages found in r1 are 0.068, 0.056, 0.084, and 0.074 Å, respectively, and in r2 0.013, 0.013, 0.044, and 0.026 Å. The 14N quadrupole coupling constants were determined by selective 15N substitution for most of the nitrogen sites in the trimers. The effects of charge redistribution in the trimers were separated from those of torsional oscillations in several instances including N2 in N2–(HCN)2.
High resolution S1←S fluoescence excitation spectra of the 1- and 2-hydroxynaphthalenes. Distinguishing the c
i
s and t
r
a
n
s rotamers
Authors: Johnson J. R. , Jordan K. D. , Plusquellic D. F. , Pratt D. W.
Published: July 2002

Both fluorescence excitation and dispersed emission techniques have been used to study the S1←S0 electronic spectra of 1- and 2-hydroxynaphthalene (1/2HN) in the collision-free environments of a supersonic jet and a twice-skimmed molecular beam, using both pulsed and high-resolution cw lasers operating in the ultraviolet. In the jet experiments, we observe that each molecule exhibits two electronic origins, separated by 274 cm−1 in 1HN and by 317 cm−1 in 2HN. In the beam experiments, we resolve the rotational structure of each of the four bands and determine the inertial constants of all eight zero-point vibrational levels, accurate to ±0.1 MHz. We also determine the orientations of the four optical transition moments in the molecular frame. Significant differences in both the inertial constants and the transition moment orientations are observed in each band. Similar experiments have been performed on the hydroxy-deuterated 1/2HN (1/2DN).

A comparison of the results obtained for 1/2DN with those for the corresponding bands in 1/2HN makes possible the determination of the center-of-mass coordinates of the hydroxy hydrogen in both electronic states, accurate to ± 0.02 Å. Differences in these coordinates reveal that the two electronic origins in each spectrum are caused by the presence of two N–O–H(D) rotamers in both 1H(D)N and 2H(D)N, one with a cis (or syn) geometry and one with a trans (or anti) geometry with respect to the naphthalene frame. We make an unambiguous assignment of each origin to a specific rotamer. The lower energy origin in the spectrum of 1HN is that of the cis rotamer, whereas the lower energy origin in the spectrum of 2HN is that of the trans rotamer. We then use these results, together with those of ab initio calculations on the ground electronic states of all four isomers, to explore the reasons for the differences in their energies, to account for the orientations of their transition moments, and to specify other features of the S0 and S1 potential energy surfaces along the cis–trans isomerization coordinate. Motion along this coordinate requires significant displacement of the oxygen atom and selected ring hydrogens as well as rotation about the C–O bond, in both electronic states.
Role of local configurations in a Langmuir–Hinshelwood surface reaction: Kinetics and compensation
Authors: Kang H. C. , Jachimowski T. A. , Weinberg W. H.
Published: July 2002

We have employed Monte Carlo sampling to calculate the rate coefficient of a Langmuir–Hinshelwood reaction between species A and B on a square lattice. The experimental situation that is simulated is the reaction between a preadsorbed overlayer of species A with species B. The preadsorbed overlayer of A is allowed to equilibrate prior to the adsorption of B. Upon adsorption of B, the initial reaction rate is calculated assuming that A is irreversibly adsorbed and immobile, and that the equilibrium between adsorbed B and gas-phase B is established much more rapidly than the time scale of the reaction between A and B. Reaction is allowed only between nearest-neighbor AB pairs. We examine the parametrization of the reaction rate coefficient into an effective activation energy and an effective preexponential factor.

We find that correlations between nearest-neighbor particles affect the reaction rate coefficient significantly. We also find that if the distribution of local configurations of nearest-neighbor pairs of reactant particles changes with temperature, the corresponding Arrhenius plot is nonlinear. The effective activation energy and the effective preexponential factor vary strongly with the fractional coverage of A and show a large compensation effect, similar to that observed experimentally in many desorption and surface-reaction systems. We conclude that variations in the distribution of local configurations of pairs of reactant molecules is a function of temperature and fractional surface coverage can be responsible for these experimentally observed compensation effects.
Instabilities of hydrogen bond network in liquid water
Authors: Sasai Masaki
Published: July 2002

A new model of the hydrogen bond network in liquid water is developed. Using the technique of polymer science, the grand canonical partition function of water is expressed by the functional integral of the field variables. The method is applied to anomalies of liquid water. Using the mean-field lattice-gas approximation it is shown that there are two instability lines, one in the supercooled region and the other in the stretched (P<0) region. Approaching the instability line in the supercooled region, the fluctuation of ice-likeness grows anomalously large and liquid water becomes unstable to be ice. Approaching the instability line in the stretched region, the fluctuation of the concentration of non-hydrogen-bonded sites grows large and liquid water becomes unstable to be gas. Divergence of the thermodynamic response functions and the mean-field values of exponents are discussed.
Wave functions with terms linear in the interelectronic coordinates to take care of the correlation cusp. II. Second-order Mo/ller–Plesset (MP2-R12) calculations on closed-shell atoms
Authors: Termath Volker , Klopper Wim , Kutzelnigg Werner
Published: July 2002

The MP2-R12 method in approximations A and B as outlined in part I of this series is applied to the ground states of the closed-shell atoms He, Be, Ne, Mg, Ar, Ca, Cu+, Zn2+, and Kr, in terms of both STO and GTO basis sets. For He, Be, and Ne the partial wave increments of the various pairs are documented and compared with their conventional counterparts. The fast convergence of the partial wave increments, that go as (l+ (1)/(2) )−8 in the MP2-R12/B scheme, is demonstrated. From the MP2-R12 calculations more accurate estimates of the exact MP2 energies are possible than from the conventional partial wave expansion. The actually calculated values differ generally by a fraction of a 1% from the estimated basis sets limits if STO basis sets with l≤5 (in some cases l≤6) are used, while errors of typically 1% are obtained with GTO basis sets and l≤3.
Lattice dynamics and vibrational spectra of polythiophene. I. Oligomers and polymer
Authors: Lopez Navarrete J. T. , Zerbi G.
Published: July 2002

The vibrational dynamics of oligomers of thiophene is treated theoretically and the experimental infrared and Raman spectra are studied with the main purpose to derive information for the interpretation of the spectra of polythiophene. The optimized geometries and the vibrational force field are calculated for the oligomers Thn (n=1, 2, 3, 4, and 6) and the parameters are used for PTh. For PTh and two isotopic derivatives dispersion curves and k=0 phonons frequencies are calculated and a vibrational assignment is proposed.
A local projection method for the linear combination of atomic orbital implementation of density-functional theory
Authors: Yang Weitao
Published: July 2002

A local projection method is proposed for obtaining the solution of the Kohn–Sham equation within the density-functional theory of molecules. The method uses the conventional linear combination of atomic orbitals as the approximate orbitals. However, it does not require multicenter integration in the construction of algebraic eigenvalue equations, and thus significantly reduces the corresponding computational effort. The method is tested on the self-consistent calculations of nitrogen molecules, N2, with basis sets of various accuracies and found to give results which are as accurate as the conventional approach. It is also shown that the local projection method can be applied to the non-self-consistent calculation scheme and provides a most efficient method for carrying out this type of electronic structure calculations.
Volume relaxation of glasses
Authors: Mansfield Marc L.
Published: July 2002

The rate of volume relaxation of glasses is computed by assuming that the volume changes only because of the diffusion of free volume in and out of the sample through the surface. The relaxation curves are strongly affected by the assumed diffusion law, which means that such experiments provide a potentially powerful tool for determining the process by which free volume diffuses in glasses. For ordinary, i.e., Fick’s law, diffusion, the relaxation curves have exponential tails, while for anomalous diffusion of the sort introduced by Shlesinger and Montroll, the curves are predicted to have inverse-power law tails. Comparison with experimental results on glassy polymers permits us to rule out Fick’s law diffusion completely, and strongly supports the anomalous diffusion law of Shlesinger and Montroll.
The effect of impact angle and corrugation on gas–surface energy transfer and sticking: A semiclassical study
Authors: Jackson Bret
Published: July 2002

A semiclassical model is derived for the scattering of an atom from a metal surface. The surface and bulk vibrations are treated quantum mechanically, and the translational motion of the gas particle is represented by a Gaussian wave packet. Equations of motion are derived for the average position and momentum of this wave packet in terms of the parameters which describe the gas–surface and gas–phonon interaction potentials. Multiphonon interactions are included to all orders, and the motion of the gas particle parallel to the surface is coupled to the lattice vibrations. Probabilities for energy transfer and trapping are computed. The way in which the normal and parallel (to the surface) components of the beam energy are mixed by the corrugation and the thermal fluctuations is examined. The effect this has on the variation of the trapping probability with beam energy and angle of incidence is discussed.
Tunable far-infrared laser spectroscopy of deuterated isotopomers of Ar–H2O
Authors: Suzuki Sakae , Bumgarner R. E. , Stockman Paul A. , Green Peter G. , Blake Geoffrey A.
Published: July 2002

Several far-infrared vibration-rotation-tunneling transitions have been measured in deuterated isotopomers of Ar–H2O for the first time. These experimental results will enable the generation of improved intermolecular potential energy surfaces for the Ar–H2O system when combined with existing microwave, far-infrared, and infrared data.
Unimolecular dissociation of cyclopentadiene and indene
Authors: Yi Whikun , Chattopadhyay Arun , Bersohn Richard
Published: July 2002

The dissociation of hydrogen atoms from the methylene group of cyclopentadiene (CP) and indene (ID) excited with a 193 nm photon has been studied by hydrogen atom laser induced fluorescence. The rate of dissociation of IND was 7.4×106 s−1 but that of CP was too fast to measure. The ratio of H atoms to D atoms generated from 5-deuteriocyclopentadiene (5-dCP) was 3.91±0.46. Rice–Ramsberger–Kassel–Marcus theory was used to calculate the rates of dissociation of CP and 5-dCP. The quantum yield for dissociating H atoms from CP was 0.85±0.07. The ejected H atoms have a Maxwell velocity distribution with temperatures which are equal to the vibrational temperatures, 3690 and 2479 K for CP and IND, respectively. The most important result of the work is this confirmation of an earlier finding on a different set of molecules that the translational temperature of the fragments after the dissociation is equal to the vibrational temperature before the dissociation. This is explained by the assumption that the motion of the fast, light hydrogen atom is partly decoupled from that of the heavier, slower atoms.
Resonances and bound rovibrational levels in the interacting X, A, C, and D states of HeH, HeD, 3HeH, and 3HeD
Authors: van Hemert Marc C. , Peyerimhoff Sigrid D.
Published: July 2002

Potential energy curves are calculated for the ten lowest states in HeH which correlate with the hydrogen asymptote in the n=1, 2, 3 occupation; these are X, A, C, D, 5 2Σ+, 6 2Σ+, and B, E, 3 2Π as well as the 1 2Δ states. Multireference configuration interaction calculations are employed thereby in an atomic orbital (AO) basis of contracted Gaussians. Extensive calculations of the ∂/∂R, ∂2/∂R2, Lx, and L2 matrix elements are carried out to account explicitly for the effects beyond the Born–Oppenheimer approximation. The positions of rovibrational levels are thereby determined in pairwise close-coupling calculations for the X/A and C/D states of 2Σ+ symmetry for the four isotopomers 4HeH, 3HeH, 4HeD, and 3HeD. Radial, angular, and mass polarization corrections affect the A and C states differently, so that the A–C energy gap increases by 39 cm−1 in 3HeD and by 53 cm−1 in HeH upon introduction of these terms, e.g., whereby the contribution of the mass polarization is by far the smallest. By employing a two-parameter correction function to the calculated electronic potential energy and making use of the calculated non-Born–Oppenheimer terms, a large number of levels for the A, C, and D states as a function of (v,J) quantum numbers are computed which agree with those, which are experimentally available for the C–A and D–A transitions within wave number accuracy.
Nonadiabatic electronic interactions in the ion-pair states of NeICl
Authors: Stephenson Thomas A. , Hong Yujian , Lester Marsha I.
Published: July 2002

Nonadiabatic interactions in the NeICl van der Waals complex have been explored in the lowest energy triad of ICl ion-pair states (∼39 000 cm−1). Dispersed fluorescence measurements reveal emission characteristic of multiple ion-pair electronic states, with the relative contributions from the E(0+), β(1), and D′(2) states changing with the initial ICl vibrational excitation (vICl). Emission directly from NeICl (vICl=0) complexes indicates that the initially prepared NeICl levels have mixed electronic character and that the ICl electronic parentage changes with the initial van der Waals vibrational level selected. NeICl complexes prepared with 1–4 quanta of ICl stretch undergo rapid vibrational predissociation with a strong propensity for ΔvICl=−1 relaxation. The electronic state(s) populated in the ICl fragments differ from the mixed electronic character of the initially prepared level, demonstrating that vibrational predissociation is accompanied by nonadiabatic electronic state changing processes. The observed final state selectivity may be attributed to the relative strength of the nonadiabatic couplings between the initial NeICl bound state and the final ICl states or a momentum gap rationale based on the overlap between the NeICl bound state wave function and the highly oscillatory continuum wave function of the separating fragments.
Rovibrational states of Ar–HCN van der Waals complex: A localized representation calculation
Authors: Mladenović M. , Bac̆ić Z.
Published: July 2002

All bound rovibrational states of Ar–HCN and Ar–DCN van der Waals complexes for J=0–10 are calculated, assuming frozen HCN (DCN). The calculations are performed using the recently developed approach for accurate and efficient computation of highly excited rovibrational states of floppy triatomic molecules [M. Mladenović and Z. Bac̆ić, J. Chem. Phys. 93, 3039 (1990)]. Matrix representation of the Hamiltonian in body-fixed Jacobi coordinates is formed by combining the discrete variable representation of the angular coordinate and distributed Gaussian basis for the radial degree of freedom. The coupled low-frequency large amplitude vibrations are treated accurately, without any dynamical approximation. Model 2-D (R,θ) potential surface by Dykstra, having two minima at collinear Ar–HCN and Ar–NCH geometries, is employed. Besides energy levels and wave functions, for each state we calculate expectation values of Jacobi coordinates, 〈R〉 and 〈θ〉, degree of wave function delocalization, and effective rotational constants (for some states). Majority of states of Ar–H/DCN are delocalized over both potential minima. Comparison is made with available experimental data and possible refinements of the present potential surface are briefly discussed.