bluuebunny/crossref_metadata_2025 huggingface dataset:
Dual Lanczos vector space. II. Lanczos transformations and contracted descriptions of relaxation processes
Authors: Wassam W. A.
Published: July 2002

We consider the relationship of the dual Lanczos transformation method, introduced in paper I, to the projection operator methods of Mori and Zwanzig for treating relaxation processes. It is shown that Mori’s memory function formalism and ‘‘generalized’’ Langevin description of relaxation processes represents a special case in the reversible limit of the ‘‘global’’ dual Lanczos transformation method. Unlike Mori’s memory function formalism, the present results are not limited to the description of autocorrelation functions for reversible systems. Our results apply to auto- and cross-correlation functions, as well as, the ensemble average of observables for both reversible and irreversible systems. In fact, we are able to construct a continued fraction representation of both auto- and cross-correlation functions. Introducing a ‘‘bath’’ dual Lanczos transformation scheme, we show that the first bath dual Lanczos projection gives rise to the Zwanzig projection operator method for constructing an equation of motion for a reduced distribution function. Additional bath dual Lanczos projections allow us to go beyond the usual results obtained in applications of the Zwanzig method.
Rayleigh–Brillouin scattering studies of segmental fluctuations in poly(siloxanes)
Authors: Wang C. H. , Fytas G. , Zhang J.
Published: July 2002

New results from a detailed temperature dependent Rayleigh–Brillouin study of siloxane polymers are reported. The study includes the examination of the effect of the relaxation of the longitudinal stress modulus on the propagation behavior of the thermally driven acoustic wave in the 109 Hz frequency range. The Rayleigh–Brillouin spectra of poly(diphenyl siloxane), poly(phenylmethyl siloxane) as well as their model analogs, 1,3-tetraphenyl 1,3-dimethyl disiloxane, and 1,3-diphenyl 1,2-tetramethyl disiloxane are measured over a wide range of temperature covering from Tg to 240 K above Tg. The theory for Rayleigh–Brillouin scattering has also been reformulated in such a way that the parameters which enter the theory are in terms of easily recognizable thermodynamic and transport coefficients.
Low-frequency Raman investigation of lead-containing fluorozirconate glasses and melts
Authors: Walrafen G. E. , Hokmabadi M. S. , Guha S. , Krishnan P. N. , Tran D. C.
Published: July 2002

Raman measurements of fluorozirconate glasses and melts were conducted from −265 (8.5 K) to ∼800 °C for numerous compositions, e.g., from 30 mol % BaF2 (no PbF2) to 35 mol % PbF2 (no BaF2), at nearly constant ZrF4 content. The normalized amplitude of the nominal 45 cm−1 peak was found to increase linearly with increasing PbF2 from 0–35 mol %, as the peak frequency decreased slightly, from ∼47 to ∼43 cm−1.

From these observations, depolarization ratios, Pb–F force constants, etc., it was concluded that the 45 cm−1 peak involves a correlated motion of second-neighbor Pb2++Zr4+ or Ba2++Zr4+ cations vibrating against their edge- (or corner-) shared coordination cages of F− anions. Martin–Brenig-type analysis of the band shape below ∼50 cm−1 (at −265 °C) yielded a structural correlation length, SCL=2σ=6.2±0.2 Å (0–15 mol % PbF2). This SCL corresponds to a repeat distance, e.g., the Ba–Zr separation of the Ba(F2)Zr edge-sharing, 4.1–4.2 Å, plus the Zr–F bond length, 2.1 Å, of the ZrF4−nn polyhedra, 5<n<7, and n=CN. Normalized Bose–Einstein corrected difference spectra, I35%–I0% (mol % PbF2), and for 5 mol % PbF2, I635–I25 (°C), show common frequencies between ∼200–500 cm−1 which refer mainly to edge or corner sharing between the ZrF4−nn , PbF2−ll , and BaF2−mm polyhedra, l, m=CN. The 45 cm−1 peak frequency decreases sigmoidally with temperature rise, dropping steeply near TG, and reaching a ∼5 cm−1 limit in the melt. This and the rough density modulation observed for the mode frequency are indicative of viscoelasticity. The virtual absence of the low-frequency mode in the melt due to the Pb+Zr and Ba+Zr vs F cage motions occurs simultaneously with the disappearance of the F sharing modes. For a total stoichiometric F/Zr mole ratio of 5.5 for the melt, nearly free ZrF−5 , ZrF2−6 , and ZrF3−7 anions are indicated.
Electron transmission spectroscopy of matrix-isolated N2
Authors: Sanche L. , Perluzzo G. , Michaud M.
Published: July 2002

Pseudovibrational levels of the 2Πg state of N−2 are detected via electron transmission through solid xenon films doped with varying concentrations of nitrogen molecules. The resonance is found to produce a significant increase in transmitted current arising from electrons having lost energy to vibrational excitation. The relaxation shifts of the anion in condensed N2 (0.7 eV), argon (0.83 eV), and xenon (1.45 eV) are discussed and compared with calculations made within the framework of the Mott–Littleton and Born models.
Comparison between approximate (perturbation) and exact (close-coupling) three-dimensional quantal methods in reactive scattering
Authors: Suck Salk S. H. , Lutrus C. K.
Published: July 2002

Presently the validity of DWBA methods has been largely unchecked with respect to exact methods. In the present study, direct comparison between the DWBA and exact close-coupling calculations is made by examining reactive scattering angular distributions of H+H2 → H2+H for the same range of collision energy as Schatz and Kuppermann chose. We have found from the DWBA study that (1) the structures of angular distributions between these two methods are remarkably similar at most collision energies; (2) the effect of coupling strongly affects the absolute magnitude of cross sections but not the structures of normalized angular distributions; and (3) the DWBA predicted state-to-state integrated (total) cross sections σDWBA are much smaller than the exact close-coupling results σEXACT, due to the dominant effect of coupling which is found to rapidly increase with collision energy EK, observing the relationship of σEXACT∝E2/3K σDWBA. In addition, the cause of agreement between the approximate DWBA and exact close-coupling calculations in the structures (shapes) of angular distribution is explained.
On the validity of the quasiresonant approximation for molecular infrared-multiphoton excitation
Authors: Quack M. , Sutcliffe E.
Published: July 2002

Quantum dynamical calculations of the IR-multiphoton excitation of two molecular model systems are presented. One is an anharmonic oscillator and the other the rovibrational structure of the lowest three excitation steps in the stretching manifolds of the ozone molecule. Systematic comparisons are made between solutions using either the Floquet approximation or the weak field quasiresonant approximation (WF-QRA). The WF-QRA is shown to be adequate for typical transition moments in molecular many level systems and intensities up to the TW cm−2 range in agreement with previous analytical considerations. The implications of these findings for an efficient approach to the computational treatment of IR-multiphoton excitation and for the mechanism of molecular IR-multiphoton excitation as a series of stepwise one photon transitions are discussed.
Theoretical study of small silicon clusters: Cyclic ground state structure of Si3
Authors: Raghavachari Krishnan
Published: July 2002

The ground state structure of Si3 is found to be cyclic (1A1, C2v symmetry) with an apex bond angle of ≂80°. The linear form, analogous to the known structure of C3, is found to be a saddle point which transforms without activation to the cyclic structure. There is a low-lying triplet state (3A′2, D3h symmetry) which lies only a few kcal/mol higher in energy. The nature of the bending potential surface of the ground state structure is also discussed.
The effect of 3d shell back bonding on the binding of chlorine containing molecules
Authors: Pettersson Lars G. M. , Siegbahn Per E. M.
Published: July 2002

The description of the binding in molecules containing chlorine is studied with special emphasis on Cl 3d shell effects. Two difficult test examples were selected, ClF and CCl, where very poor results for the equilibrium bond distance have been reported earlier, even though large multireference CI calculations were performed. The earlier errors of nearly 0.10 a.u. in the bond distance are essentially removed by including the chlorine 3d shell in the CASSCF active space. The incorporation of the 3d shell in the active space allows for better backbonding from F 2p to Cl, and a better description of angular correlation. The best multireference contracted CI results for ClF are re =3.075 a.u. (expt. 3.077 a.u.), μe =0.885 D (0.888 D), and De =2.53 eV (2.67 eV). For CCl the ground state potential curve was calculated and essentially perfect agreement with experimental spectroscopic parameters was obtained. The C 2s orbital was not correlated in these calculations and the results for CCl may therefore be fortuitously good due to cancellation of errors. The poor performance of the multireference cluster correction for re in ClF and CCl is also discussed in the paper.
Parametric and nonparametric four-wave mixing in pentacene: p-terphenyl
Authors: Steehler J. K. , Wright J. C.
Published: July 2002

Three tunable lasers are used to generate a fully resonant nonparametric process that generates new coherent output at a higher energy than fluorescence interference in condensed phase molecular systems. The pentacene:p-terphenyl system is used as a model to test the basic ideas of the method and to compare the results with comparable spectra obtained with a parametric process. The resonance enhancement is shown to reflect intermode coupling between different vibrational and vibronic modes. Interference is observed between transitions of different pentacene sites that reflects the differences in the parametric and nonparametric processes. These differences become the dominant characteristic determining the potential for four-wave nonlinear mixing methods to provide resolution within inhomogeneously broadened electronic bands.
Collision-induced IR absorption in gas phase Hg(6 3P1)+HgBr2(1 1Σ+g) reactions: Enhancement of the HgBr(B 2Σ+) product channel
Authors: Helvajian H. , Marquardt C. L.
Published: July 2002

We have investigated laser-assisted reactions in Hg/HgBr2 gas mixtures by measuring fluorescence from the product, HgBr radical, following simultaneous IR (1064 nm) and UV (253.7 nm) laser excitation. The product emission has one-photon dependence on both IR and UV laser power, and the results indicate that the excited HgBr product channel is accessed through single photon absorptions by the {Hg–HgBr2} complex. This constitutes the first experimental evidence for an electronically excited state of the {Hg–HgBr2} collision complex, having an energy level >4.9 eV but <6.1 eV. With the addition of tunable IR lasers this technique could provide an important spectroscopic method for investigation of the homologous series of collision complexes, {Hg–HgX2} where (X=F, Cl, Br, I).
Zeeman effects in excited states of SO2 investigated with sub-Doppler resolution
Authors: Kullmer R. , Demtröder W.
Published: July 2002

Zeeman splittings in the spectrum of SO2 around 320 nm have been accurately measured at different magnetic fields in a collimated supersonic SO2 beam with a spectral resolution of 10 MHz. The analysis of the Zeeman patterns proves that the magnetic moment of the vibronically coupled 1B1 and 1A2 states is mainly due to spin-orbit coupling with a triplet state while Coriolis coupling with the 1A1 ground state does not significantly contribute to the Zeeman effect.
Relativistic configuration interaction calculations of low-lying states of PbF
Authors: Balasubramanian K.
Published: July 2002

Relativistic configuration interaction calculations are carried out on the low-lying states [1/2, 3/2, 1/2(II), 3/2(II), 4Σ−1/2, B2Σ+1/2 ] of PbF, together with comparison calculations on the five low-lying λ-s states (2Π, 2Σ+, 4Σ−, 2Δ, 4Π). Spectroscopic properties and dissociation energies of these states are computed and compared with the available experimental results. The ground state spin-orbit splitting is calculated to be 7895 cm−1. Our calculations enable the assignment of experimentally observed X1, X2, A, and B states to ‘u2Π1/2, 2Π3/2, 2Σ+1/2, and 2Σ+1/2(II), respectively. The B state is assigned to the lowest Rydberg state arising from σ2σ′R π4 configuration. Calculations of 4Σ−1/2 (in type C coupling) and 3/2(II) state are reported which are yet to be observed.
Infrared absorption and predissociation of NO dimer
Authors: Brechignac Ph. , De Benedictis S. , Halberstadt N. , Whitaker B. J. , Avrillier S.
Published: July 2002

The infrared absorption spectrum of the ν4 fundamental band of (NO)2 has been recorded at 0.3 cm−1 resolution in a cell cooled down to 110 K. From the band contour analysis and comparison with a simulated spectrum accurate values of the rotational constants in the ν4 state are derived, showing a reduction of the van der Waals bond on vibrational excitation. Absorption measurements performed using the various lines emitted by an isotopic CO laser suggest an unresolved rotational fine structure. A lower limit of the time for energy redistribution of 50 to 100 ps is proposed. Also, infrared photopredissociation of (NO)2 has been directly observed in a crossed laser-molecular beam experiment using bolometric detection of the beam attenuation. An upper limit of the dissociation lifetime can be set to 10−3 s.
Microwave rotational spectra, hyperfine interactions, and structure of the hydrogen fluoride dimers
Authors: Gutowsky H. S. , Chuang Carl , Keen John D. , Klots T. D. , Emilsson Tryggvi
Published: July 2002

The hyperfine structure has been resolved for the two J=0→1 transitions of (DF)2, for the low frequency J=0→1 transition of (HF)2, and for the J=0→1 transition of the mixed species HFDF. Also, the J=0→1 transition and hyperfine structure were observed for the other mixed species DFHF, not found previously. The relative intensities of the transitions for HFDF and DFHF, and their zero-point vibrational frequencies, correspond to an effective equilibration temperature between the two species of ∼50 K in the gas expansion. Resolution and assignment of the (DF)2 and (HF)2 hyperfine structure was aided by a method for suppressing the Doppler doubling in a Flygare spectrometer; it improved resolution twofold to a FWHM of 5 kHz at 11–14 GHz. The results bear on several features of the angular structure of this unusual system. At equilibrium, the end H/DF in HFDF and DFHF is bent 60±2° from the F⋅⋅⋅F axis, while the H/DF in the hydrogen bond is bent 7±3° in the opposite direction. The tunneling in (DF)2 gives an average angle between the DF’s and the a axis (39.3° and 38.7° for the two transitions) which is 1.5° larger than the composite (37.5°) of the DF’s in DFHF and HFDF. For the (HF)2 transition, the observed average angle (40°) is also 1.5° larger than the composite (38.5°) of the HF angles in the mixed species. The hydrogen bonding in HFDF and (DF)2 decreases the electric field gradient at the deuterium by ∼15% from its value in free DF.
Effect of J dependence of Raman cross sections on CARS product quantum state distributions for the H+D2 → HD+D reaction
Authors: Veirs D. Kirk , Rosenblatt Gerd M. , Valentini James J.
Published: July 2002

HD product quantum state distributions are recalculated for published CARS measurements using J-dependent Raman cross sections determined from accurate, ab initio calculations. The use of J-dependent cross sections lowers calculated number densities by 5%–10% at high J and therefore systematically shifts the relative product-state distribution somewhat from high J to low J. This shift does not noticeably affect discrepancies between CARS results and multiphoton ionization experiments on the same reaction. The results indicate that J-dependent Raman cross sections should be used in reducing vibrational Q-branch CARS and other Raman data for molecular hydrogen and its isotopes.
Rovibrational analysis of ν3 HCN---HF using Fourier transform infrared spectroscopy
Authors: Wofford B. A. , Bevan J. W. , Olson W. B. , Lafferty W. J.
Published: July 2002

The gas phase rovibrational spectrum of the ν3 band arising from the C≡N stretching vibration in the hydrogen bonded heterodimer HCN---HF has been observed at 0.004 cm−1 instrumental resolution using a Fourier transform infrared spectrophotometer. Analysis of the spectrum gave the following molecular parameters (in cm−1): ν3=2120.935(12), α3=+5.06(19)×10−4, B′=0.119 283(19), D′J=2.30(7)×10−7. Excited state amplitude lifetimes of observed transitions are demonstrated to be 5.6(4)×10−10 s.
Picosecond CARS as a probe of ground electronic state intramolecular vibrational redistribution
Authors: Tannor David J. , Rice Stuart A. , Weber Peter M.
Published: July 2002

We describe a representation of coherent anti-Stokes Raman spectroscopy (CARS) suitable to the description of time-dependent measurements. This representation includes the standard energy frame formulation as a special case when cw fields are involved. The traditional CARS cw field frequency matching condition ω0=ω1−ω2+ω3 must be generalized for non-cw fields; in that case it refers to the Fourier component at ω0 of the convolution of the radiation field with the wave packet recurrences. The influence of resonance, both in the ground and electronically excited states, on the decay of time delayed CARS signals is discussed. As expected, intramolecular vibrational redistribution on the ground state potential surface of a molecule causes the CARS signal to decay in time. Model calculations show that quantum beats in the CARS signal may be observed, reflecting either a small number of coupled states (strong and regular recurrences) or sequential coupling of states (weak and early recurrences).
Critical exponents and scaling functions of a self-avoiding walk interacting with a defect plane
Authors: Kremer Kurt
Published: July 2002

We analyze the behavior of a single self-avoiding walk (single polymer in good solvent) in the presence of a penetrable (d−1) dimensional defect layer by the use of Monte Carlo simulations. The layer can be either neutral to the bonds or can be attractive or repulsive. We analyze the data by means of a scaling picture and show that there is an excellent agreement with the deGennes, Bray, and Moore conjecture φ=1−ν for the crossover exponent. This corrects a recent estimate of φ made by a series analysis of enumeration data of short chains.
Exponentially generated wave functions
Authors: Nakatsuji Hiroshi
Published: July 2002

We consider several generalizations of the exponential ansatz in a rather formal way, giving several new wave functions which we call exponentially generated (EG) wave functions. There are three distinct ways of the exponential-type generations of the wave functions, two of which are new. They are named ESAC (extended symmetry-adapted-cluster) wave function and exponentially generated CI (EGCI) wave function. The ESAC wave function is a simple extension of the SAC wave function and is applicable even when the Hartree–Fock reference configuration is not dominant. The EGCI wave function is a CI wave function constructed in the spirit of the cluster expansion theory. Formally, it has the merits of both the CI theory and the cluster expansion theory; for example, the upper bound nature, size consistency, and the applicability to quasidegenerate states and excited states. We then introduce several new wave functions by a multiple and mixed use of the exponential-type operators. We call such a class of wave functions multiexponentially generated (MEG) wave functions. There are many possibilities for the MEG wave functions, and the MR-SAC wave function proposed previously is one of them. When the system involves several classes of electron correlations, the MEG wave function permits an optimal (physically and practically) use of the exponential-type operators for the distinct classes of electron correlations. We described the method of solution of the EG and MEG wave functions and examined size consistency and some other properties.
Molecular dynamics test of the Brownian description of Na+ motion in water
Authors: Wilson Michael A. , Pohorille Andrew , Pratt Lawrence R.
Published: July 2002

The autocorrelation function of the velocity of an infinitely dilute Na+ ion in aqueous solution, and the autocorrelation function of the force exerted on a stationary Na+ under the same conditions are evaluated by molecular dynamics calculations. The results are used to test the accuracy of Brownian motion assumptions which are basic to hydrodynamic models of ion dynamics in solution. The self-diffusion coefficient of the Na+ ion predicted by Brownian motion theory is (0.65±0.1)×10−5cm2/s. This value is about 60% greater than the one obtained for the proper dynamics of the finite mass ion, (0.4±0.1)×10−5cm2/s. The numerically correct velocity autocorrelation function is nonexponential, and the autocorrelation of the force on the stationary ion does not decay faster than the ion velocity autocorrelation function. Motivated by previous hydrodynamic modeling of friction kernels, we examine the approximation in which the memory function for the velocity autocorrelation function is identified with the autocorrelation function of the force on the stationary ion. The overall agreement between this approximation for the velocity autocorrelation function and the numerically correct answer is quite good.
Interaction of vibrating H atoms on the surface of platinum particles by isotope dilution neutron spectroscopy
Authors: Rush J. J. , Cavanagh R. R. , Kelley R. D. , Rowe J. M.
Published: July 2002

The first results of a study of the vibrational spectrum of hydrogen, chemisorbed on the surface of platinum by isotope dilution neutron spectroscopy are presented. (AIP)
Collinear hydrogen transfer from hydride reactants to nondegenerate products: The F+DBr (v=0) →FD (v′≤7)+Br reaction
Authors: Gertitschke P. L. , Manz J. , Römelt J. , Schor H. H. R.
Published: July 2002

Exact quantum reaction probabilities P vs energy yield the following dynamical effects of the title reaction: (A) oscillating reactivity with (B) a superimposed ‘‘spectrum’’ of low energy Feshbach and higher energy shape resonances, and (C) conservation of translational energy. The propensity rule (C) implies dominant selection of the nonreactive (P≳0.95) and of the highly excited (v′=6) product level channels (P<0.05) but not at shape resonances, which provide highly reactive doorway states to several product channels. The results are compared with hydrogen exchange dynamics for the case of (quasi-) degenerate hydride levels, and related to experimental observations.
Phase space considerations for interacting particles obeying the Fokker–Planck equation
Authors: Rodger P. Mark , Sceats Mark G.
Published: July 2002

The N-particle Fokker–Planck equation has been cast into a form which is invariant to coordinate transformations. For a pair of particles interacting through a radial potential V(R), the equation of motion may be projected onto a one-dimensional problem in R when the conjugate momenta of the center of mass and internal coordinates have symmetric distributions, as in Boltzmann distributions. However, the R2 dependence of the density of configuration space demands that V(R) be supplemented with an effective repulsive potential −2kT ln R, a result which has consequences for Langevin simulations of chemical processes. For N interacting particles a subset of coordinates can often be identified as having such symmetric distributions, and if these are not of interest, they can be eliminated with the result that the potential containing the remaining coordinates is supplemented by −kT ln(‖gkl‖)1/2, where ‖gkl‖ is the determinant of that portion of the covariant metric tensor pertaining to unwanted coordinates k and l.
On the viscoelastic properties of n-alkane liquids
Authors: Sceats Mark G. , Dawes Judith M.
Published: July 2002

The extant experimental data for the n-alkanes (pentane–hexadecane) derived from ultrasonic absorption, Brillouin and Rayleigh light scattering, and low frequency shear viscosity measurements are analyzed in terms of a model which decomposes the frequency dependent shear and longitudinal viscosities into contributions arising from trans–gauche rotational isomerism, rotational reorientation, and structural relaxation. The results yield values of the transverse and longitudinal structural relaxation times, and indicate that there is a strong coupling of molecular reorientation to adiabatic compression as well as to shear, presumably because the existing orientational correlations between chains are perturbed by the temperature variations during adiabatic compression.
Spectroscopic study of the afterglow excited by intense electrical discharges in high-pressure helium hydrogen mixtures
Authors: Motret O. , Pouvesle J. M. , Stevefelt J.
Published: July 2002

In this work, we report the study of afterglow plasmas with initial electron density over 1015 cm−3 excited by repetitive 3 kA, 7 ns duration discharges in He–H2 mixtures with helium pressure in the range 310–760 Torr and partial pressure of hydrogen from 20 to 400 mTorr. The afterglow radiation was emitted in He2 transitions as well as in atomic hydrogen spectral lines, with Lα being particularly intense. The ground state H concentration was estimated from the profile of the self-absorbed Lα line, indicating that dissociation of hydrogen occurs mainly in the afterglow by electron–ion recombination. He (2 3S) destruction frequencies have been determined from measurements of the optical absorption of the 2 3S→3 3P helium transition and were analyzed in terms of rate coefficients for various quenching processes. Among these, the Penning reaction with atomic hydrogen was found to be important.
Mean number of distinct sites visited by correlated walks. I. Perfect lattices
Authors: Kehr Klaus W. , Argyrakis Panos
Published: July 2002

The mean number of distinct sites visited by correlated walks on one-, two-, and three-dimensional lattices is studied by numerical simulations and by generating-function techniques. The random walks include correlations over two consecutive steps. The asymptotic behavior is derived analytically in d=1, and in d=2, 3 for the model with restricted reversals, and good agreement with the simulations is found. The model with increased probability for forward steps is studied numerically in d=2, 3 and analyzed. It is found in all cases that the mean number of visited sites cannot be simply obtained by rescaling the step number n with the correlation factor f, but there are additional correction terms that do not obey scaling.
Reaction dynamics of O(1D2)+H2, HD, D2: OH, OD(X 2Πi) product internal energy distributions
Authors: Butler J. E. , Jursich G. M. , Watson I. A. , Wiesenfeld J. R.
Published: July 2002

Partial internal energy distributions of the hydroxyl reaction products of O(1D)+H2, HD, and D2 reactions are presented. Inverted rotational distributions, preferential population of the π+ lambda doubling sublevels, and statistical population of the spin sublevels are observed. A slight preferential formation of the OD vs OH reaction products observed is measured for the reaction of O(1D)+HD. Surprisal analysis of these results indicates both dynamical and kinematic constraints on the reaction dynamics. Comparison of these results with published model calculations suggest that an insertion mechanism to form a highly energetic collision complex dominates the reaction dynamics.
Dynamics of overtone excitation processes for a polyatomic model
Authors: Holme Thomas A. , Hutchinson John S.
Published: July 2002

The excitation dynamics of a polyatomic system are presented in terms of both classical and quantum dynamics. These studies show that classical mechanics is qualitatively inaccurate in its description of the overtone excitation process. The quantum excitation is therefore an example of dynamic tunneling, which like most tunneling processes is slow. In a polyatomic molecule, intramolecular vibrational relaxation occurs much faster than does excitation, by several orders of magnitude. The disparity of time scales results in the excitation being very eigenstate specific in the small to medium molecule limit. We discuss the importance of these observations to photochemical experiments (possible preparation of long-lived localized excitations) and to theoretical interpretations of molecular wave functions (possible interference effects in the molecular state function).
Bistable and chaotic behavior in a damped driven Morse oscillator: A classical approach
Authors: Lie George C. , Yuan Jian-Min
Published: July 2002

Dynamical behavior of a damped Morse oscillator driven by an external sinusoidal field has been studied by solving a classical equation. Bistability of dynamical origin has been found for damping constants ranging from 0.001 to 0.4. In the weaker damping limit, corresponding to a liquid or high-pressure gas medium, bistability shows up for a large range of driving frequency and both the lower and upper branches belong to the type of period-one orbits. At the stronger damping limit, simulating a solid-state system, and in the presence of an intense field, we have found a sequence of period-doubling bifurcations leading to chaos for both the lower and upper branches. The stability diagram in the driving amplitude and frequency space show two cusp regions of bistability. This shape suggests that each of them can be described locally as a cusp catastrophe. Possible relevance of the molecular bistability to stimulated Raman scattering experiment is discussed.
Theory of reorientational motional broadening of vibrational spectral bands in solid state
Authors: Cho Yonghae , Kobayashi Masamichi , Tadokoro Hiroyuki
Published: July 2002

A theory of reorientational motional broadening of vibrational bands in solid state was developed by using the site model. The equation describing the correlation function of the transition moment and the Raman tensor was derived in a matrix form. In the case of an equivalent site model, the obtained correlation function indicated that the band profile was represented by the overlap of Lorentzian functions located at the same in peak position but different in half-width. The theory was applied to the Raman bands of n-C20H42 in the urea inclusion adduct, and used to interpret semiquantitatively the observed reorientational motional broadening of the bands. It was found that the amount of broadening of the infrared and Raman bands induced by molecular motion depended systematically on their symmetry species. This was interpreted theoretically by a group theoretical consideration of the reorientational relaxation.
Multireference-state Rayleigh–Schrödinger perturbation theory applied to the electronic states X 1Σ+g and E
F 1Σ+g of H2
Authors: Hose Gabriel
Published: July 2002

The effect of the reference space on the convergence of Rayleigh–Schrödinger perturbation series within the molecular-orbitals framework is studied for the ground X 1Σ+g and excited EF 1Σ+g electronic states of H2 over a wide range of internuclear separations. Near the ground-state equilibrium distance of 1.4 bohr each state is well described by a single spin-adapted configuration function. This no longer holds for the EF 1Σ+g state around 3.0 bohr because of forbidden curve crossing. In general, the quasidegeneracy increases with the internuclear distance due to improper dissociation of molecular orbitals. A rigorous approach to define a proper reference space is discussed. It is based on analysis of convergence with emphasis on identifying intruder states. A reference space of nine spin-adapted functions is adequate in the range 1.4–8.0 bohr; giving third-order results within less than 1×10−3 hartree from the basis limit. Other findings are: (i) Epstein–Nesbet breakup of the Hamiltonian usually gives faster convergence as compared with the Mo/ller–Plesset scheme. (ii) Padé approximants improve the results but only when the reference space is capable of describing the state. When this is not the case the Padé sequence is erratic and physically meaningless. (iii) With suitably defined reference space there is only a marginal difference in the results between different breakups of the Hamiltonian, and little improvement is gained by employing Padé approximants.
A comparison of force fields and calculation methods for vibration intervals of isotopic H+3 molecules
Authors: Carney Grady D. , Adler-Golden Steven M. , Lesseski David C.
Published: July 2002

This paper reports (a) improved values for low-lying vibration intervals of H+3, H2D+, D2H+, and D+3 calculated using the variational method and Simons–Parr–Finlan representations of the Carney–Porter and Dykstra–Swope ab initio H+3 potential energy surfaces, (b) quartic normal coordinate force fields for isotopic H+3 molecules, (c) comparisons of variational and second-order perturbation theory, and (d) convergence properties of the Lai–Hagstrom internal coordinate vibrational Hamiltonian. Standard deviations between experimental and ab initio fundamental vibration intervals of H+3, H2D+, D2H+, and D+3 for these potential surfaces are 6.9 (Carney–Porter) and 1.2 cm−1 (Dykstra–Swope). The standard deviations between perturbation theory and exact variational fundamentals are 5 and 10 cm−1 for the respective surfaces. The internal coordinate Hamiltonian is found to be less efficient than the previously employed ‘‘t’’ coordinate Hamiltonian for these molecules, except in the case of H2D+.
Rydberg–valence interactions in the Πg states of O2
Authors: Sur Abha , Ramana C. V. , Chupka William A. , Colson Steven D.
Published: July 2002

Multiphoton ionization spectroscopy is used to characterize the (3sσ) 3Πg Rydberg state of O2. It is found that the 3Πg Rydberg state is very diffuse and has rotationally resolved structure for the v′=2 level only. The FWHM of rotational lines for both the (3sσ) 3,1Πg states is greater than the excitation bandwidth, indicating strong radiationless decay of the two states. Interactions with the nearby 3Πg and 1Πg valence states are invoked to explain the observed behavior.
Electron and cation transport in gaseous carbon dioxide: Density and temperature effects
Authors: Jacobsen Finn M. , Freeman Gordon R.
Published: July 2002

In low density CO2 gas, the density normalized mobility of electrons is nμe =1.88×1024 molecule/m V s, independent of temperature. It is an order of magnitude lower than the value in ethene, a hydrocarbon of similar molecular size and anisotropic polarizability. Scattering of thermal electrons by carbon dioxide molecules might involve a transient anion state. At densities n>5×1025 molecule/m3 along the vapor/liquid coexistence curve, electron attachment to molecular clusters occurs, e−+(CO2)q⇄(CO2)−q, where q≊6. At n>14×1025 molecule/m3 in the coexistence vapor the negative and positive charges have the same mobility, so the electrons are permanently attached. Electron detachment can occur when the gas is heated at constant density; heating decreases the average cluster size and provides activation energy for detachment. At a constant density ≥30×1025 molecule/m3 the value of nμe can be changed 1000-fold by changing T. At n=31 and 56 (1025 molecule/m3) the values of nμe for quasifree (unattached) electrons are 1.65 and 1.50 (1024 molecule/m V s), respectively. The equilibrium between attachment and detachment has ΔH0a =−90 kJ/mol and ΔS0a =−270 J/mol K. The value of nμ+ of the cations in the coexistence vapor is larger at temperatures below the triple point than above it. Perhaps the clustered ions also undergo a structure transition near the triple point. Positron attachment to (CO2)q clusters occurs at similar temperatures and densities as electron attachment.
Nascent rotational and vibrational product state distribution in the charge transfer reaction of N++CO→CO++N at near thermal energy
Authors: Lin Guang Hai , Maier Jürgen , Leone Stephen R.
Published: July 2002

An improved ion beam apparatus is used to measure the nascent product state distribution in the charge transfer reaction N++CO→CO++N under single-collision conditions at 0.16 eV energy. At this energy, the major vibrational channel in the CO+ products is v=1, in contrast to the predominant formation of v=0 at thermal energy. The relative vibrational distribution for the N++CO reaction is (0.40±0.07)v=0 : (0.57±0.04)v=1 : (0.03±0.01)v=2. In the v=0 channel, the rotational distribution under single-collision conditions can be characterized by a Boltzmann distribution with a temperature of T=410±40 K. In the v=1 channel the rotational distribution is highly excited and non-Boltzmann. The lower rotational states approximate a very high temperature of 2000 K. For high rotational quantum numbers (K>23), the rotational temperature is about T=810±20 K. The dramatic differences in the rotational distributions are clear evidence that these two vibrational channels are formed by different reaction mechanisms, most likely a direct reaction for v=0, and an intimate collision for v=1.
Methyl and t
e
r
t-butyl reorientation and distributions of activation energies in molecular solids. A nuclear spin-relaxation study in 2,4- and 2,5-di-t
e
r
t-butylhydroxybenzene
Authors: Beckmann P. A. , Cheung A. M. , Fisch E. E. , Fusco F. A. , Herzog R. E. , Narasimhan M.
Published: July 2002

We have measured proton Zeeman relaxation rates R in the 2,4- and 2,5-isomers of di-tert-butylhydroxybenzene (DTHB) in the solid state. R was measured as a function of temperature T at proton Larmor frequencies of ω/2π=8.50, 22.5, and 53.0 MHz. The T ranges were from 78 K to just below the melting points of 2,4- and 2,5-DTHB, 385 and 323 K, respectively. The 2,5-DTHB R vs T and ω can be interpreted qualitatively in terms of three Bloembergen–Purcell–Pound (BPP) spectral densities, one for each of the three types of rotors in the molecule. The quantitative agreement is poor but a good fit is obtained using either a Davidson–Cole (DC) or Frölich spectral density, still preserving the three rotor types. The implications of this are discussed. The BPP and DC spectral densities fail completely in interpreting R vs T and ω for 2,4-DTHB whereas good quantitative fits are obtained using a Frölich spectral density. The distributions of activation energies characterizing the three rotor types are so wide for the Frölich spectral density fit of the 2,4-DTHB data that the individual rotor types lose their identity.
An exact correspondence between Eigen’s evolution model and a two-dimensional Ising system
Authors: Leuthäusser Ira
Published: July 2002

It is shown that for the case of point mutations Eigen’s model for macromolecular evolution can be mapped on a two-dimensional Ising system with nearest-neighbor interaction in one direction. The analogies and differences between the two systems are discussed.
Channel specific rate constants for reactions of O(1D) with HCl and HBr
Authors: Wine P. H. , Wells J. R , Ravishankara A. R.
Published: July 2002

Time resolved resonance fluorescence detection of O(3P) and H(2S) has been employed in conjunction with pulsed laser photolysis of O3/HX/He mixtures to determine absolute rate coefficients and product yields for reactions of O(1D) with HCl(1) and HBr(2) at 297 K. Total rate coefficients for O(1D) removal are found to be, in units of 10−10 cm3 molecule−1 s−1, k1=1.50±0.18 and k2=1.48±0.16, where the quoted errors are 2σ and represent precision only. The absolute accuracy of these rate coefficients is estimated to be ±20%. Formation of O(3P)+HX is found to account for 9%±5% of the overall rate of reaction (1) and 20%±7% of the overall rate of reaction (2). Formation of H(2S)+XO is found to account for 24%±5% of the overall rate of reaction (1) but <4.5% of the overall rate of reaction (2). By difference, the OH+X(2P) channel accounts for 67%±10% of the overall rate of reaction (1) and 80%±12% of the overall rate of reaction (2).
Multiphoton ionization studies of ultracold nitric oxide
Authors: Miller John C. , Compton R. N.
Published: July 2002

The multiphoton photoionization mechanisms in ultracold nitric oxide have been investigated via one- and two-color laser excitation in a supersonic expansion. Photoelectron spectroscopy is used to identify the roles of direct ionization, autoionization, and Rydberg-valence mixing for multiphoton ionization via several Rydberg states (A, E, H, F, N) of nitric oxide. Accidental double resonance with mixed Rydberg-valence states in the third-photon region of a nominally (2+2) multiphoton ionization is shown to yield non-Franck–Condon photoelectron distributions. However, non-Franck–Condon peaks can also appear when the third-photon region is either eliminated by frequency doubling or uniquely determined by using two-color excitation.
Studies of chemical reactivity in the condensed phase. I. The dynamics of iodine photodissociation and recombination on a picosecond time scale and comparison to theories for chemical reactions in solution
Authors: Harris A. L. , Berg M. , Harris C. B.
Published: July 2002

Picosecond transient absorption measurements from 1000–295 nm are used to monitor the recombination dynamics of iodine after photodissociation in a variety of inert solvents. The high time resolution and signal-to-noise ratio of these measurements permits the development of a detailed model of this reaction, which should resolve disagreements over the time scales of geminate recombination and vibrational relaxation and over the role of excited electronic state trapping. Most of the atoms which undergo geminate recombination do so in ≤15 ps, in agreement with the predictions of existing molecular dynamics simulations. The subsequent vibrational and electronic energy relaxation of the recombined molecule is relatively slow and accounts for most of the transient absorption dynamics. The relaxing X-state vibrational population distribution is extracted with an approximate method using calculated spectra of the excited vibrational levels and is compared to recent models. Vibrational relaxation times vary from ∼15 ps near the middle of the ground state well to ∼150 ps for complete relaxation to v=0. The vibrational relaxation rates do not provide support for the predicted role of resonant vibration-to-vibration energy transfer to chlorinated methane solvents, but some evidence for this mechanism is found in alkane solvents. B-state predissociation times of 10–15 ps and A′-state lifetimes of 65–2700 ps are found depending on the solvent. Current theory is not able to satisfactorily explain the large variation of the A′-state lifetime in various solvents.
Photoelectron spectroscopy of the excited states of the CH radical
Authors: Pallix Joan B. , Chen Peter , Chupka William A. , Colson Steven D.
Published: July 2002

We report the first photoelectron spectra of the excited states of the CH radical. The spectra reveal the source of perturbation of the v=2 vibrational level of the D state. Photoelectron partial cross sections have been used to enhance specific features in regions where spectral congestion results from the overlap of two or more electronic absorption bands.
Chemisorption of O and H on an Al13 cluster
Authors: Partridge Harry , Bauschlicher Charles W.
Published: July 2002

Chemisorption of oxygen and hydrogen atoms onto a 13 atom aluminum cluster is investigated theoretically. Oxygen atoms are found to remain on the outside of the cluster, and at low coverage, to expand the Al–Al bond length by 0.42 bohr. Hydrogen atoms penetrate into the cluster resulting in a sizeable, 0.57 bohr expansion at high coverage. The difference between chemisorption on aluminum and beryllium clusters is discussed.
A quantum mechanical investigation of vibrational energy transfer in O(3P)+H2O collisions
Authors: Johnson B. R.
Published: July 2002

Cross sections for the vibrational excitation of H2O in collision with O(3P) are calculated for relative collision energies of 0.5 to 3.0 eV by the vibrational close-coupling rotational infinite order sudden method using an accurate potential energy surface. The excitation cross sections obtained by this quantum mechanical calculation are compared to results of a recently published quasiclassical trajectory study which used the same potential surface. Very large differences between the quantum mechanical and classical trajectory results are found.
A molecular dynamics study of an aqueous LiI solution between Lennard-Jones walls
Authors: Spohr E. , Heinzinger K.
Published: July 2002

An MD simulation of a 2.2 m aqueous LiI solution between 12-6 Lennard-Jones walls has been performed which extended over 19 ps at an average temperature of 283 K. The basic tetragonal cell contained 200 ST2 water molecules and eight ions of each kind described as Lennard-Jones spheres with an elementary charge at the center. The effect of the walls on the hydration shell of the ions is investigated by means of ion–water radial distribution functions. Anisotropic water pair correlation functions are reported in order to show specific structural features near the walls. Changes in translational and rotational motions of the water molecules near the walls are discussed by means of velocity and angular velocity autocorrelation functions and their spectral densities, respectively.
Density dependence of the dephasing and energy relaxation times by computer simulations
Authors: Chesnoy J. , Weis J. J.
Published: July 2002

The energy relaxation time (T1) and dephasing time (T2) of a system of molecules interacting by spherically symmetric intermolecular and vibrational potentials have been evaluated by the molecular dynamics method. The computational ease introduced by the spherical symmetry of the potentials allows an extensive study of the density variations of T1 and T2 from the gas phase to the highly compressed liquid and solid, as well as of the influence of a long range component in the vibrational potential on the relaxation times. The general trends which proceed from this study are compared with recent theoretical approaches and experimental observations.
Intramolecular vibrational relaxation in n-alkyl benzene chromium tricarbonyls: State selective production of chromium atoms
Authors: Hossenlopp Jeanne M. , Samoriski Brian , Rooney Dan , Chaiken J.
Published: July 2002

We have measured one color multiphoton dissociation/ionization (MPD/MPI) spectra for a series of n-alkyl substituted arene chromium tricarbonyls (ACTs). Our data indicate that intramolecular vibrational relaxation (IVR) rates for methyl, ethyl, and propyl ACTs increase relative to the benzene analog in the ratio of 3:6:34. In contrast, the net relaxation rates of the p-xylene (dimethyl) and mesitylene (trimethyl) analogs are two and three times faster than benzene, respectively. The behavior of these latter analogs probably also reflects differences in electronic structure relative to the benzene ACT. Taken with the results of experiments on other analogs and the independently obtained results on the uncomplexed arenes, we have found a strong correlation between the presence of low frequency vibrations and the rate of IVR. The rate of IVR determines the distribution of neutral chromium atoms formed by MPD of the n-alkyl ACTs.
An a
b i
n
i
t
i
o study of the molecular electric polarizabilities of N2, HCN, acetylene, and diacetylene
Authors: Jameson Cynthia J. , Fowler P. W.
Published: July 2002

The independent components of the α, β, γ, A, and B tensors were computed for the ground states of N2, HCN, acetylene, and diacetylene molecules from electric moments induced in a perturbing static electric field. The results are used to comment on trends in nonlinear susceptibilities of polyacetylenes and polydiacetylenes.