bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanism of the ultraviolet photodissociation of chloroethylenes determined from the Doppler profiles, spatial anisotropy, and power dependence of the photofragments
Authors: Mo Yuxiang , Tonokura Kenichi , Matsumi Yutaka , Kawasaki Masahiro , Sato Tetsuya , Arikawa Tatsuo , Reilly Peter T. A. , Xie Yongjin , Yang Yung-an , Huang Yibo , Gordon Robert J.
Published: July 2002

Doppler profiles of chlorine and hydrogen atomic fragments produced in the photodissociation of mono- and dichloroethylenes at 193 nm have been measured in a pump-and-probe experiment using 2+1 resonance-enhanced multiphoton ionization. In a second experiment, the angular distributions of the Cl fragments produced from chloroethylenes at 235 and 238 nm were measured using a perfect-focusing mass spectrometer. In a third experiment, we measured the power dependence of the relative yields of H, Cl, HCl, and HCl+ produced from vinyl chloride at 193 nm. For Cl detachment, two primary processes have been confirmed. One produces an isotropic angular distribution of photofragments, while the other produces an anisotropic distribution. For H atom detachment, an isotropic angular distribution and a Boltzmann velocity distribution were found. The ratio of yields of the Cl and H fragments was found to be 4±1 for CH2CCl2 and higher than 10 for t-CHClCHCl and CCl2CClH. The H, Cl, and HCl yields were found to be first order in laser intensity, while the HCl+ yield was found to be third order. Saturation measurements of the ion yield indicate that the latter results from a 1+1+1 resonance-enhanced process involving a bound state of the parent molecule. This intermediate state may also be responsible for producing the statistical component of the Cl atom product.
Ab initio calculation of the vibronically averaged values for the hyperfine coupling constants in NH2, NHD, and ND2
Authors: Perić M. , Engels B.
Published: July 2002

Vibronically averaged values for K=0 and K=1 bending levels in the energy range between 0 and 25 000 cm−1 are computed for the 14N, H, and D atoms in NH2, NHD, and ND2. The pure ab initio electronic potentials, as well as those derived by fitting of experimentally observed band positions are employed. Effects of vibronic coupling and local perturbations of close-lying levels belonging to different electronic states are discussed.
On the performance of large Gaussian basis sets for the computation of total atomization energies
Authors: Martin J. M. L.
Published: July 2002

The total atomization energies of a number of molecules have been computed using an augmented coupled-cluster method and [5s4p3d2 f1g] and [4s3p2d1f] atomic natural orbital (ANO) basis sets, as well as the correlation consistent valence triple zeta plus polarization (cc-pVTZ) and correlation consistent valence quadrupole zeta plus polarization (cc-pVQZ) basis sets. The performance of ANO and correlation consistent basis sets is comparable throughout, although the latter can result in significant CPU time savings. Whereas the inclusion of g functions has significant effects on the computed ΣDe values, chemical accuracy is still not reached for molecules involving multiple bonds. A Gaussian-1 (G1) type correction lowers the error, but not much beyond the accuracy of the G1 model itself. Using separate corrections for σ bonds, π bonds, and valence pairs brings down the mean absolute error to less than 1 kcal/mol for the spdf basis sets, and about 0.5 kcal/mol for the spdfg basis sets. Some conclusions on the success of the Gaussian-1 and Gaussian-2 models are drawn.
Xenon enhancement and third harmonic generated lines in the 3+m-photon ionization spectroscopy of CO (A 1Π←X 1Σ+) in CO+Xe mixtures
Authors: Jiang Bo , Sha Guohe , Sun Weizhong , Zhang Cheng , He Jinbao , Xu Su , Zhang Cunhao
Published: July 2002

A study of 3+m-photon ionization spectra of CO (A←X) transitions in CO+Xe mixtures shows strong Xe enhancement and the appearance of extra ionization ‘‘lines’’ to the violet end of the respective CO (A←X) vibrational bands. Both facts are adequately interpreted as the result of reabsorption of laser-induced third harmonic generation (THG) in CO+Xe mixtures. The shift of the THG line from the CO (A←X) band origin and its profile, together with their dependence on the CO/Xe pressure ratio, have all been satisfactorily predicted by calculations based on the phase-matching requirement for THG, in which a semiempirical expression for the refractive index of CO is proposed and used with success.
A test of the possibility of calculating absorption spectra by mixed quantum-classical methods
Authors: Haug Kenneth , Metiu Horia
Published: July 2002

Some of the most efficient methods for studying systems having a large number of degrees of freedom treat a few degrees of freedom quantum mechanically and the remainder classically. Here we examine how these methods fare when used to calculate the cross section for photon absorption by a quantum system imbedded in a medium. To test the method, we study a model which has two degrees of freedom and mimicks the properties of a one-dimensional alkali atom–He dimer. We treat the electron motion quantum mechanically and the distance between the He atom and the alkali ion classically. Light absorption occurs because the electron is coupled to radiation. The calculation of the absorption cross section by quantum-classical methods fails rather dramatically−at certain frequencies, the absorption coefficient is negative. By comparing with exact quantum calculations, we show that this failure takes place because the time evolution of the classical variables influences the dynamics of the quantum degree of freedom through the Hamiltonian only; important information, which a fully quantum treatment would put in the wave function, is missing. To repair this flaw, we experiment with a method which uses a swarm of classical trajectories to generate a ‘‘classical wave function.’’ The results are encouraging, but require substantial computer time when the number of classical variables is large. We argue that in the limit of many classical degrees of freedom, accurate calculations can be performed by using the time-dependent Hartree method and treating some degrees of freedom by exact numerical methods (e.g., a fast Fourier transform procedure) and the others by Gaussian wave packets or any other propagation method that is accurate for a very short time. This procedure leads to a simple time domain picture of dephasing and line broadening in the case of a localized quantum system imbedded in a medium with heavy atoms.
Molecular-beam laser spectroscopy of lanthanum monofluoride: Magnetic hyperfine and dipole moment measurements
Authors: Simard Benoit , James Andrew M.
Published: July 2002

The lanthanum monofluoride molecule (LaF) was generated in a pulsed molecular beam by chemical reaction in a laser-produced plasma. The (0,0) and (1,0) bands of the B 1Π–X 1Σ+ system of LaF (ν00=16 184.52 cm−1), and the 0+–X 1Σ+ band (ν00=16 637.95 cm−1), were investigated at sub-Doppler resolution (120 MHz) using a ring dye laser to excite fluorescence. The electron orbital-nuclear spin interaction parameter (magnetic hyperfine a parameter) was determined to be +138(5) MHz and +149(5) MHz for the v=0 and v=1 levels of the B 1Π state, respectively (2σ error bounds). The observed hyperfine structure is interpreted in terms of ligand field theory. Molecular rotational constants for all three bands were found to be in good agreement with previous work [Schall et al., J. Mol. Spectrosc. 100, 437 (1983)]. The permanent electric dipole moments of the X 1Σ+ and 0+ states of LaF were determined by molecular-beam Stark spectroscopy to be 1.808(21) D and 3.43(10) D (2σ errors). Results are compared with recent experimental determinations of the dipole moments of the other group III monofluorides.
Ionization potential and electron affinity of the Au atom and the AuH molecule by all-electron relativistic configuration interaction and propagator techniques
Authors: Pizlo Artur , Jansen Georg , Heß Bernd Artur , von Niessen Wolfgang
Published: July 2002

We present the results of all-electron relativistic and nonrelativistic ab initio calculations on ionization potentials, electron affinities, and excitation energies for the gold atom and the gold hydride molecule. Values obtained from selecting multireference configuration interaction (CI) methods, full-class singles–doubles multireference CI, averaged coupled pair functional (ACPF), and various Green’s function methods {outer valence Green’s function (OVGF), algebraic diagrammatic construction in third order [ADC(3)]} are compared and generally found to be in good agreement. Relativistic effects are taken into account by means of the spin-free no-pair Hamiltonian obtained from a second-order Douglas–Kroll transformation. The influence of relativistic effects and electron correlation on the investigated properties is discussed. As far as experiments are available, our calculations are in good agreement with the measurements. Our best estimate for the ionization potential of the gold atom is 9.08 eV (experiment 9.22 eV) and 2.28 eV for the electron affinity (experiment 2.31 eV).
Effects of the orientational pair correlation on second order nonlinear optical coefficients
Authors: Wang C. H.
Published: July 2002

The orientational order parameter associated with the second order nonlinear optical (NLO) coefficient is calculated by using a statistical mechanics method that includes the angular dependent intermolecular interaction potential. It is shown that in addition to the alignment of dipoles due to the poling field, the orientational pair correlation (OPC) factor is also important in determining the orientational order parameter. When OPC is included, the order parameter becomes a function of the NLO chromophore concentration. The concentration dependent part of the order parameter is shown to be proportional to the Kirkwood g factor associated with the orientational correlation of the electric dipoles. In the weak field limit, it is shown that the ratio of two nonlinear optical coefficients, χ33/χ31, remains equal to 3, independent of the specific nature of the anisotropic intermolecular potential.
Photon echo measurements in liquids: Numerical calculations with model systems
Authors: Cho Minhaeng , Fleming Graham R.
Published: July 2002

Two- and three-pulse photon echo signals are calculated for various model systems. The use of an experimental solvation correlation function as the solvent fluctuation correlation function leads to two conclusions. First, inertial solvent motion plays a major role in the electronic dephasing process. Second, simple models such as Markovian or exponential models for the solvent fluctuation correlation function may not provide an adequate description of the echo signal. The real and imaginary parts of the echo response, which may be measured via heterodyne detected stimulated photon echoes, are compared with conventional photon echo signals.
Interatomic potential of the HgNe van der Waals complex in the E(3Σ+) Rydberg state
Authors: Okunishi Misaki , Yamanouchi Kaoru , Onda Ken , Tsuchiya Soji
Published: July 2002

The lowest Rydberg state E(3Σ+) of the HgNe van der Waals complex has been investigated by optical–optical double resonance (OODR) spectroscopy using two intermediate electronic states of A 30+ and B 31. The E–B band exhibits an oscillatory free–bound continuum, which reflects a Franck–Condon projection of the wave function of the B state onto the repulsive part of the E state potential. In the E–A band, two relatively sharp peaks are observed together with a free–bound continuum showing an asymmetric interference structure. The observed intensity patterns of the E–B and E–A bands are interpreted by a potential barrier characteristic of the E state potential, which traps two quasibound vibrational states in the inner well. Based on (i) the observed Franck–Condon pattern of the free–bound transitions; (ii) the transition’s wave number of the bound–bound transitions in the E–A band; and (iii) the rotational constants of the quasibound (v=0 and v=1) levels in the E state, the interatomic potential of the E state is derived by a numerical simulation. The height of the potential barrier measured from the dissociation limit of Hg (7 3S1)+Ne is determined to be 153 cm−1 and the interatomic distance at the top of the barrier to be located at 3.9(1) Å.
Photodissociation spectroscopy of the Mg+–CO2 complex and its isotopic analogs
Authors: Yeh C. S. , Willey K. F. , Robbins D. L. , Pilgrim J. S. , Duncan M. A.
Published: July 2002

Mg+–CO2 ion–molecule cluster complexes are produced by laser vaporization in a pulsed nozzle cluster source. The vibronic spectroscopy in these complexes is studied with mass-selected photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer. Two excited electronic states are observed (2) 2Σ+ and 2Π. The 2Π state has a vibrational progression in the metal–CO2 stretching mode (ωe′=381.8 cm−1). The complexes are linear (Mg+–OCO) and are bound by the charge–quadrupole interaction. The dissociation energy (D0″) is 14.7 kcal/mol. Corresponding spectra are measured for each of the 24, 25, and 26 isotopes of magnesium. These results are compared to theoretical predictions made by Bauschlicher and co-workers.
The van der Waals approach to adsorption on heterogeneous surfaces
Authors: Bakaev V. A. , Steele W. A.
Published: July 2002

The van der Waals approach is widely used in the theory of liquids and nonlocalized adsorption on homogeneous surfaces. It takes as a reference system the system of hard or soft repulsive spheres and considers attractive interaction between adatoms as a perturbation. In the present paper, this approach is extended to the case of localized adsorption where adsorbed molecules are located near deep minima of the adsorption potential (at adsorption sites) on a surface. The high temperature expansion of thermodynamic potentials is presented in a form convenient for adsorption applications. With the help of this method, a mean field correction to the Langmuir isotherm on a general (homogeneous or heterogeneous) surface is obtained. This correction is much simpler than that described in the literature (for the lattice model) because it refers not to adsorption, but to chemical potential. It is shown that for the case of a one-dimensional model, the corrected Langmuir isotherm deviates considerably from the exact isotherm obtained by the transfer matrix method for a homogeneous surface (line), but almost coincides with that obtained for a strongly heterogeneous surface. Computer simulation of adsorption isotherms of Ar atoms deprived of mutual attraction on two irregular and one regular surface also shows that the mean field correction (van der Waals approach) makes it possible to take into account attractive interaction between adatoms and works better on heterogeneous (irregular) surfaces than on a homogeneous (regular) one.
Mobility of stretched water
Authors: Vaisman Iosif I. , Perera Lalith , Berkowitz Max L.
Published: July 2002

To study the mobility of stretched SPC/E water and its dependence on temperature and density, five molecular dynamics computer simulation runs were performed. Three runs were performed at temperature 300 K and densities 1.0, 0.9, and 0.8 g/cc. Two more runs were performed at temperature 273 K and densities 1.0 and 0.9 g/cc. At temperature 300 K, the translational diffusion coefficient of the stretched SPC/E water increased with the stretch, at temperature 273 K the translational diffusion decreased with the stretch. This behavior is correlated with the observed changes in the hydrogen bonding pattern of water.
A critical comparison of time dependent models for gas–surface sticking
Authors: Jackson Bret
Published: July 2002

A number of theories describing gas–surface energy transfer are examined. In these theories the gas particle can interact and exchange energy with the vibrational modes of the solid, which are treated quantum mechanically. All of the theories are applied to the same set of model systems, and the results are compared. Probabilities for energy transfer and sticking are computed for He, Ne, and Ar incident on a Cu surface, for a wide range of particle energies and surface temperatures. A recently developed close-coupling approach which includes only single-phonon processes is presented. It is accurate for light particles at low energies and temperatures. Two common approaches to the problem based on an uncorrelated particle–bath state, the forced oscillator and time dependent Hartree methods, are discussed. They are compared with each other and with the close-coupling calculation in the one-phonon limit. Mean field approaches which attempt to improve upon these zero-correlation models are presented and compared with the above models. A new one-phonon multiconfiguration self-consistent field model is also examined.
Dissociative electron attachment to S2F10, S2OF10, and S2O2F10
Authors: Olthoff J. K. , Stricklett K. L. , Van Brunt R. J. , Moore J. H. , Tossell J. A. , Sauers I.
Published: July 2002

The absolute cross sections for dissociative electron attachment to the molecules S2F10, S2OF10, and S2O2F10 were measured in an electron transmission experiment. The corresponding negative-ion fragments were identified in a separate mass spectrometric measurement. For S2F10, the attachment of thermal electrons (energy less than 0.1 eV) appears to result primarily in the formation of F− and SF5− with possibly a small fraction of SF4− and SF6−. The ions F− and SF5− are also produced from two attachment resonances at electron energies of about 4.5 and 9.5 eV. Both S2OF10 and S2O2F10 have unusually large dissociative attachment cross sections (on the order of 10−12 cm2) at energies near 0.1 eV. Electron attachment to S2OF10 yields primarily SOF5−, while S2O2F10 yields both SF5− and SOF5− with possible minor fractions of F− and SOF3−. Self-consistent-field calculations have been carried out on the neutral molecules and the corresponding anions to aid in the description of the observed dissociative attachment.
Coherent Raman observations of factor group splittings and lattice vibrations in nanocrystals formed in supersonic expansions
Authors: Lee K. H. , Brown K. W. , Triggs N. E. , Richardson A. D. , Rich N. H. , Nibler J. W.
Published: July 2002

Coherent anti-Stokes Raman spectra are reported for solid, nanometer-sized C2H2 and CO2 aggregates formed in free jet expansions. From comparisons with spectra of equilibrium samples, the clusters are seen to be face centered cubic with a crystalline character indicated by the appearance of vibrational factor group splittings and, for CO2, by the observation of two external lattice librations.
A quasiparticle derivation of a zeroth-order Hamiltonian for use in multiconfigurational perturbation theories
Authors: Murphy Robert B. , Messmer Richard P.
Published: July 2002

A derivation of a zeroth-order Hamiltonian for use in multiconfigurational perturbation theories is presented. This derivation is based on a quasiparticle Green function formalism. It is shown that this formalism can naturally describe the physical picture of the zeroth-order Hamiltonian as an approximate one-particle potential set up by the multiconfigurational reference wave function. The derivation also provides a physically meaningful definition of single-particle eigenvalues which arise from the multiconfigurational perturbation theory.
State-specific vibrational predissociation and interconversion tunneling quenching at 3ν1 and 3ν2 of (HF)2
Authors: Chang Huan-C. , Klemperer William
Published: July 2002

We report the second overtone (Δv=3←0) spectra of the free-HF (ν1) and bound-HF (ν2) stretches of (HF)2 using laser induced fluorescence. Subbands of K=0←0 and K=1←0 are detected near 900 nm with linewidths spanning almost two orders of magnitude. The line broadening (Δνpd) due to vibrational predissociation is not only mode specific but also is state specific. A fit of the spectral lines to a Voigt profile reveals Δνpd=10 GHz for the parallel band of 3ν2, and 0.10 and 1.9 GHz for the parallel and the perpendicular bands of 3ν1, respectively. The linewidths of these subbands are J and tunneling state independent. The K-dependent vibrational predissociation is attributed to near-resonant centrifugal interaction of the K=1 state with the K=1 combination mode of the bound HF stretch (3ν2) and the antisymmetric bend (ν5). The exceedingly state-specific behavior is at variance with elementary density of states arguments. Spectroscopic constants of these two K subbands and two tunneling states (A+ and B+) of 3ν1 are determined from their rotationally resolved manifolds. For the parallel band, we obtain band origins ν0=11 273.501 cm−1 (A+), 112 73.499 cm−1 (B+), rotational constants B̄=0.221 177 cm−1 (A+), 0.221 179 cm−1 (B+), and centrifugal distortion constants D=2.02×10−6 cm−1 (A+), 2.05×10−6 cm−1 (B+).

For the perpendicular band, ν0=11 299.850 cm−1 (A+), 11 299.847 cm−1 (B+), and B̄=0.222 02 cm−1 (A+), 0.222 04 cm−1 (B+). The interconversion tunneling splitting is found to be 0.0024 cm−1, showing that the tunneling motion of the dimer could be quenched entirely. For the 3ν2 where only the R branch is resolved, the breadth of the lines prevents accurate determination of its spectroscopic constants. The band is estimated to center at 11 043.09 cm−1 with a rotational B̄ constant of 0.2240 cm−1. All the constants indicate that a stronger hydrogen bond is formed at higher valence vibrational states. The shifts of the free- and the bound-HF stretching frequencies from that of the monomer are −99.306 and −329.72 cm−1, respectively. Finally, we present an analysis for the rotational dependence of the tunneling in states of v1, which suggests that the transition state, under the assumption of C2h geometry, has the HF units oriented at 33° with respect to the F–F axis.
Large spectral bandwidth stimulated Rayleigh-wing scattering in CS2
Authors: Wang Dadi , Rivoire Geneviève
Published: July 2002

Large spectral bandwidth stimulated Rayleigh-wing scattering (SRWS) induced by picosecond laser pulse has been investigated in bulk liquid CS2. The spectral broadening range of SRWS can reach up to 200 cm−1 on Stokes side. Many features of this scattering, such as the shift of the spectral maximum from excitation line and the temperature dependence, are presented. In order to explain the spectral broadening phenomenon, a possible mechanism based on collision-induced light scattering is also discussed.
Effects of methyl substitution on magnetic field effects on fluorescence of isolated molecules. 4- and 5-methylpyrimidine
Authors: Ohta Nobuhiro , Yamazaki Iwao
Published: July 2002

Magnetic field dependence of fluorescence of jet-cooled 4-methylpyrimidine (4-MP) and 5-methylpyrimidine (5-MP) has been examined following excitation into the individual rotational lines belonging to lower vibronic bands of the S0→S1 transition. This work extends our earlier study of magnetic field effects on photophysical processes of pyrimidine vapor. Rovibrational state dependence of the magnetic field effects on fluorescence similar to pyrimidine is observed both in 4-MP and in 5-MP. Methyl substitution remarkably enhances the efficiency of magnetic quenching, and the efficiency at the ‘‘1e’’ methyl internal rotor level is higher than that at the ‘‘0a1’’ level in both compounds. Further, the efficiency of magnetic quenching of 4-MP is much higher than 5-MP, suggesting that a high barrier height to methyl internal rotation in 4-MP leads to a drastic increase of the effective number of the triplet state coupled to S1 as a result of a strong vibration/internal rotation coupling in the triplet states. In both 5-MP and 4-MP, fluorescence lifetime of the slow component becomes shorter with increasing the field strength on any excitation, which is interpreted in terms of the field-induced enhancement of mixing between T1(nπ*) and T2(ππ*) states.
Theoretical study of the lowest 5Π and 5Σ+ states of CO
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Partridge Harry
Published: July 2002

The lowest 5Π and 5Σ+ states of CO are characterized using very large one-particle basis sets and the internally contracted multireference configuration–interaction treatment of electron correlation. It is found that the (1)5Σ+ state is weakly bound (De=572 cm−1) with an re of 4.76 a0, in good agreement with earlier theoretical calculations by Rosenkrantz et al. [Theor. Chim. Acta 82, 153 (1992)]. The (1)5Π state (hereafter designated a″ 5Π) has a weakly bound outer well (De=18 cm−1, re=5.80 a0) and a significantly bound inner well (De=6043 cm−1, re=2.76 a0). It is shown that the emission spectrum between about 360 and 520 nm observed by Bahrdt et al. [Chem. Phys. 144, 273 (1990)] can be assigned to the a″ 5Π−a′ 3Σ+ and a″ 5Π−d 3Δ transitions. Franck–Condon factors are presented to aid in the experimental identification of the a″ 5Π state by absorption from the low-lying triplet states of CO.
Dissociation dynamics of high-v Rydberg states of molecular hydrogen
Authors: McCormack E. F. , Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

A high-resolution, state-selective investigation of the decay of (X 2Σg+)np, v′≥9 Rydberg states of H2 into the H(1s)+H(3l) and H(1s)+H(4l) dissociation continua has been performed. The (X 2Σg+)np Rydberg states were excited by double-resonance excitation via the E,F 1Σg+, v=6, J=0 and 1 states, and the H(3l) and H(4l) dissociation products were detected by monitoring the energy-resolved photoelectrons produced by nonresonant photoionization of the fragments. The energy region of interest (138 800–140 000 cm−1) includes the X 2Σg+, v+=9, N+=0–3 ionization thresholds, the H(1s)+H(4l) dissociation threshold, and the H++H− ion-pair threshold. Evidence of both direct and indirect dissociation involving both singly and doubly excited electronic states is observed. Comparisons to the previously observed X 2Σg+, v+=8 constant-ionic-state photoelectron spectrum and the ion-pair spectrum observed in the same energy region reveal strong competition between the ionization and dissociation processes through both rovibrational and electronic interactions.
Distribution of the widths of resonances for strong overlap and numerous decay channels: Influence on the average survival probability
Authors: Desouter-Lecomte M. , Culot F.
Published: July 2002

The distribution of the widths of N quasidegenerate metastable states decaying into a large number K of dissociation continua is studied in the framework of the random matrix theory. For strong overlap, the distribution of the imaginary parts of the eigenvalues of the effective Hamiltonian H−iΓ/2 is that of the eigenvalues of the width matrix Γ. The latter is found to belong to a model of random matrices proposed by Wigner and developed by Dyson. The analytical expression of the asymptotic density ρ(γ) for equal partial widths and N=K→∞ is a semicircular law centered at twice the mean width γ̄ times a function 1/γ. It predicts extensive fluctuations around the mean with a high density of small widths. As a result, the average survival probability of the metastable states lying within a narrow energy range decays more slowly than the exponential law which is assumed in the RRKM theory.
k dependence of Brillouin halfwidths
Authors: Gomperts Stephen N. , Variyar Jayasankar E. , Kivelson Daniel
Published: July 2002

We find that at a given temperature the Brillouin linewidths in triphenylphosphite are well described by the two-parameter expression Bkα, where the scattering wave number k is varied over two and a half orders of magnitude. The data include measurements in all states of the system from low viscosity liquid to a glass; the parameters α(T) and B(T) exhibit extrema at similar temperatures.
Self-diffusion in colloidal suspensions. Intermediate times
Authors: Carbajal-Tinoco M. D. , Arauz-Lara J. L.
Published: July 2002

A theoretical approach to the study of self-diffusion properties of interacting colloidal particles in suspensions is developed. It is based on the assumption of a model for the memory function Ms(q,t), whose parameters are calculated by using exact short-time conditions for the self-diffusion propagator. Expressions, in terms of microscopic quantities of the system, for the time-dependent self-diffusion coefficient D(t) are derived from three different functional forms for Ms(q,t): a single-exponential function (already introduced in previous communications [J. L. Arauz-Lara and M. Medina-Noyola, J. Phys. A 19, L117 (1986); G. Nägele, M. Medina-Noyola, R. Klein, and J. L. Arauz-Lara, Physica A 149, 123 (1988)], an algebraic function, and a combination of the two. The latter one involves an adjustable parameter (the relative weight of the two modes) which can be determined by fitting the calculated values of D(t) to experimental data at long times. Explicit calculations of D(t) were carried out for systems of hard spheres interacting via a Derjaguin–Landau–Verwey–Overbeek type pair potential. Comparison of our results with Brownian dynamics data [K. J. Gaylor, I. K. Snook, W. van Megen, and R. O. Watts, J. Chem. Soc. Faraday Trans. 2 76, 1067 (1980)] shows that the bimodal model reproduces quite accurately the computer simulation data for D(t) with just one adjustable parameter, suggesting in this way that the actual functional form of the memory function may consist of a fast and a slow decaying mode.
Frequency dependent hyperpolarizabilities of polyynes
Authors: Jaszuński Michal , Jo/rgensen Poul , Koch Henrik , Ågren Hans , Helgaker Trygve
Published: July 2002

Ab initio calculations have been performed for the static and dynamic polarizability and hyperpolarizability for a series of polyynes C2nH2 using self-consistent field (SCF) (n=1–6) and multiconfiguration self-consistent field (MCSCF) (n=1–4) wave functions. We have considered the longitudinal component of the static hyperpolarizability and the same component of the dynamic hyperpolarizability measured in electric-field induced second harmonic generation γESHG=γ(−2ω;ω,ω,0). The frequency dependence of the polarizability and hyperpolarizability has been rationalized in terms of the coefficients in expansions in ω2. The static hyperpolarizabilities vary smoothly with the chain length and satisfy γ(C2nH2)=γ (C2H2)×nX, where X≊3.0. The dynamic hyperpolarizability satisfies a similar relation where X increases slowly with ω.
van der Waals vibrational states of atom–large molecule complexes by a 3D discrete variable representation method: Naphthalene⋅Ar
Authors: Mandziuk Margaret , Bačić Zlatko
Published: July 2002

We present an accurate and efficient method for calculating highly excited 3D van der Waals (vdW) vibrational states of structurally nonrigid M⋅R complexes between an atom R and a large, arbitrarily shaped molecule M. Our method combines the atom–molecule Hamiltonian of Brocks and van Koeven, in which Cartesian components of the vector connecting R and the center of mass of M are used as internal coordinates, with the 3D discrete variable representation (DVR) of all three intermolecular large amplitude degrees of freedom. Our 3D DVR method is aimed at highly anisotropic M⋅R complexes, in which the size of the molecule is typically larger than the average atom–molecule distance. The symmetry of the complex (if any) is exploited by constructing symmetry adapted 3D DVRs which transform under the irreducible representations of the symmetry group, and bring the Hamiltonian matrix to a block diagonal form. The 3D DVR is particularly well suited for description of excited and strongly coupled, delocalized vdW states, and internal motions on very anharmonic intermolecular potentials with multiple minima. We use this method to calculate vdW vibrational energy levels and wave functions of a floppy complex naphthalene⋅Ar. The lower-lying vdW states are assigned by inspection of the wave function plots.
Infinitesimal differential diffusion quantum Monte Carlo study of CuH spectroscopic constants
Authors: Belohorec Peter , Rothstein Stuart M. , Vrbik Jan
Published: July 2002

We show how to extend the formalism of infinitesimal differential diffusion quantum Monte Carlo to the case of higher derivatives of the ground-state energy of a molecule with respect to the molecular geometry for a large-Z molecule. We propose a new all-electron approach based on the idea of using different simulation time-scales for different shells of the constituent atoms, viz., a substantially smaller time-step for innermost shells, and relatively large time-steps for the valence electrons. We avoid the approximations inherent in pseudopotential methods while at the same time, when used in conjunction with variational Monte Carlo, the sampling is done on a firm theoretical basis. We use CuH as an example, obtaining properties with an accuracy on par with those from SDCI, despite our using a crude single-determinant wave function with only two adjustable parameters.
A theory for calculating the surface-adsorbate bond dissociation energy from collision-induced desorption threshold measurements
Authors: Szulczewski Gregory , Levis Robert J.
Published: July 2002

A model is presented for determining the bond dissociation energy, D0, of a surface-adsorbate complex from collision-induced desorption measurements. D0 is calculated from the minimum collision energy required for desorption by correcting for two energy transfer events using a model based on classical mechanics. The model is tested with measurements made for the CH4/Ni{111} system [Beckerle et al., J. Chem. Phys. 93, 4047 (1990)] and a D0 of 118 meV is found.
Infrared external reflection spectroscopy of adsorbates on dielectric substrates: Determining adsorbate orientation in Langmuir monolayers
Authors: Buontempo Joseph T. , Rice Stuart A.
Published: July 2002

Infrared external reflection spectroscopy has enjoyed enormous success in the characterization of adsorbates on metal substrates, but has been used only rarely to study adsorbates on nonmetallic substrates. In this report we present the theory of infrared external reflection spectroscopy as applied to the study of adsorbates on dielectric substrates. We begin by reviewing the well-established general macroscopic theory in which Maxwell’s equations are applied to the model system ambient-phase/adsorbate/substrate. Then we explicitly consider the case of external reflection from an arbitrary dielectric substrate and derive simple, intuitive equations for the infrared external reflection intensity as a function of macroscopic optical parameters. Our interest is in formulating a theory which will allow us to extract from experimental data information about the microscopic structure of the monolayer, such as the orientation of adsorbates on dielectric substrates, without the necessity for concomitant use of other spectroscopic techniques, such as infrared transmission or internal reflection spectroscopies, which may be impossible or inconvenient to implement; such is the case for Langmuir monolayers of long chain amphiphile molecules at the air–water interface. Unfortunately, the use of only external reflection spectroscopy to study a system results in more unknown optical constants than experimental data. To overcome this difficulty we propose for Langmuir monolayers a microscopic model which relates the optical parameters of the adsorbate monolayer to select molecular parameters, those which will allow us to extract the adsorbate orientation from experimental data. To illustrate the method we model the external reflection spectroscopy of long chain amphiphile CH2 stretching modes of typical fatty acids and alcohols. We also derive for this system a very useful dichroic ratio which facilitates the direct interpretation of experimental data. We demonstrate the behavior of the reflection spectrum as a function of adsorbate orientation using simulated spectra.
Calculations of inelastic collisions of excited states of Xe with He and Ar
Authors: Hickman A. P. , Huestis D. L. , Saxon R. P.
Published: July 2002

Coupled channel calculations of thermal collisions of Xe(5p5nl) (nl=6s,6p,5d) with He and Ar have been performed using potential curves and coupling matrix elements that we previously calculated. Coupling among all of the closely spaced 6s′, 6p, and 5d fine structure levels of Xe is fully included. These excited levels lie in the range 76 000–84 000 cm−1 above the Xe ground state, and the spacing of adjacent levels ranges from 84 to 1460 cm−1. Rate constants for level-to-level transitions are calculated at T=300 and T=800 and compared with recent experiments; agreement is generally good. The rate constants are very sensitive to the specific initial and final levels and to the collision partner. This sensitivity can be explained by the pattern of crossings and avoided crossings of potential curves corresponding to individual fine structure levels. The calculations thereby illustrate the importance of spin–orbit effects in heavy atom systems and provide insight into a large amount of data.
Laser-induced gratings in free jets. II. Photodissociation dynamics via photofragment transient gratings
Authors: Butenhoff Thomas J. , Rohlfing Eric A.
Published: July 2002

We describe the use of photofragment transient gratings to measure populations, velocities, and translational anisotropies of nascent, state-selected photofragments. This technique is a variant of two-color laser-induced grating spectroscopy (LIGS); scanning the probe laser gives spectra that reveal fragment-state populations and scanning the grating laser, with the probe laser monitoring a specific fragment state, produces photofragment excitation (PHOFEX) spectra. The temporal behavior of the photofragment grating is used to determine speeds and translational anisotropies of the photofragments. We have demonstrated the photofragment grating method on the NO(2Π1/2, v″=0, J″) fragments produced from the near-threshold photolysis of jet-cooled NO2. The grating PHOFEX spectrum yields the threshold for production of NO(2Π1/2, v″=0, J″=0.5, e)+O(3P2) from NO2 (N″=0) as 25 128.5±0.2 cm−1. For photolysis at 126 cm−1 above threshold, the NO rotational distribution obtained from the probe spectrum is in good agreement with that determined from a LIF spectrum recorded simultaneously. In addition, analysis of the grating decays for state-selected NO fragments gives their speeds and anisotropies; the speeds are in excellent agreement with the known values, even for fragments with as little as 7 cm−1 of translational energy.
Auger electron spectroscopy of molecules: Theory for spin polarization following photoabsorption in rotating linear molecules
Authors: Chandra N. , Sen S.
Published: July 2002

This paper develops theoretical expressions to study angular distribution and spin polarization of those Auger electrons which are emitted in the decay of a vacancy created by the absorption of a photon in a rotating linear molecule. Identical expressions except, of course, for different decay amplitudes, in both the Hund’s coupling schemes (a) and (b), are obtained for the differential Auger current emitted in the transition J→Jf measured by an electron spectrometer sensitive to spin detection. The structure of these angular distributions is exactly the same as that of the spin-resolved photoelectrons from unoriented atoms and molecules. The present paper thus puts the angle- and spin-resolved Auger and photoelectron spectroscopies on the same footing wherein identical geometrical and kinematical analysis is applicable. The four parameters needed to completely characterize such distributions depend, in the present case, on rotational orientation and/or alignment of the photoexcited molecule, in addition to its Auger decay amplitudes. The use of parity-adapted molecular states separates the Auger spectra into even and odd partial wave components of the ejected electron continuum in both of the coupling schemes. Our analysis shows that the integrated Auger current is spin resolved provided it is produced in the decay of oriented vacancies. We further find that Auger electrons which leave the molecular ion in Jf=0 state may have nonzero degree of spin polarization if they follow absorption of only circularly polarized light. In this case, both the angular distribution and spin polarization of emitted electrons become totally independent of Auger dynamics. Thus, angle- and spin-resolved Auger electron spectroscopy can be used to produce polarized electrons, to determine rotational orientation and alignment of linear molecules, to study their structure and dynamics, and to prepare ions of such molecules in selective ro-vibronic states.
The spatial distribution of relaxation times in a model glass
Authors: Foley Michael , Harrowell Peter
Published: July 2002

One approach to exploring the mechanisms which give rise to the characteristic distribution of relaxation times in a glass is to examine the correlation between this distribution and the spatial distribution of the relaxation rates. Detailed maps are presented of the spatial distribution of relaxation times in the facilitated kinetic Ising model of the structural glass transition. A statistical analysis of these spatial distributions leads to a set of correlation lengths associated with different aspects of the cooperative dynamics. One such length scale, ξ2, is found to be closely associated with the average relaxation time τL, the two quantities being related by a power law over 6 orders of magnitude variation in τL.
Intramolecular screening of intermolecular forces
Authors: Liang Ying Q. , Hunt K. L. C.
Published: July 2002

By use of nonlocal polarizability densities, we analyze the intramolecular screening of intermolecular fields. For two interacting molecules A and B with weak or negligible charge overlap, we show that the reaction field and the field due to the unperturbed charge distribution of the neighboring molecule are screened identically via the Sternheimer shielding tensor and its generalizations to nonuniform fields and nonlinear response. The induction force on nucleus I in molecule A, derived from perturbation theory, results from linear screening of the reaction field due to B and nonlinear screening of the field from the permanent charge distribution of B. In general, at first or second order in the molecular interaction, the screening-tensor expressions for the force on nucleus I involve susceptibilities of one order higher than the expressions derived from perturbation theory. The first-order force from perturbation theory involves permanent charge moments, while the first-order screened force involves linear response tensors; and the second-order screened force depends on hyperpolarizabilities, while second-order induction effects are specified in terms of static, lowest-order susceptibilities. The equivalence of the two formulations for these forces, order by order, is a new illustration of the interrelations we have found among permanent moments, linear-response tensors, and nonlinear response. This work also provides new insight into the dispersion forces on an individual nucleus I in molecule A by separating the forces into two distinct terms—the first term results from changes in the reaction of A to the fluctuating charge distribution of the neighboring molecule B, when nucleus I shifts infinitesimally, and the second term stems from changes in correlations of the fluctuating charge distribution of A itself. Changes in the fluctuation correlations are determined by changes in the classical Coulomb field of nucleus I and by the imaginary part of the hyperpolarizability density of A. The full dispersion force on nucleus I in A is equivalent to the screened force of an effective fluctuating field due to B at imaginary frequencies.
Isotope and microstructure interactions in blends of random copolymers
Authors: Rhee Jangweon , Crist Buckley
Published: July 2002

Combined isotope and microstructure effects on thermodynamic interactions are examined by small-angle neutron scattering from binary blends of random copolymers having one partially deuterated component. These copolymers are based on linear and ethyl branched C4H8 units and are models for poly(ethylene-co-butene-1). The Flory–Huggins interaction parameter χ is found to be ∼100% larger for blends in which the more branched copolymer is labeled with deuterium. This ‘‘switching’’ effect, where the chemical microstructure of the labeled component affects χ, is confirmed by critical temperatures in blends having all possible isotope combinations. Parallel studies by Graessley et al. show the same effects with copolymers having higher branch concentrations. Analysis in terms of copolymer/copolymer theory indicates that the repulsive interactions between monomers differing in both chemical microstructure and isotope depend on which repeat unit, linear or branched, is labeled with deuterium. A simple model based on molar volumes of linear and branched, hydrogenous and deuterous repeat units accounts for the observed effects in a semiquantitative manner. The contribution to χ from chemical microstructure does not obey simple copolymer theory; the strength of repulsive interactions depends on the average composition of the chains in addition to the composition difference between components.
Exchange interactions in trinuclear basic chromium(III) clusters: Direct observation of the magnetic spectrum by inelastic neutron scattering
Authors: Jayasooriya Upali A. , Cannon Roderick D. , White Ross P. , Stride John A. , Grinter Roger , Kearley Gordon J.
Published: July 2002

Incoherent inelastic neutron scattering spectra are reported for salts of the complex [Cr3O(OOCCH3)6(OH2)3]+. The data are consistent with predominantly antiferromagnetic coupling between pairs of chromium ions. The complete spectrum of transitions between successive states with total spin S=1/2, 3/2, 5/2, 7/2, and 9/2 has been observed for the first time. Splittings of the ground state S=1/2 have been observed directly and attributed to lowering of symmetry of the triangular cluster. For the chloride salt the data confirm that two sets of complex cations with different degrees of symmetry lowering are present in the crystal, at least at the lowest temperatures used (T=1.4 to 50 K). In principle, the relationship of J values for the symmetry-lowered case could be described as ‘‘isosceles,’’ with Jab≳Jbc=Jac or Jab<Jbc=Jac; or as ‘‘scalene,’’ with all three J values different. We find that, for at least one of the two sets of metal ion clusters, the scalene case applies, with J values of −11.5±0.2, −10.5±0.2, and −9.7±0.2 cm−1. Using appropriate linear combinations of J values, analogous to symmetry coordinates for a triatomic molecule, these values can be regarded as being midway between the two limiting isosceles cases. With these parameters, the Heisenberg spin-only coupling model accounts well for the energies and intensities of the magnetic transitions.
Bond-distance and bond-angle constraints in reaction-path dynamics calculations
Authors: Lu Da-hong , Truhlar Donald G.
Published: July 2002

Projection operator techniques for enforcing bond-distance and bond-angle constraints in reaction-path dynamics calculations are presented. These techniques provide a systematic method for carrying out reduced-dimensionality calculations in generalized normal mode coordinates with constrained values for selected internal coordinates. The methods are illustrated with generalized transition-state theory and multidimensional semiclassical tunneling calculations for the reactions OH+H2→H2O+H and CH3+H2→CH4+H in which one or more bond lengths and/or bond angles is held fixed.