bluuebunny/crossref_metadata_2025 huggingface dataset:
A high-resolution analysis of the Ã 2A′–X̃ 2A′ transition of CaSH by laser excitation spectroscopy
Authors: Jarman C. N. , Bernath P. F.
Published: July 2002

The high resolution spectrum of the Ã 2A′–X̃ 2A′ transition of CaSH has been recorded near 650 nm using laser excitation spectroscopy with selected fluorescence detection. While both a-type and b-type rotational transitions have been observed, extensive measurements have been made for the b-type transitions up to K′a = 4 and K″a = 5. Altogether over 3000 rotational lines have been measured and fitted with an A-reduced Hamiltonian. The X̃ 2A′ state has rotational constants A=9.693 322(46) cm−1, B=0.141 864 7(33) cm−1, and C=0.139 581 0(33) cm−1. The Ã 2A′ state has a band origin at 15 380.284 7(2) cm−1, and effective values for the rotational constants A=9.090 808(78) cm−1, B=0.147 459 8(34) cm−1, and C=0.144 804 2(34) cm−1. An approximate r0 structure for CaSH is discussed. The Ã 2A′ state is the lower energy Renner–Teller component of the ‘‘Ã 2Π’’ state of the hypothetical linear CaSH molecule, and consequently was found to have a relatively large positive value for the spin–rotation parameter εaa at 3.445 69(26) cm−1. The upper asymmetry component of the F1 spin component of the Ka=1 stack and the F2 spin component of the Ka=0 stack in the Ã 2A′ state perturb each other with an avoided crossing between J=37.5 and J=38.5. These two spin components interact through the off-diagonal ‖εab+εba‖/2 element of the spin–rotation tensor. For CaSH, the Ã 2A′ state has ‖εab+εba‖/2=0.065 915(46) cm−1.
Dipolar interactions between NH3 molecules trapped in solid argon. I. Kinetics of the nuclear spin species conversion
Authors: Gauthier-Roy B. , Abouaf-Marguin L. , Boissel P.
Published: July 2002

The rovibrational ν2 and ν4 absorption spectra of NH3 trapped in solid argon at low temperatures are re-examined. They provide a quantitative basis to study the spin species conversion, which occurs in the matrix at T=5 K, after a fast cooling down from 30 K where Boltzmann’s equilibrium has been ensured. The kinetics for this conversion is monitored by recording the integrated absorption of the aQ(1,1) ‘‘M’’ triplet, at different times after cooling down. A decay law of the (v=0, a, J=K=1) level population, on the form exp−(αt)1/2, is found and this shows that intermolecular proton–proton interactions, of magnetic dipolar type, dominate the conversion process in the concentration range 1/200–1/1000. This interaction can be characterized by a distance dependent rate constant kr=κ r−6, with κ=1800 s−1 Å6.
A quantum molecular dynamics simulation of an excess electron in methanol
Authors: Zhu Jianjun , Cukier R. I.
Published: July 2002

The structure, energetics, and dynamics of a ground-state, excess electron in the polar solvent methanol are simulated. Two pseudopotentials describing the interaction of the excess electron and the methanol molecules are developed. An adiabatic simulation method is used whereby the Schrödinger equation for the electron is solved in the presence of a fixed solvent configuration and the solvent configuration is advanced with the forces arising from the methanol interactions and the expectation value of the electron–methanol interaction. We find that the electron is localized with average radii of 3.1 and 2.6 Å, depending on which pseudopotential is used, and both show a fairly strong solvation structure. The methanols are on average methoxyl bond-dipole oriented toward the electron in one model and hydroxyl bond-dipole ordered in the other. The binding energy (kinetic plus potential) of the electron fluctuates about the value −2.2 eV. The electron solvates on about a 400 fs time scale with a fast decay component of ∼30 fs. The dynamics of the electron’s center of mass can be described by a diffusion process with a diffusion constant ≂1.9×10−5 cm2 s−1.
Structure and dynamics in hexagonal ice: A molecular dynamics simulation with an ab initio polarizable and flexible potential
Authors: Sciortino Francesco , Corongiu Giorgina
Published: July 2002

We perform a molecular dynamics simulation of the hexagonal solid phase of water, using a new polarizable and flexible potential which has been derived from quantum mechanical calculations. We calculate and compare with available experimental data the molecular structure, the density of states in the translational, librational, and vibrational regions as well as the acoustic dispersion relations. The molecular structure in the condensed phase is found to be in agreement with the recent interpretation of the neutron scattering data of Kuhs and Lehmann [W. F. Kuhs and M. S. Lehmann, J. Phys. (Paris) 48, C1, 3 (1987)]. The collective low frequency modes in the simulated ice are in good agreement with the experimental data in the c-axis direction but 20% softer in the hexagonal plane.
The Franck–Condon structure of the 1A→1B transition of cis- and trans-hexatriene: An ab initio modeling
Authors: Zerbetto Francesco , Zgierski Marek Z.
Published: July 2002

The vibronic structure of the lowest lying intense polyenic electronic transition of cis- and trans-hexatriene is investigated with the recently introduced ab initio configuration interaction singles (CIS) procedure using the 6-31+G basis set. First, the molecular structures are optimized and the vibrational frequencies are calculated at the stationary points along the central CC torsional curve. Selected points along such a curve are also optimized to furnish a better understanding of the isomerization process in the 1B state. Then, the displacement parameters are obtained and used to simulate the Franck–Condon activity in the 1A–1B transitions of the two isomers of hexatriene. A simple scaling procedure of the displacement parameters is proposed to improve the agreement between theory and experiment. The model calculations of the Franck–Condon structure of both the harmonic and the anharmonic degrees of freedom simulate very well the absorption spectra of both isomers and explain the origin of the different vibronic linewidth in the two electronic transitions.
Control of proton-transfer reactions with external fields
Authors: Morillo M. , Cukier R. I.
Published: July 2002

The possibility of controlling the tunneling of a proton in a condensed phase with the use of static or time varying external fields, which couple to the transition dipole moment of the tunneling proton, is investigated. Starting from a Hamiltonian, an equation of motion describing the tunnel dynamics of the proton as a stochastically modulated, externally driven, two-level system is derived under suitable restrictions. For external fields that satisfy a precise connection between frequency and amplitude, whereby the resulting Floquet eigenvalues (quasienergies) are degenerate, tunneling can be suppressed in the absence of the medium. With the medium present, we examine the consequences to this tunnel suppression. Static fields, if sufficiently strong, can also suppress tunneling. Expressions are derived for the effect of a static external field on the medium-influenced, tunnel-rate constant. The rate constant can be enhanced or decreased, depending on the sizes of the medium-reorganization energy and external field and the latter’s direction relative to the tunnel system. It is demonstrated that proton tunneling in dicarboxylic acids would be a good candidate to exhibit a proton-transfer rate dependent on the relative orientation of the external field and proton tunnel system.
Isotope effect in vibration-mediated ultraviolet photodesorption: A short-time-limit harmonic approximation
Authors: Zhu X.-Y.
Published: July 2002

Excitation and de-excitation of the electronic states of an adsorbed molecule on a solid surface result in intramolecular vibrational excitation. The internal vibrational energy can couple to the adsorbate–substrate bond and lead to molecular desorption. In this mechanism, the isotope effect in molecular desorption is determined not by the mass ratio of the leaving particles, but by that in the internal coordinate. This process is modeled within the context of semiclassical wave packet dynamics, which, in the short-time-limit harmonic approximation, allows analytical solution. Results are obtained for the ammonia/GaAs(100) system and are compared with experimental observations.
Three-vector correlation study of orientation and coherence effects in Na(3p,2P1/2←2P3/2)+He: Semiclassical and quantum calculations
Authors: de Vivie-Riedle Regina , Driessen Jan P. J. , Leone Stephen R.
Published: July 2002

‘‘Multistructure’’ cross sections, of both conventional and coherence types, are calculated for the fine structure transition Na(2P1/2←2P3/2)+He in an energy range of 10–200 meV. The cross sections are related to conditions of a crossed beams experiment in which the three controlled vectors are the initial relative velocity and the two polarization directions of excitation and probe laser beams. Both semiclassical and quantum mechanical calculations are performed. The semiclassical method is employed to interpret and visualize the collision mechanism leading to the multistructure cross sections. In addition, the validity of the approximations used in the semiclassical model is investigated. For the conventional cross sections, the semiclassical and quantum-mechanical results are in good agreement. We find that both initial sodium states 3 2P3/2,3/2 and 3 2P3/2,1/2 preferentially populate the 3 2P1/2,−1/2 final state, i.e., a final state in which the sign of the magnetic quantum number has changed. Vector evolution diagrams are introduced to interpret the conventional cross sections and the pattern of the Stückelberg-like oscillations. The semiclassical model gives the correct order of magnitude for the coherence cross sections, but in this case, is inadequate in the quantitative description; thus the coherence cross sections are mainly studied with full quantum calculations. The coherence cross section is especially sensitive to small variations in the energy difference and shape of the potential curves. The value of the coherence cross section is strongly correlated to the position and well depth of the potential minimum of the 2Π state.
Emission spectroscopy of nitrogen in a supersonic discharge. Evidence of excited neutral nitrogen clusters
Authors: Graham Daniel J. , Hurst Scott M. , Luo Jing-Chen
Published: July 2002

Emission spectra indicating transient clusters in a supersonic nitrogen discharge are presented. Select spectral bands are attributed to emission from N2(C 3Πu,v′=0,1)–N2(X 1Σg+). The cluster transitions manifest as blue-degraded, blue-shifted sidebands of the second positive group due to emission from N2(C 3Πu). Results and significance concerning these previously undiscovered nitrogen clusters are discussed.
Rotational energy transfer in the Na2 b 3Πu state: Propensity rules for rotation, spin–orbit component, and e/f-parity changing collisions
Authors: Li Li , Zhu Qingshi , Lyyra A. M. , Whang Thou-Jen , Stwalley W. C. , Field R. W. , Alexander M. H.
Published: July 2002

State-to-state collision-induced transitions within the Na2 b 3Πu state have been studied by a sub-Doppler optical–optical double resonance (OODR) technique employing cw, single mode dye lasers for the PUMP and PROBE steps. The main experimental results are (1) strong Ω-conservation is observed when a low-J 3Π0 or 3Π1 parent level is initially prepared (we are unable to prepare low-J 3Π2 levels). This Ω-conservation relaxes as J increases. (2) e/f parity conservation (i.e., propensity for even ΔJ changes in homonuclear molecules) is observed at low-J within the 3Π0 manifold. This propensity for e/f-conservation relaxes rapidly as J increases. (3) The e/f parity conservation rule observed within the 3Π0 manifold is observed not to apply to the collision-induced transitions within the 3Π1 manifold. At low-J, ΔJ=±1, e→f transitions are even stronger than ΔJ=±2, e→e transitions. (4) At higher-J, for example J=16, neither Ω-conservation nor e/f-conservation within the 3Π0 manifold are observed. These results are compared with theoretical predictions for a case (a) 3Π state [Alexander and Pouilly, J. Chem. Phys. 79, 1545 (1983)] and with experimental observations on another homonuclear molecule, N2 in the B 3Πg state, which also exhibits intermediate coupling between cases (a) and (b) [Ali and Dagdigian, J. Chem. Phys. 87, 6915 (1987)].
Molecular dynamics simulations with a flexible and polarizable potential: Density of states for liquid water at different temperatures
Authors: Corongiu Giorgina , Clementi Enrico
Published: July 2002

Molecular dynamic simulations for liquid H2O and D2O are reported in the temperature range 238–368 K. We have used the NCC–vib interaction potential, which is obtained from ab initio data, allows vibrations and includes explicitly polarizability. We report on the translational and rotational velocity autocorrelation functions and on the density of states in the range 0–4500 cm−1. Special attention is given to the computation of the translational, librational, bending, stretching and overtones spectra comparing liquid H2O with D2O, temperature dependences, and the positions of the simulated spectra with those from infrared, Raman and neutron scattering experiments. The overall agreement is satisfactory; minor discrepancies in the bending and stretching frequency positions have suggested improvements for the interaction potential.
Resonance enhanced multiphoton ionization photoelectron spectroscopy and pulsed field ionization via the F 1Δ2(v′=0) and f 3Δ2(v′=0) Rydberg states of HCl
Authors: de Beer E. , Buma W. J. , de Lange C. A.
Published: July 2002

In this paper, we report the first rotationally resolved one- and two-color resonance enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PES) study of the HCl molecule. The agreement between our experimental branching ratios and theoretical investigations is excellent. We also report the first zero kinetic energy pulsed field ionization (ZEKE-PFI) experiments carried out in a ‘‘magnetic bottle’’ electron spectrometer. A direct comparison is made between ZEKE-PFI and REMPI-PES spectra for ionization via several rotational levels of the F 1Δ2(v′=0) and f 3Δ2(v′=0) Rydberg states of HCl. Large differences in both the spin–orbit and rotational branching ratios are found between the ZEKE-PFI and REMPI-PES spectra. These differences can be understood qualitatively on the basis of rotational and spin–orbit autoionization mechanisms.
Vibrational spectrum of linear C3H in solid argon
Authors: Jiang Qian , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

A Fourier transform infrared isotopic study has been made of the deuterium and carbon-13 substituted C3H radical produced by trapping in solid Ar the products of the vacuum ultraviolet photolysis of CH2CHCH3 (propylene) or a mixture of methane and acetylene. The 1824.7 and 1159.8 cm−1 absorptions, which in earlier work using only deuterium isotosopic data could be assigned to either the cyclic or linear isomer, can now be definitely identified on the basis of extensive 13C isotopic measurements as the ν2(σ+) and ν3(σ+) fundamentals of linear C3H. The frequencies of the fundamentals and D and 13C isotopic shifts are in good agreement with the predictions of preliminary ab initio calculations for the linear isomer.
Adsorption of fluids in disordered porous media from integral equation theory
Authors: Vega C. , Kaminsky R. D. , Monson P. A.
Published: July 2002

We consider the application of the integral equation theory developed by Madden and Glandt to a recently developed model of methane adsorbed in a silica xerogel. At higher temperatures, above the bulk fluid critical temperature, the theory yields very good predictions of the adsorbate–matrix and adsorbate–adsorbate distribution functions. At lower temperatures, where the attractive intermolecular forces play a more dominant role in determining the microstructure, the agreement between simulation and theory deteriorates somewhat. It also becomes increasingly difficult to obtain solutions of the integral equations at low temperatures. We have found that for this system and others where the matrix particles are much larger than the adsorbate particles the adsorbate–matrix and adsorbate–adsorbate correlation functions differ only slightly from those of an equilibrium mixture of matrix and adsorbate particles. We offer an explanation for this result on the basis of cancellation of diagrams in the cluster expansion of the total correlation function. We discuss the determination of adsorption isotherms from integral equation theory using expressions developed in the context of density functional theory.
Theory of continuum-Raman spectroscopy with pulses
Authors: Shapiro Moshe
Published: July 2002

A uniform theory of Raman scattering and resonance fluorescence from an intermediate dissociative manifold for excitation with pulses is developed. It is shown that transient effects lead to the appearance of additional terms, not included in the Kramers–Heisenberg formula. The role of ‘‘true’’ Raman scattering vs ‘‘resonance fluorescence’’ in contributing to the observed signal is elucidated. It is shown that the relative importance of these two processes is strongly dependent on the pulse parameters and the spontaneous emission lifetimes. ‘‘True’’ Raman is shown to dominate at very short times, during the rise of the pulse. At longer times resonance fluorescence sets in and dominates the observed signal. The important implications of the above to the time-averaged excitation-emission spectrum (the emission signal as a function of the excitation wavelength), with real ns pulses is discussed. The present study explains recent experiments showing rich structure in the excitation emission spectrum of IBr [I. Levy et al., J. Chem. Phys. 96, 1858 (1992)].
Photoabsorption, photoionization, and neutral-dissociation cross sections of SiF4, SiCl4, and Si(CH3)4 in the extreme-ultraviolet range
Authors: Kameta Kosei , Ukai Masatoshi , Numazawa Toshiyuki , Terazawa Norihisa , Chikahiro Yuji , Kouchi Noriyuki , Hatano Yoshihiko , Tanaka Kenichiro
Published: July 2002

The absolute photoabsorption cross sections (σt) and photoionization quantum yields (η) of SiF4, SiCl4, and Si(CH3)4 have been measured in the 50–100 nm range, from which we have evaluated the absolute photoionization cross sections (σi) and the total cross sections for nonionizing processes, i.e., the neutral-dissociation cross sections (σd) following superexcitation. In addition to the broad structures in the σt curve for SiF4 around 95, 83, 78, 74.5, and 70 nm, oscillating structures which have not been found previously are revealed around 65 nm. These structures, which appear also in the present σi curve, are tentatively ascribed to the vibrational progressions of Rydberg states converging to the (4t2)−1C̃ ionic state. Other oscillating structures in the 57–62 nm range are ascribed to the Rydberg states converging to the (5a1)−1D̃ ionic state. Broad peaks around 95, 92, 75, and 58 nm are observed in the σt and σi curves of SiCl4. Rydberg structures converging to the 7a1−1 limit are observed in the σt and σi curves in the 68–74 nm range. A general appearance of the cross sections of Si(CH3)4 resembles that of CH4.
Simulation of ionic diffusion in solid polymer electrolytes with correlated chain motion
Authors: Chang W. , Xu G.
Published: July 2002

Based on the dynamic bond percolation model, we try to regenerate the non-Arrhenius temperature dependence for the ionic conductivity of the polyethylene oxide type polymer electrolytes by considering continuous, correlated motions of the host polymer matrix. To simulate the chain movements, periodic rotations of bond groups of several sizes were introduced into a two-dimensional square lattice. Although the Arrhenius temperature dependence of the rotation frequencies were assumed such that the rotation periods ∝ exp(E/kT), where E increases with the bond group size, the resulting logarithmic diffusivity plotted against reciprocal temperature produces a convex curvature—the typical feature observed by the experiments.
Solvent effects on molecular spectra. II. Simulations of hydrated clusters and dilute solutions of pyrimidine in its lowest (n,π*) singlet excited state
Authors: Zeng J. , Hush N. S. , Reimers J. R.
Published: July 2002

Hydrogen bonding between pyrimidine in its lowest (n,π*) singlet excited state and water in dilute solution is investigated using NPT-ensemble Monte Carlo simulations, and the properties of the pyrimidine–water complex are examined using molecular dynamics. The 1(n,π*) excitation, known experimentally to be delocalized in the gas phase, is shown to remain delocalized when pyrimidine undergoes hydrogen bonding. Assuming that the intermolecular interactions are electronic state independent, Kollman’s Lennard-Jones interactions are combined with molecular charges obtained in four different ways, generating a variety of intermolecular pair potentials. It is found that, in solution, both pyrimidine–water–hydrogen bonds are considerably weakened in the excited state; however, on average, one hydrogen bond per pyrimidine remains. The excited state hydrogen bonding gives rise to structures in the liquid more like those found in van der Waals bonded systems than in strongly hydrogen-bonded systems such as ground-state pyrimidine in water. A blue shift of the absorption band origin is correctly predicted and its magnitude and Franck–Condon contribution are in reasonable agreement with experiment. For pyrimidine–water clusters, minor changes in the intermolecular potential surfaces are shown to cause major qualitative differences in the excited state cluster dynamics, mimicking the observed spectral properties of several different hydrogen-bonded pyrimidine clusters.
Solvent effects on molecular spectra. III. Absorption to and emission from the lowest singlet (n,π*) state of dilute pyrimidine in water
Authors: Zeng J. , Hush N. S. , Reimers J. R.
Published: July 2002

A model is developed for the solvent shift of the center of an electronic absorption or emission band and is applied to the interpretation of the spectra of dilute pyrimidine in water. It is based on the use of standard simulation techniques such as molecular dynamics and Monte Carlo to calculate the liquid structure around the chromophore in its initial electronic state; the solvent shift is then deduced by considering the changes in the electrostatic distribution of the chromophore on vertical Franck–Condon excitation. During the solvent-shift evaluation only, spherical boundary conditions are used and the sample, containing both the solute and a large number of explicitly polarizable solvent molecules, is placed inside a dielectric continuum. The results show that the solvent shifts of both the absorption and fluorescence spectra of pyrimidine in water are comprised of approximately equal contributions from specific hydrogen-bonding interactions and long-range–plus–nonspecific dipole solvation effects. The solvent shift is shown to be very sensitive to the structure of the liquid. A specific aim of our approach is to develop a method applicable to molecular electronic devices, and to inorganic complexes.
Photoabsorption and photoionization cross sections of gas phase GeH4 and Rydberg structures of GeH4 and SiH4
Authors: Wu C. Y. Robert , Chen F. Z. , Judge D. L.
Published: July 2002

The photoabsorption and photoionization cross sections of GeH4 from the ionization threshold to 1060 Å have been measured using synchrotron radiation as the continuum light source. The cross sections for the production of neutral products in the same spectral region have also been determined. Weak diffuse structures were observed in both the photoabsorption and photoionization spectra. The steplike ionization structures in the ionization spectrum were observed. The adiabatic ionization potential was determined to be at 1177.6±0.5 Å which is in excellent agreement with a value recently reported by Ruscic et al. [J. Chem. Phys. 92, 1865 (1990)] using a photoionization mass spectrometry technique. Rydberg series in the 1060–1180 Å region converging to the ground state of GeH4+ have been tentatively assigned. In addition, the assignments of Rydberg states of SiH4 reported in the literature have been re-examined in light of the recently reported ionization potential [Berkowitz et al., J. Chem. Phys. 86, 1235 (1987)].
The molecular parameters of the antimony monodeuteride radical from diode laser spectroscopy
Authors: Urban Rolf-Dieter , Essig Kay , Jones Harold
Published: July 2002

Extensive measurements of the 3Σ ground state infrared spectrum of the antimony monodeuteride radical (SbD) have been carried out using a diode laser spectrometer. Transitions of the fundamental band of all three components of two isotopomers of SbD were measured with a nominal accuracy of ±0.001 cm−1. Only transitions of the O+ component of the v=1←2 hotband were intense enough to be observed. The entire data set was fitted as a single 3Σ state and accurate molecular parameters determined. The LMR data of Stackmann et al. [Mol. Phys. (in press)] on SbH provided invaluable aid with the assignment. Measurements were also carried out on two isotopomers of SbH in order to improve the molecular parameters available for these species.
Femtosecond four wave mixing spectroscopy of conjugated polymers
Authors: Hartmann M. , Mukamel S.
Published: July 2002

Time-resolved four wave mixing (4WM) signals of conjugated polymers are calculated using a density matrix approach and the Hartree–Fock approximation. The influence of the Coulomb interaction as well as size effects on the nonlinear optical response are examined. For the Hückel model (no Coulomb interaction), quantum beats due to the coherent excitation of several band states are found for small sizes, and photon echo-like behavior is obtained for infinite size. The Coulomb interaction changes the nature of the response and gives rise to a broad delayed signal whose temporal profile is determined by the dephasing time.
Approximation of molecular electrostatic potentials
Authors: Köster Andreas M. , Kölle Christian , Jug Karl
Published: July 2002

A new method is presented for the approximation of molecular electrostatic potentials. The method is designed to fit the asymptotic values of the exact molecular electrostatic potential at the nuclei and at large distances where it approaches the multipole expansion. The method is applied to LiF, H2O, NH3, and C6H6 to demonstrate its suitability.
Interacting distributed approximating functions for real-time quantum dynamics
Authors: Hoffman David K. , Arnold Mark , Zhu Wei , Kouri Donald J.
Published: July 2002

The distributed approximating function (DAF) approach to quantum real-time dynamics is generalized to include the effects of the potential. The ‘‘interacting’’ DAF (IDAF) is introduced as the identity for a certain class of functions that can be chosen to approximate as closely as desired any wave packet of interest. Free propagation of the IDAF yields the free propagator for the IDAF class in the coordinate representation, and substitution of this result into the Trotter form for the short-time full propagator, G(x,x′‖τ), yields the IDAF class full propagator, G(x,x′;{p}‖τ), in the coordinate representation. Here {p} denotes the set of parameters that determine the IDAF class. The IDAF class full propagator can be used to develop discretized path integral-based algorithms for real-time quantum dynamics. Use of G(x,x′;{p}‖τ) in the Feynman path integral formalism leads to a new result with interesting features compared to the standard path integral. Specifically, the IDAF class full propagator incorporates the classical force, and (1) automatically biases the dynamics toward the neighborhood of classical trajectories (but without relying on destructive and constructive interferences in that no recourse is made to stationary phase arguments), (2) automatically concentrates the wave packet in highly classical regions and attenuates the wave packet in highly nonclassical regions. Of the many possible IDAF-based algorithms two are presented as examples. One illustrates a Monte Carlo approach and the other a discretized matrix multiplication approach.
The effects of collision mass and potential on the energy transfer in thermal collisions of gas phase clusters
Authors: Schulte J. , Lucchese R. R. , Marlow W. H.
Published: July 2002

We have studied the collisional energy transfer between molybdenum clusters and the rare-gas atoms Ne, Ar, and Xe. We have chosen these systems as nontrivial models of the thermalization process of metal clusters in a background gas. The mean energy transfer cross sections and energy transfer rate constants for collisions of molybdenum clusters (Mo5) with rare gas atoms are computed as functions of relative collision energy (gas temperature) and mass. The dynamics of gas phase molybdenum clusters are simulated by classical trajectories whose initial conditions are sampled from a distribution appropriate to thermal collisions. For the interaction of the molybdenum cluster atoms with the background gas, a Buckingham-type potential for unlike atoms was fitted to energies obtained using standard quantum chemistry techniques. The molybdenum cluster atoms interact among themselves by a Lennard-Jones potential. The simulation shows that the energy transfer rate constants are dominated by the characteristic collision velocity, i.e., within the domain of internal cluster temperature and background gas temperature investigated here, the mass and the interaction force do not change the energy transfer rate constants very much. The mean energy transfer cross sections, however, are coupled to the collision mass as well as to the actual interaction force. The coupling is nonlinear, and there is some evidence that in the energy transfer, for small clusters, complex collisions with more than one cluster atom are involved.
A computer simulation of the classic experiment on osmosis and osmotic pressure
Authors: Murad S. , Powles J. G.
Published: July 2002

A novel computer simulation technique for studying fluids in confined geometries has been developed and used to replicate Pfeffer’s experiment on osmosis in semipermeable membranes in 1877. Our results confirm the validity of van’t Hoff’s famous relationship for osmotic pressure over a wide range of concentrations, and also clearly establish its validity even for molecular systems. We believe this is the first theoretical validation of this result for such a wide range of concentrations, where no explicit assumption of ideality is made for the interactions of the solute molecules.
Polyelectrolyte solutions: Excluded-volume considerations
Authors: Mattoussi Hedi , Karasz Frank E.
Published: July 2002

We provide experimental evidence for the electrostatically related excluded-volume effects on the colligative properties and the single chain behavior of polyelectrolyte solutions in the dilute regime. The data are compared to the theory developed by Fixman, Skolnick, Odijk, and Houwaart. Good agreement between these theoretical considerations and the experimental data is observed.
A three stage model for adsorption of nonionic surfactants
Authors: Cory H. , Rodgers G. J.
Published: July 2002

A three stage model for the adsorption of nonionic surfactants is proposed which makes use of existing theory from studies of random sequential adsorption. The model is simulated and the adsorption curves are found. The theory of random sequential adsorption is used to calculate the coverage exactly at the end of each of the three stages.
The role of the anisotropic interaction on collision induced absorption of systems containing linear molecules: The CO2–Ar case
Authors: Borysow Aleksandra , Moraldi Massimo
Published: July 2002

Numerical calculations are presented of spectral moments of collision induced absorption (CIA) coefficient of CO2–Ar mixtures at various temperatures. Three spherical components have been used for the induced dipole moment all of which arise from pure multipolar induction. The calculations have been performed with six different potentials which differ with respect to the amount of anisotropy. The comparison of experimental and numerical results shows that the potential proposed by Preston and Pack [J. Chem. Phys. 66, 2480 (1977)] is the most suitable to describe CIA spectra. In addition, it is shown that the role of the anisotropy of the potential with regard to its contribution to the spectral moments, particularly the amount of mixing of dipole components of different symmetry, cannot be neglected.
Ab initio calculations of circular dichroism for (S)-2-deuteriopropanoic acid and (R)-2-deuterioglycine
Authors: Chen Qing , Powers D. , Pinkus A. G.
Published: July 2002

Ab initio calculations using the random-phase approximation are reported for the natural circular dichroism spectra (ellipticity vs wavelength) of (S)-2-deuteriopropanoic acid (CH3CHDCOOH) and (R)-2-deuterioglycine (NH2CHDCOOH). The calculated results agree reasonably well with previously reported experimental measurements when consideration is given to the environment in which the calculation (vapor) and measurement (liquid) are made. The results also show that the structure of the molecules under study deviate slightly from the planar states of their parent compounds propanoic acid (CH3CH2COOH) and glycine (NH2CH2COOH). The latter feature has not been tested experimentally.
Electronic spectroscopy of the niobium dimer molecule: Experimental and theoretical results
Authors: James Andrew M. , Kowalczyk Pawel , Fournier René , Simard Benoit
Published: July 2002

Rotationally resolved electronic spectra of the niobium dimer molecule are reported for the first time. The molecules were produced by laser vaporization of a niobium target rod and cooled in a helium supersonic expansion. The molecular beam containing niobium dimer molecules was interrogated in the range 400–900 nm using a pulsed dye laser to excite fluorescence. Numerous Ω=0←Ω=0 and Ω=1←Ω=1 vibronic transitions were discovered in the region 630–720 nm and investigated at 200 MHz resolution using the cw output of a single mode ring dye laser. The principal features were classified into five Ω=0←Ω=0 systems originating from a common lower state of 0+g symmetry, and three Ω=1←Ω=1 systems originating from a common lower state of 1g symmetry. The two lower states were assigned as the Ω=0 and Ω=1 spin–orbit components of the X 3Σ−g ground state, which is derived from the electron configuration 1π4u1σ2g2σ2g1δ2g. The two spin–orbit components are split by several hundred cm−1 due to a strong, second-order isoconfigurational spin–orbit interaction with the low-lying 1Σ+g state. Evidence for significant 4d orbital participation in the Nb2 bond is furnished by the short bondlength [re=2.077 81(18) Å] and large vibrational frequency [ωe=424.8917(12) cm−1] determined for the X 3Σ−g(0+g) state (2σ error bounds). The electronic structure of niobium dimer was investigated using density functional theory. For the electronic ground state, the predicted spectroscopic properties were in good agreement with experiment. Calculations on excited states reveal congested manifolds of triplet and singlet electronic states in the range 0–3 eV, reflecting the multitude of possible electronic promotions among the 4d- and 5s-based molecular orbitals. The difficulties of correlating the experimentally observed electronic transitions with specific valence electronic promotions are addressed. Comparisons are drawn between Nb2 and the isoelectronic molecule V2.
Dynamic mean-field models with correlated modes
Authors: Vekhter B. , Ratner Mark A. , Gerber R. B.
Published: July 2002

We discuss a generalized dynamic mean-field method combining the advantages of explicit pair correlations and of configuration interaction. The approximate dynamical method, which we call time-dependent self-consistent-field configuration interaction (TDSCF2-CI), is constructed by including N(N−1)/2 TDSCF2 configurations. In each configuration a given pair of N coupled modes is directly (not in the mean-field approach) correlated; the N(N−1)/2 configurations include all such choices of pairs. As such, it has both the usual advantages of TDSCF and improvements due to some inclusion of correlations (exact results for any two-mode problem, improved descriptions of dynamical corrections, and greater accuracy). A three-mode model Hamiltonian is analyzed using five approximate treatments, i.e., the usual TDSCF, the three TDSCF2 forms, and the TDSCF2-CI one. The quantities for comparison with the exact results include the decay P(t) of the initial state, the time dependencies of the energies e(i) of individual modes, and the overlap S(t) of the corresponding approximate wave function with the exact one. We find, indeed, that explicit inclusion of pair correlations improves the description of the quantum dynamics of the system.
Reaction of O(1D) with ethylene: Vibrational and rotational state distribution of product OH
Authors: Honma Kenji
Published: July 2002

Reaction of O(1D) with ethylene was studied under low pressure flow conditions. The O(1D) was formed by photodissociation of N2O by an ArF laser and rotational and vibrational state distributions of product OH were determined by laser induced fluorescence. The rotational distributions of the v=0 and 1 levels showed a bimodal feature. A major part of the OH had a rotational energy higher than the statistical expectation, and about 20% of total available energy appeared as the rotation of OH. About 20% of the total OH was characterized by a rotational distribution that corresponded to a specific temperature, i.e., 546 K and 526 K for v=0 and 1, respectively. The relative populations of the first two vibrational levels were measured to be 1.00 and 0.30, and no inversion was observed. No propensity was observed between formation of the two spin–orbit states, and the Π(A′) state was slightly more favored than the Π(A″) state. These results were explained by a mechanism in which O(1D) inserts into a CH bond of ethylene and OH is eliminated from a bend geometry. The same reaction was studied under cluster conditions where a van der Waals complex of N2O–C2H4 was converted into the reactant pair, O(1D)–C2H4. The rotational distribution of OH formed under these conditions showed little difference from that from the bimolecular reaction.
Differential and integral cross sections for the inelastic scattering of NO (X 2Π) by Ar based on a new ab initio potential energy surface
Authors: Alexander Millard H.
Published: July 2002

New ab initio potential energy surfaces for the Ar–NO (X 2Π) system are reported based on correlated electron pair approximation (CEPA) calculations. The fitted, rigid-rotor surface was then used in full close-coupling calculations of differential and integral cross sections for excitation of NO at a center-of-mass energy of 442 cm−1 (0.0548 eV), as well as differential cross sections at lower energies of 119 and 149 cm−1 (0.0145 and 0.0185 eV). The calculated cross sections are compared with those determined using earlier electron-gas potential energy surfaces and with the results of available experimental measurements. In general, the new CEPA potential energy surfaces yield very good agreement with available experimental integral and differential cross sections. Both theory and experiment reveal a significant tendency for population of final rotational states of Π(A″) reflection symmetry.
Large order dimensional perturbation theory for complex energy eigenvalues
Authors: Germann Timothy C. , Kais Sabre
Published: July 2002

Dimensional pertubation theory is applied to the calculation of complex energies for quasibound, or resonant, eigenstates of central potentials. Energy coefficients for an asymptotic expansion in powers of 1/κ, where κ=D+2l and D is the Cartesian dimensionality of space, are computed using an iterative matrix-based procedure. For effective potentials which contain a minimum along the real axis in the κ→∞ limit, Hermite–Padé summation is employed to obtain complex eigenenergies from real expansion coefficients. For repulsive potentials, we simply allow the radial coordinate to become complex and obtain complex expansion coefficients. Results for ground and excited states are presented for squelched harmonic oscillator (V0r2e−r) and Lennard-Jones (12-6) potentials. Bound and quasibound rovibrational states for the hydrogen molecule are calculated from an analytic potential. We also describe the calculation of resonances for the hydrogen atom Stark effect by using the separated equations in parabolic coordinates. The methods used here should be readily extendable to systems with multiple degrees of freedom.
First observation of the quasibound levels and tunneling line broadening in the 3 1Πg state of Na2 using an ultrasensitive ionization detector
Authors: Tsai Chin-Chun , Bahns John T. , Stwalley William C.
Published: July 2002

All quasibound vibrational energy levels v=27 and 28 of the Na2 3 1Πg state were accessed by using optical–optical double resonance and detected with a shielded ionization detector. The line broadening due to the tunneling effect was observed to be larger than 1 cm−1 at the rovibrational level v=28, J=31. The potential curve has been determined by the Rydberg–Klein–Rees method and modified using Jeung’s calculation for the large distance barrier and comparison of the calculated and the measured line widths. The barrier height was found to be ∼126 cm−1 above its asymptotic limit (3s+4p) at 5.98 Å. There also exists an unobserved shallow outer well with depth ∼35 cm−1 at 9.5 Å according to our modification of Jeung’s calculated potential. One way to possibly probe this outer well is also discussed.
Rovibrational coupling in the S1 excited state of 1-methylindole
Authors: Leach Gary W. , Demmer David R. , Bickel Grant A. , Wallace Stephen C.
Published: July 2002

We have undertaken an extensive study of the spectroscopy and dynamics of the S1–S0 transition in jet-cooled 1-methylindole and 1-methyl(d3)indole. The energy-resolved fluorescence decays resulting from picosecond excitation of S1 vibrational modes display unusual quantum interference effects which have been attributed to the excitation of a large number of quasidegenerate states even at relatively low levels of vibrational excitation. This has been attributed to rovibrational coupling in which the selection rules are much less restrictive than is normally the case with rigid planar molecules. A significant rotational temperature dependence of the vibrational dynamics has been measured which serves to corroborate this model.
Ab initio theoretical predictions of C28, C28H4, C28F4, (Ti@C28)H4, and M@C28 (M=Mg, Al, Si, S, Ca, Sc, Ti, Ge, Zr, and Sn)
Authors: Guo Ting , Smalley Richard E. , Scuseria Gustavo E.
Published: July 2002

Recent experiments have demonstrated that C28 is the smallest fullerene cage that successfully traps elements in its inside. In this work, we have studied the electronic structures, equilibrium geometries, and binding energies of the title molecules at the self-consistent field (SCF) Hartree–Fock level of theory employing basis sets of double-zeta quality. The empty C28 fullerene is found to have a 5A2 open-shell ground state and behaves as a sort of hollow superatom with an effective valence of 4, both toward the outside and inside of the carbon cage. The theoretical evidence suggests that C28H4 and C28F4 should be stable molecules. The possibility of simultaneous bonding from the inside and outside of the C28 shell, as in (Ti@C28)H4, is also explored. Our calculations show that the binding energy of the M@C28 species is a good indicator of the success in experimentally trapping the metal atoms (M) inside the fullerene cage. Based on these results, we propose that elements with electronegativities smaller than 1.54 should form endohedral fullerenes larger than a minimum size which depends on the ionic radius of the trapped atom. This qualitative model, correctly reproduces the available experimental evidence on endohedral fullerenes.
Analysis of the torsional potential of 9,10-dihydrophenanthrene in three electronic states: S, S1, and cation ground state
Authors: Smith Jonathan M. , Knee Joseph L.
Published: July 2002

Vibronic spectra are measured for the ground, first excited, and ion ground state of 9,10-dihydrophenanthrene and each is dominated by a progression in a single vibrational mode. The ion vibrational spectrum is obtained using zero electron kinetic energy photoelectron spectroscopy (ZEKE-PES) and the I.P. is determined to be 63 645 cm−1. Semiempirical calculations including normal modes analysis were used to determine the vibrational motion responsible for the observed progression. The vibration is primarily a phenyl torsion, but is more complicated than a simple rigid motion. The specific shape of the potentials are determined from the frequencies of the observed vibrations and the relative shifts are obtained from a Franck–Condon analysis. The problem is solved using a one dimensional potential in the normal coordinate. A fluorescence depletion experiment is used to confirm the single-welled nature of the potential energy surfaces. The phenyl dihedral angles are found to be 18°, 6°, and 14° in the ground, S1, and cation ground electronic states, respectively.
Ion-pair formation from saturated hydrocarbons through photoexcitation of an inner-valence electron
Authors: Mitsuke Koichiro , Hattori Hideo , Yoshida Hiroaki
Published: July 2002

Ion-pair formation from the superexcited states of saturated hydrocarbons has been studied by negative-ion mass spectrometry using synchrotron radiation in the 15–35 eV photon energy range. Negative ion H− has been observed from CH4, C2H6, C3H8, n-C4H10, iso-C4H10, and neo-C5H12. The maximum cross section ranges from 1.6×10−21 to 1.0×10−20 cm2. Strong peaks observed in the photodissociation efficiency curve of H− are assigned as resulting from transitions to the Rydberg states formed by promotion of an electron in a carbon 2s-type molecular orbital. In contrast, the valence-Rydberg transitions from a carbon 2p-type orbital have little (C2H6) or no contribution [CH4 and CmH2m+2 (3≤m≤5)] to the H− formation. This difference can be interpreted as that the latter Rydberg states have short lifetime with respect to autoionization to lower ionic states on account of relatively large overlap between two carbon 2p-type orbitals involved in an electron exchange process.
Relaxation theory for curve-crossing corrections to electronic absorption line shapes in condensed phases
Authors: Evans Deborah G. , Coalson Rob D.
Published: July 2002

A quantum mechanical relaxation theory is developed to enable approximate computation of electronic absorption line shapes of condensed phase systems where nonadiabatic coupling effects are important. At the simplest level, these computations require a time kernel (termed a memory kernel) which can be obtained from a sequence of wave packet propagations, each carried out on a single Born–Oppenheimer potential surface. Complications associated with the need to evolve wave packets on several nonadiabatically coupled surfaces are thereby avoided. Moreover, for many condensed phase problems the memory kernel can be computed via semiclassical techniques which rely on classical trajectories and simple Monte Carlo methods. The promise of the theory is demonstrated by numerical applications to the spectroscopic spin boson model [R. D. Coalson, J. Chem. Phys. 86, 995 (1987)], a nontrivial multimode model of electronic absorption lineshapes involving two nonadiabatically coupled excited state surfaces. The relevant quantum dynamics for the spectroscopic spin boson model can be computed exactly via path integration techniques. In this way, the accuracy of the proposed relaxation theory can be benchmarked, and the applicability of various semiclassical prescriptions for computing the memory kernel ascertained.