bluuebunny/crossref_metadata_2025 huggingface dataset:
Using relaxation theory to compute the electronic absorption spectrum of a chromophore coupled to a condensed phase environment
Authors: Evans Deborah G. , Coalson Rob D.
Published: July 2002

A time-dependent method for computing the electronic absorption spectrum of a chromophore in a condensed phase environment is presented. The cumulant expansion-based relaxation theory of Nitzan and Silbey [J. Chem. Phys. 60, 4070 (1974)] is adapted to enable approximate calculation of the absorption spectrum of a general system coupled to an arbitrary bath. The system–bath interaction potential operator is expanded to second order in a time-dependent perturbation series and then exponentiated in an attempt to capture the long-time dynamics. The dynamics of the bath is contained in Heisenberg correlation functions of the bath operators which are related to their classical counterparts. The formalism is applied to a harmonic and an anharmonic (Morse) system oscillator coupled to a harmonic bath. The exactly solvable harmonic model, a two-dimensional model of a Morse oscillator coupled to a single bath oscillator and a comparison to the time-dependent Hartree approximation are used to illustrate the accuracy of the formalism.
Quasiclassical trajectory study of HF(v) by CO
Authors: Wallis Eric P. , Thompson Donald L.
Published: July 2002

Quasiclassical trajectories were used to study the energy transfer rates and mechanisms in collisions of HF(v,J) with CO. A potential-energy surface was formulated by using spectroscopic and ab initio information. We have computed state-to-state rates for HF (vi=3,5,7,9; Ji=2)+CO(vi=0)→HF(vf,Jf)+CO for a thermal distribution of translational energies and CO rotational states at 300 K. The relaxation is due predominantly to vibration-to-rotation energy transfer with Δv=−1. As the initial vibrational state is increased, multiquantum transitions (Δv≤−2) become increasingly important. The computed results are in good agreement with experimental results.
The distribution of velocities for Ba+ ions in Ar gas
Authors: Viehland Larry A. , Hampt Denise S.
Published: July 2002

The ground-state potential for singly-charged barium ions interacting with argon atoms is inferred from laser-induced fluorescence measurements of the gaseous ion transport coefficients. The potential is used to compute velocity component distribution functions and fluorescence spectra. Comparison with the measured spectra provides insight into the accuracy with which the potential can be inferred and with which the distribution function and spectra can be calculated. Failure of a similar study of barium ions in helium gas is possibly due to inelastic collisions that invalidate a single-potential description of this system.
Solitonic Bloch functions and others in the Fulton–Gouterman frame of exciton-phonon systems
Authors: Sonnek Mathias , Wagner Max
Published: July 2002

A one-dimensional model for Frenkel excitons interacting with acoustical phonons is considered. The generalized Fulton–Gouterman (FG) transcription reduces the problem to a pure vibrational one and simultaneously guarantees the Bloch nature of any exciton-phonon eigenfunction. A general variational ansatz is established with regard to the specific topological structure of the FG equations. Simplified versions of this ansatz are shown to yield lower ground-state energies than any previously used dressed exciton or localized solitonic (‘‘non-Blochian’’) trial functions for all coupling-transfer regimes. This evinces the practicability of the FG conception also in extremal coupling-transfer situations. Since the calculation is easily extended to excited states, the FG approach seems to be an adequate means to investigate energy transport in exciton-phonon systems. In particular, the solitonic problem in these systems may be analyzed on a discrete footing.
Orientational and conformational correlations in deformed polymer chains with fixed end-to-end separation: A Brownian dynamics simulation study
Authors: Haliloglu Turkan , Bahar Ivet , Erman Burak
Published: July 2002

Static and dynamic correlations between bond conformations and reorientations are examined by the Brownian dynamics simulation technique for polymer chains with fixed ends. Polyethylenelike model chains are considered. Rates of rotational isomeric transitions and time evolution of orientational correlations are analyzed for various extensions of the chain. The more extended chains exhibit the highest mobility in the short-time scale but possess the lowest effective rate of rotational isomerization as follows from a hazard analysis covering time ranges up to 10 ns. The time decays of bond orientational correlations are reproducible by stretched exponential functions with exponent almost independent of chain extension. The imposition of fixed deformation at chain ends affects the orientational mobility of the chain down to the scale of individual bonds which may be observed from the biased evolution of time-dependent distribution functions for bond spatial reorientations.
Time-dependent probability distribution functions for orientational motions of segments in polymer chains
Authors: Haliloglu Turkan , Erman Burak , Bahar Ivet
Published: July 2002

The spatial orientation of vectors rigidly embedded in polymer chains are described in terms of time-dependent joint probability distribution functions. Serial expansion in terms of double spherical harmonics is adopted for the probability distribution functions, with the coefficients therein evaluated from Brownian dynamics simulations. Truncation of the series after the second-order harmonics accurately reproduces the results of Brownian dynamics simulations for a 50 bond polyethylene chain whose ends are held fixed at various extensions.
The velocity and recoil of DNA bands during gel electrophoresis
Authors: Keiner Louis E. , Holzwarth G.
Published: July 2002

The velocity and recoil of bands of DNA containing 48.5 to 4800 kilobasepairs (kb) were measured during pulsed-field gel electrophoresis by a video imaging and analysis system. When a 10 V/cm electric field was first applied, the velocity showed an initial sharp peak after approximately 1 s whose amplitude depended on the molecular weight of the DNA and the rest time and polarity of the previous pulse. For example, G DNA (670 kb) exhibited an initial velocity peak of 13 μm/s. The velocity then oscillated through a shallow minimum and small maximum before reaching a 5.0 μm/s plateau. After the field was turned off, the bands moved backward (recoiled). The band position obeyed a stretched-exponential relation, x = x0 exp[ − (t/τ)β] with amplitude x0 equal to approximately 1/10th of the DNA contour length and β≊0.6; for S. pombe DNA, x0 was a remarkable 165 μm. Both the initial velocity spike and the recoil arise from the presence of a significant fraction of U-shaped molecules with low configurational entropy. The initial velocity spike is exploited in field-inversion gel electrophoresis to generate the ‘‘antiresonance,’’ which is the basis of size-dependent mobility. Recent computer simulations which include tube-length fluctuations and tube leakage are in excellent accord with the measured velocities.
Surface properties of semi-infinite diblock copolymer melts
Authors: Tang Hai , Freed Karl F.
Published: July 2002

The quartic free-energy contribution for a semi-infinite diblock copolymer system is demonstrated to be important near the bulk spinodal, even when the bulk is disordered. The retention of the quartic term leads to a finite-density profile decay length at the spinodal, contrary to the prediction of a divergent decay length at the spinodal that is derived with a quadratic theory. Variational methods are introduced to treat the quartic theory and are used to show that bulk ordering induces formation of surface structures, such as terraces, islands, and holes, even in the absence of surface interactions. We also present a necessary modification to the Ohta–Kawasaki Green’s function for diblock copolymers.
Femtosecond real-time probing of reactions. VIII. The bimolecular reaction Br+I2
Authors: Sims I. R. , Gruebele M. , Potter E. D. , Zewail A. H.
Published: July 2002

In this paper, we discuss the experimental technique for real-time measurement of the lifetimes of the collision complex of bimolecular reactions. An application to the atom–molecule Br+I2 reaction at two collision energies is made. Building on our earlier Communication [J. Chem. Phys. 95, 7763 (1991)], we report on the observed transients and lifetimes for the collision complex, the nature of the transition state, and the dynamics near threshold. Classical trajectory calculations provide a framework for deriving the global nature of the reactive potential energy surface, and for discussing the real-time, scattering, and asymptotic (product-state distribution) aspects of the dynamics. These experimental and theoretical results are compared with the extensive array of kinetic, crossed beam, and theoretical studies found in the literature for halogen radical–halogen molecule exchange reactions.
Molecular-orbital theory of a solute in a continuum with an arbitrarily shaped boundary represented by finite surface elements
Authors: Wang Bingze , Ford George P.
Published: July 2002

A complete formalism is presented for the molecular-orbital study of a solute in a continuum. The Schrödinger equation of the solute in the solvent is derived from that for the entire solute–solvent system. An arbitrarily shaped cavity boundary is constructed using finite element techniques based on hexagonal and pentagonal surface elements and the induced charge on its surface calculated using analytical formulas for the electrostatic field strength. The Fock operator, which differs from one in widespread use, is modified by two terms resulting from variations in both the electrostatic field of the solute and the induced charges. An Austin Model 1 (AM1) version of the theory is developed with the addition of no new semiempirical parameters and illustrated with calculations on dimethyl ether and propane.
The applicability of binary collision theories to complex molecules in simple liquids
Authors: Schultz Karen E. , Russell D. J. , Harris Charles B.
Published: July 2002

The relaxation dynamics of vibrationally excited ground-state azulene molecules have been examined by picosecond transient absorption spectroscopy in a variety of different solvents including hexane, chloromethanes, methanol, CClF3, Xe, and Kr. A high pressure optical cell was used to liquify gases for use as solvents and to change their density and temperature independently over the entire liquid density range. Experimental results indicate that the vibrational cooling rate is strongly solvent dependent, with cooling rates of approximately 20 ps in molecular solvents and approximately 150 ps in atomic solvents. Comparison of the rates in Xe and Kr at constant number density demonstrates the strong effect of solvent mass on energy transfer. The effect of solvent temperature on vibrational cooling is minimal, as is the effect of solvent density. The latter result is quite surprising in light of earlier experiments on simpler molecular systems, such as I2 in Xe, and predictions of isolated binary collision theories. An explanation is offered from large scale molecular dynamics simulations of the system. In effect, azulene forms an ‘‘ordered Xe cluster’’ with xenon atoms; the xenon number density normal to the azulene molecular plane is independent of solvent density.
Resonance effects of diabatic surface crossing within the torsional spectrum of 9-(N-carbazolyl) anthracene observed by supersonic jet fluorescence spectroscopy
Authors: Monte Ch. , Roggan A. , Subaric-Leitis A. , Rettig W. , Zimmermann P.
Published: July 2002

Using the supersonic jet technique and laser-induced fluorescence spectroscopy, the ground and excited state surface of isolated 9-(N-carbazolyl) anthracene (C9A) is investigated. Ground and excited state torsional potentials of high accuracy are deduced from excitation and fluorescence spectra, considering characteristic patterns of Franck–Condon factors within the dispersed fluorescence. S0 exhibits a very flat double minimum potential (equilibrium twist angle 77.5°, barrier 17 cm−1); the barrier for perpendicularity in S1 is approximately 1050 cm−1 and the equilibrium angle is shifted towards coplanarity (64°). An unusual intensity profile of the long progression found in the fluorescence excitation spectrum is ascribed to a resonant nonradiative decay channel within the excited state surface. State selective fluorescence decay rates vs excess vibrational energy confirm this resonant relaxation process. This uncommon observation leads to a model of diabatic surface crossing along the torsional coordinate where the crossing ‘‘dark’’ state is discussed as a predicted charge transfer state or a higher lying triplet state, mediating further electronic relaxation. Although extended intermolecular vibrational redistribution (IVR) is present in the fluorescence spectra from high vibrational levels, this process is of secondary importance for the resonant nonradiative relaxation.
Scaling law for the glass and Ferry temperatures in the Gaussian random energy model
Authors: Young Robert D.
Published: July 2002

The Gaussian random energy model (REM) implies a non-Arrhenius temperature dependence for the characteristic relaxation time in the high-viscosity (supercooled) regime of glass-forming liquids. For example, the temperature dependence of the viscosity of the supercooled liquid is given as η(T)≊η0 exp[(TF/T)2]. TF is denoted as the Ferry temperature. The REM exhibits a (glass) transition at a temperature Tg. The REM predicts a scaling law for the two characteristic temperatures, TF=α(log ℜ)1/2Tg where N is the total number of energy levels of the system and α≊1. The scaling law has been successfully applied to a large number of glass-forming liquids as well as biopolymers.
Methods for calculating the bound state energies of van der Waals trimers: Applications to Ar3
Authors: Cooper Adam R. , Jain S. , Hutson Jeremy M.
Published: July 2002

Several different methods for calculating the energy levels of van der Waals trimers are compared for the case of Ar3. These include solution of the coupled equations in a basis set of hyperspherical harmonics and various three-dimensional basis set expansions. It is found that a basis set expansion in Jacobi coordinates gives results that are adequate for calculating the energy levels of the lowest few vibrational states, and that this method is computationally cheap enough to be extended to handle atom–atom–diatom complexes. The transition dipole moments for the lowest allowed transitions of Ar3 are estimated, and found to be about 7×10−5 D. The effects of three-body forces on the energy levels are investigated, and found to be large enough to determine from high-resolution spectroscopic experiments.
van der Waals theory of two-component melting
Authors: Young David A.
Published: July 2002

A van der Waals model based on accurate free energies for solid and liquid hard-sphere two-component mixtures is described. The model is computationally simple, it readily describes the variety of solid–liquid phase diagrams observed in metallic, ionic, and molecular systems, and in addition it permits the study of phase diagrams as a function of pressure. Several well-known empirical regularities in two-component phase diagrams are found to be predicted by the model calculations. Quantitative comparison of theory and experiment for metallic mixtures shows good agreement.
Scattering experiments with sodium clusters: Differential cross sections and classical trajectory calculations
Authors: Goerke Alexander , Palm Hartmut , Schulz C. P. , Spiegelmann Fernand , Hertel Ingolf V.
Published: July 2002

Scattering of sodium clusters Nan (n =3,8) with argon, neon and molecular nitrogen is investigated in a crossed molecular beam experiment. The clusters are scattered by a pulsed gas beam and subsequently ionized by a pulsed laser and detected in a time-of-flight mass spectrometer. Total differential cross sections for Nan+ Ar, Ne and N2 (n=3−8) are measured. The angularly resolved cross sections show a significant amount of energy transfer to the clusters in the collision process. Classical trajectory calculations are performed using an distance-dependant Hückel-type Hamiltonian with added pair potentials for the sodium–rare gas interaction. The results show good agreement with the measurements in the range of small impact parameters. It is observed that the different isomers of the clusters play an important role in the process. Discrete excitations are analyzed for Na3 and Na5. The shape of the measured angular distributions is explained from the superposition of rotational rainbows.
Open-system Monte Carlo simulations of Xe in NaA
Authors: Van Tassel Paul R. , Davis H. Ted , McCormick Alon V.
Published: July 2002

The grand canonical ensemble Monte Carlo method is used to determine the adsorption isotherm, energy, entropy, occupancy distribution, and density distribution of Xe atoms adsorbed in zeolite NaA, which is modeled as a single alpha cage and as a network of connected cages. The isotherms exhibit multiple plateaus, indicating the existence of certain favored loadings. These plateaus can be predicted by considering constant loading energy and density distributions found previously. A linear regression method of determining the constant loading entropy of Xe atoms is presented. The entropy is a smooth function of loading, which suggests that the structure of the adsorbed Xe is also smoothly varying. The pressures needed to obtain complete loading are quite large and beyond the range of conventional experiment. However, the simulated isotherm and occupancy distribution compare well with the available experimental data. Structural modifications of NaA in the form of cation removal are investigated. A cation-poor model NaA adsorbs fewer Xe at low chemical potential due to its smaller electric field, but has a larger overall adsorption capacity due to its greater accessible volume.
Time-independent quantum dynamics for diatom–surface scattering
Authors: Saalfrank Peter , Miller William H.
Published: July 2002

Two time-independent quantum reactive scattering methods, namely, the S-matrix Kohn technique to compute the full S-matrix, and the absorbing boundary Green’s function method to compute cumulative reaction probabilities, are applied here to the case of diatom–surface scattering. In both cases a discrete variable representation for the operators is used. We test the methods for two- and three-dimensional uncorrugated potential energy surfaces, which have been used earlier by Halstead et al. [J. Chem. Phys. 93, 2359 (1990)] and by Sheng et al. [J. Chem. Phys. 97, 684 (1992)] in studies of H2 dissociating on metal substrates with theoretical techniques different from those applied here. We find overall but not always perfect agreement with these earlier studies. Based on ab initio data and experiment, a new, six-dimensional potential energy surface for the dissociative chemisorption of H2 on Ni(100) is proposed. Two- and three-dimensional cuts through the new potential are performed to illustrate special dynamical aspects of this particular molecule–surface reaction: (i) the role of corrugation effects, (ii) the importance of the ‘‘cartwheel’’ rotation of H2 , and (iii) the role of the ‘‘helicopter’’ degree of freedom for the adsorbing molecule.
Infrared spectroscopy of methanol clusters adsorbed on large Arx host clusters
Authors: Huisken Friedrich , Stemmler Martin
Published: July 2002

The pick-up technique has been employed to deposite single methanol molecules as chromophores on the surface of large Arx host clusters. At higher densities of the guest molecules in the interaction region, more than one methanol molecule are adsorbed on the host cluster and, hence, dimers or even larger complexes can be formed. The methanol complexes are carried by the Arx substrate clusters to the detector and are analyzed with a mass spectrometer. It turns out that the commonly encountered protonation reaction leading to the fragmentation of the ionized methanol complexes is appreciably quenched. The technique is then combined with molecular beam depletion experiments employing the infrared radiation of a pulsed CO2 laser. In the region between 1000 and 1100 cm−1, which covers the ν8 CO stretch and the ν7 CH3 rock monomer vibrations, absorption spectra have been measured for the methanol monomer as well as the methanol dimer, trimer, and tetramer adsorbed on large Arx substrate clusters. For the monomer, both vibrations are observed. The dimer spectrum features three peaks and resembles very much the spectrum obtained in a previous study of the free gas-phase dimer, except that all lines are shifted to the red by ∼2 cm−1. From this observation it is concluded that the methanol dimer on the Arx host cluster adopts the same structure as in the gas phase. Whereas this observation has also been made for the trimer, the tetramer spectrum is distinctly different.
Rotational excitations in CH4/krypton mixtures
Authors: Asmussen B. , Press W. , Prager M. , Blank H.
Published: July 2002

In contrast to the CH4/argon system, methane and krypton form homogeneous solutions. Inelastic neutron scattering (INS) has been used to observe the concentration dependence of the rotational spectra of CH4 in krypton. The methane concentrations (0.3%, 3.2%, and 6.5%) were chosen such that the INS spectra are dominated by scattering from isolated defects and methane dimers. A model based on a Gaussian distribution of local orientational potentials reproduces the measured intensities well and enables the separation of the contributions of the different configurations to the scattering. It is demonstrated that the substitution of one krypton atom on the nearest-neighbor shell by a methane molecule reduces the energy of the J=1 level by about 5%.
Photodissociation of Arn+ cluster ions: Kinetic energy distributions of neutral fragments
Authors: Nagata Takashi , Kondow Tamotsu
Published: July 2002

The time-of-flight (TOF) spectra of fragments produced in the photodissociation of Arn+ (3≤n≤24) were measured at 532 nm. Analysis of these TOF spectra provides quantitative information on the kinetic energy distributions of the neutral Ar fragments. For Arn+ with n≤14, two types of Ar fragments were distinguished according to the kinetic energy release. One having a sizable amount of kinetic energy is ascribed to the fragments directly produced via the dissociation of the chromophoric core in the cluster ions. The other carrying a smaller amount of kinetic energy can be described by ‘‘evaporation’’ of solvent atoms in Arn+. The average translational energies of the ‘‘fast’’ and the ‘‘slow’’ fragments were estimated to be 0.35–0.38 and 0.07–0.1 eV, respectively, for n=7–11. The angular distribution of the fast fragments exhibits a preferential anisotropy with 1.5≲β≲2 along the direction of the polarization vector of the excitation laser, while an almost isotropic distribution was observed for the slow fragments. A possible photodissociation mechanism was proposed based on the theoretically predicted geometries of Arn+. In the TOF spectra for the larger Arn+ with 14≤n≤24, no indication was obtained for the production of the fast fragments. The average kinetic energy of the ejected neutral atoms is ∼0.05 eV at n=24. This finding indicates that the direct core dissociation no longer takes place in the larger Arn+ clusters, suggesting that the photophysical properties of Arn+ (n≥14) differ from those of the smaller cluster ions.
Surface scattering of C60+: Recoil velocities and yield of C60
Authors: Yeretzian Chahan , Hansen Klavs , Beck Rainer D. , Whetten Robert L.
Published: July 2002

In the impact of a C60+ beam against solid surfaces, a substantial fraction of the beam is neutralized. By means of a pulsed photoionization experiment, we have detected the neutralized scatterers from a silicon (100) surface, and found that they are mainly intact C60, with little or no fragmentation at impact energies to 200 eV. The neutralization probability is found to increase monotonically with increasing impact energy. This reionization method has been used in a time-of-flight scheme to measure the recoil velocity distributions of scattered C60. Collisions at impact energies above 50 eV are found to be highly inelastic, and the normal recoil distribution changes very little with impact energy over the range from 50 to 200 eV. The peak in the velocity distribution is near 1150 m/s (∼5 eV), with a full width at half-maximum (FWHM) of 350 m/s. By comparison with molecular dynamics simulations, an interpretation of this speed is proposed in terms of a limit to reversible deformation of the fullerene cage. A model of the scattering kinematics, based on treating the fullerene cage as a deformable, hollow sphere, with a harmonic deformation limit (15 eV) as found by theory, predicts the observed speed quantitatively.
Laser investigation of Na atoms deposited via inert spacer layers close to metal surfaces
Authors: Balzer F. , Bammel K. , Rubahn H.-G.
Published: July 2002

Na atoms have been observed on top of inert spacer layer/metal substrates at room temperature in an ultrahigh-vacuum environment via two-photon laser-induced fluorescence. As spacers, 1, 5, and 11 cadmium-arachidate (LB) and single dodecanthiol (SAM) layers were used, thereby changing the distance between atom and metal between 26 and 290 Å. A low sticking probability (≪10−6) was observed, resulting in a coverage of the spacer layers with less than 10−3 of a monolayer Na. This low coverage is due to the low binding energy of Na at the CH3 end groups of the spacers (millielectron volts). The Na intensity showed a biexponential decay as a function of time with a small time constant (40 s) due to direct evaporation of Na adsorbed on top of the layers and a large time constant (650 s) due to diffusion of the Na atoms through the fatty acid layers. The two-photon detection scheme allowed a Doppler-free (high-resolution) measurement of line broadening and frequency shift of the spectral lines due to the presence of the metal surface. A moderate broadening was observed for Na on top of LB multilayers on Pt/mica substrates, denoting that the Na atoms diffuse through the fatty acid chains with their mean distance from the surface decreasing from 290 to 28 Å. In addition, we found a red shift of the spectral lines by a few hundred Megahertz. The results are compared with previous data of sticking and line broadening of Na on a SAM/Au/Si substrate.
Polymer solutions and chemical reactions under flow: A thermodynamic description
Authors: Lebon G. , Casas-Vázquez J. , Jou D. , Criado-Sancho M.
Published: July 2002

The influence of mechanical stresses on polymer solutions and chemical reactions is examined. It is shown that the presence of a flow raises the Gibbs free energy. As a consequence, one observes in polymer solutions a shift of the critical point with respect to quiescent situations and in chemical reactions, a modification of the value of the chemical constant. The analysis is based on a recent theory of nonequilibrium thermodynamics known as extended irreversible thermodynamics.
Rearrangements of model (H2O)8 and (H2O)20 clusters
Authors: Wales David J. , Ohmine Iwao
Published: July 2002

We have calculated rearrangement mechanisms for (H2O)8 and (H2O)20 clusters by eigenvector following. For (H2O)8, two different parametrizations of a four-site, rigid water effective pair potential were considered and found to give very similar results. Hence, only one of the potentials is applied to (H2O)20. 6N−6 internal coordinates are required to describe a (H2O)N cluster in these calculations, of which 3N−6 were chosen as center-of-mass distances, angles, and dihedral angles, the other 3N being Euler angles. A wide variety of different rearrangements for both (H2O)8 and (H2O)20 are illustrated, with barrier heights ranging over three orders of magnitude. The mechanisms range from almost imperceptible changes of geometry to folding processes that result in dramatic structural transformations.
Lanczos method for the numerical propagation of quantum densities in phase space with an application to the kicked harmonic oscillator
Authors: Torres-Vega Go.
Published: July 2002

A numerical propagation scheme based on dual Lanczos transformation theory is introduced for the propagation of quantum mechanical wave functions in coordinate and phase spaces. This method is applied to the numerical propagation of the coherent state moving in the kicked harmonic oscillator potential. We also make comparisons with classical analogs and we make some comments regarding the advantage of working directly in phase space.
C 1s photoionization of H2CO and C2H4: An angle-resolved photoelectron study
Authors: Kilcoyne A. L. D. , Schmidbauer M. , Koch A. , Randall K. J. , Feldhaus J.
Published: July 2002

We have measured partial photoionization cross sections σ and asymmetry parameters β of the C 1s main line and the π→π* shake-up satellites of the isoelectronic molecules formaldehyde and ethylene in the near threshold region. In all channels we observe a strong cross section enhancement and a decrease in the β parameter due to a shape resonance in the continuum. For each molecule the variation of σ and β with photoelectron kinetic energy is nearly identical for main line and satellites indicating only relatively small modifications of the effective molecular potential induced by the additional π→π* excitation. There are no indications of conjugate shake-up processes in either molecule.
Quantum dynamics of overtone relaxation in benzene. V. CH(v=3) dynamics computed with a new ab initio force field
Authors: Wyatt Robert E. , Iung Christophe
Published: July 2002

Large-scale quantum mechanical calculations of the CH(v=3) overtone spectrum and survival probability are reported for 21-mode planar benzene. A valence coordinate hybrid force field built from the following two sets of ab initio information was used: (1) the quartic DZP/SCF force field recently reported by Maslen et al.; (2) a force field computed at the 6-311G/MP2 level for the overtone excited CH chromophore. Comparisons are made between these results and the overtone spectrum and survival probability computed using the older Pulay et al. 4-21P/SCF scaled quadratic plus cubic force field. In addition, comparisons are made with experimental spectra from two research groups. These comparisons provide information about the sensitivity of the computed results to alterations in the input force field.
Direct observation of the simultaneous transfer of vibrational energy and charge in the 15N2+(v)+14N2 reaction
Authors: Frost Michael J. , Kato Shuji , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

In this Communication, we report the use of a mass-selected flow tube, laser-induced fluorescence technique for the study of vibrationally excited ions. Using this technique, we have made measurements on the reaction of 15N2+ (v=2) with 14N2, which show conclusive evidence for the simultaneous transfer of vibrational energy and charge. We believe this to be the first direct observation of such a channel in charge transfer.
Far infrared absorption by electrolyte solutions
Authors: Dodo Taro , Sugawa Masao , Nonaka Eiji
Published: July 2002

The far infrared absorption spectra of aqueous solution of LiCl have been investigated in a frequency range 8–40 cm−1 at room temperature. In low frequency range (8–20 cm−1), the absorption is mainly controlled by Debye relaxation of the electric dipole rotation, and the absorption decreases with the increase of concentration.
Nonadditive intermolecular forces from the spectroscopy of van der Waals trimers: Calculations on Ar2–HCl
Authors: Cooper Adam R. , Hutson Jeremy M.
Published: July 2002

The spectra of van der Waals trimers formed from two atoms and a diatomic molecule are investigated. A computational method for calculating vibrational energies, rotational constants and angular expectation values for such complexes is developed and applied to the Ar2–HCl complex. All five low-frequency modes of the trimer are included. The pair potentials for Ar–Ar and Ar–HCl interactions are well known, and calculations are performed on Ar2–HCl potentials obtained from them assuming pairwise additivity. The calculations are compared with experimental results obtained from microwave and far-infrared spectroscopy. Substantial discrepancies between experiment and theory are found, and are attributed to the effects of nonadditive interactions. Several different contributions to the nonadditive interactions are investigated: dispersion forces, electrostatic induction forces, and exchange overlap forces are all found to be significant, but not large enough to explain the discrepancies. Exchange multipole forces, arising from the interaction between overlap-induced multipoles on the Ar atoms and the permanent multipoles on the HCl molecule, are found to be important, and to improve the agreement between experiment and theory substantially. It seems likely that it will be possible to obtain detailed information on nonadditive interactions from the spectroscopy of van der Waals trimers.
A molecular dynamics simulation of small benzene clusters
Authors: Del Mistro G. , Stace A. J.
Published: July 2002

Molecular dynamics calculations have been undertaken on small benzene clusters containing between two and six molecules, with each molecule being treated as a rigid rotor. As a function of kinetic temperature, calculated dynamic quantities associated with the translational and rotational motion of individual molecules, show that the clusters exhibit distinct solidlike and liquidlike forms. In all cases, the transition to a liquidlike state occurs at a temperature ∼170 K below the melting point for bulk benzene. Melting in the tetramer is complicated by the presence of a metastable planer configuration to which the more stable tetrahedral arrangement is observed to isomerize. Power spectra, derived from angular velocity correlation functions, show that melting in the clusters is accompanied by an increase in motion about the C6 molecular axis. In contrast, motions about the C2 axes remain hindered even in the liquidlike clusters.
Surface properties of solids using a semi-infinite approach and the tight-binding approximation
Authors: Ou-Yang Hui , Källebring Bruno , Marcus R. A.
Published: July 2002

A semi-infinite approach (rather than a slab method or finite number of layers) is used to treat surface properties such as wave functions, energy levels, and Fermi surfaces of semi-infinite solids within the tight-binding (TB) approximation. Previous single-band results for the face-centered cubic lattice with a (111) surface and for the simple cubic lattice with a (001) surface are extended to semi-infinite layers, while the extension to calculations of other surfaces is straightforward. Treatment of more complicated systems is illustrated in the calculation of the graphite (0001) surface. Four interacting bands are considered in the determination of the wave functions, energies, and Fermi surface of the graphite (0001) surface. For the TB model used, the matrix elements in the secular determinants for the semi-infinite solid and for the infinite bulk solid obey the same expressions, and the wave functions are closely related. Accordingly, the results for the bulk system can then be directly applied to the semi-infinite one. The main purpose of the present paper is to provide wave functions and other properties used elsewhere to treat phenomena such as scanning tunneling microscopy and electron transfer rates at electrodes.
Weak- and strong-field Stark energy levels of symmetric top dipolar molecules
Authors: Maergoiz A. I. , Troe J.
Published: July 2002

Perturbation expansions of the energy levels of symmetric top dipolar molecules in weak and strong electric fields (field strength F) are extended up to terms proportional to F3 (for weak fields) and to F−1 (for strong fields). For small quantum numbers, suitable analytical interpolations between the weak- and strong-field expansions are obtained from matrix diagonalization results. For large quantum numbers, where this approach becomes inconvenient, quasiclassical techniques provide these interpolations. The expressions are of use not only in spectroscopy but also in reaction kinetics, e.g., in adiabatic channel treatments of reactions on attractive potentials with cos θ anisotropies.
Aniline-CH4 S1 vibrational dynamics studied with picosecond photoelectron spectroscopy
Authors: Smith Jonathan M. , Zhang Xu , Knee J. L.
Published: July 2002

Intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) are measured for a number of vibronic states in the S1 electronic state of the aniline-CH4 complex. The detailed dynamics are monitored using picosecond threshold photoelectron spectroscopy which is shown to be a sensitive probe of van der Waals molecule dynamics. For the lowest vibrational states accessed, the 6a10 and 6a10+24 cm−1 bands, both IVR and VP are observed and their rates are independently determined. At higher excess energy, IVR becomes the rate limiting step and the rate for VP cannot be measured independently. The results are interpreted using a serial dissociation mechanism with a simple kinetic model description. The rates of the reaction are modeled using standard Rice–Ramsperger–Kassel–Marcus (RRKM) theory which qualitatively predicts rates consistent with the experimental observations.
The dynamics of exciton tunneling and trapping in condensed xenon on ultrafast time scales
Authors: Peterson Eric S. , Schwartz Benjamin J. , Harris Charles B.
Published: July 2002

We report the use of picosecond transient absorption spectroscopy to directly observe the dynamics of formation, tunneling, and subsequent cooling of the n=1, Γ(3/2) exciton in condensed xenon over a density range of 0.10 to 1.8 g/ml. At lower densities, only Xe*2 excimers are formed, which undergo vibrational cooling on a tens of picoseconds time scale. At densities high enough to support exciton formation, tunneling from the free to the trapped exciton state takes place in ≤3 ps, and cooling of the localized exciton takes place in 5–10 ps; neither rate is strongly dependent on Xe density over the range investigated. The results are compared to theories that describe the formation and cooling rates of the trapped exciton state, and are consistent with a resonant energy transfer mechanism in which the excitation hops between neighboring Xe atoms during the trapping process.
The two-particle one hole-Tamm-Dancoff approximation (2ph-TDA) decoupling of the dilated electron propagator with application to 2P shape resonances in e-Be, e-Mg, and e-Ca scattering
Authors: Medikeri Milan N. , Nair Jayraman , Mishra Manoj K.
Published: July 2002

Formulas for the renormalized full two-particle one hole-Tamm-Dancoff approximation (2ph-TDA) decoupling of the dilated biorthogonal electron propagator based on complex scaled bivariational self-consistent-field (SCF) procedure are derived and the diagonal 2ph-TDA approximation is implemented for the first time. The 2P shape resonances in e-Be, e-Mg, and e-Ca scattering are characterized using the diagonal 2ph-TDA, the second-order and the zeroth-order (bivariational SCF) approximations to the dilated electron propagator. A comparative investigation of these different decouplings reveals that although the resonance energies and widths depend on the level of correlation employed, greater correlation need not lead to sharper resonances.
Resonance Raman spectrum of the allyl-d5 radical and the force field analysis of the allyl radical
Authors: Liu Xianming , Getty James D. , Kelly Peter B.
Published: July 2002

Resonance Raman spectra of the allyl-d5 radical have been obtained with excitation between 247 and 223 nm. Analysis of the spectra yields the first observation of fundamental frequencies, ν4, ν5, and ν7 and overtone frequencies 2ν9, 2ν10, and 2ν12. The new vibrational data are combined with previously observed frequencies of allyl-h5 and allyl-d5 radical to produce the force field analysis for the allyl radical. This study suggests reassignment of several previously observed infrared (IR) bands. Experimental frequencies and assignments for allyl-h5 and allyl-d5 are compared with results from ab initio calculations. Force constants obtained in the present work are compared with the force constants of other sp2 hybridization molecules such as benzene, allene, and ethylene.
Guided-ion beam studies of the reactions of protonated water clusters, H(H2O)+n (n=1–4), with acetonitrile
Authors: Honma Kenji , Sunderlin L. S. , Armentrout P. B.
Published: July 2002

Reactions of protonated water clusters, H(H2O)n+ (n=1–4), with acetonitrile are studied by using guided-ion beam mass spectrometry. Absolute reaction cross sections are determined as a function of the collision energy from thermal energy to over 10 eV. Proton transfer, charge transfer, and dissociative proton transfer are the reactions seen for n=1. For n=2–4, proton transfer is accompanied by solvent transfer such that the dominant product at low collision energies is H(CH3CN)(H2O)n−1+ in each case. At higher collision energies, additional water ligands are lost and the collision-induced dissociation products, H(H2O)n-m+ where m=1 to n−1, are also observed. The total reaction cross sections are large and decrease slightly with the number of water molecules. The results indicate that energy transfer in these complexes is efficient and thus are consistent with reactions that proceed via hydrogen-bonded intermediate complexes.
Structure and predissociation dynamics of (HCCCN)2: A high resolution infrared study
Authors: Kerstel E. R. Th. , Scoles G. , Yang X.
Published: July 2002

The IR spectrum of the HCCCN dimer has been studied using the optothermal technique. Both the ν1 and ν2 fundamentals have been recorded. Several hot bands pertaining to these two bands were also observed. The ν1 band and the ν1+ν17−ν17 hot band have been analyzed to yield accurate molecular constants for both the upper and the lower states. The structure of the HCCCN dimer is determined to be linear. The redshift of the ν1 dimer band (outside C–H stretch) from the corresponding monomer band is 2.95 cm−1, while the red shift of the inside C–H stretching fundamental band ν2 is about 67 cm−1. The predissociation lifetime of the ν1=1 state is in the range from 90 μs to 16 ns, while the predissociation lifetime of the ν2=1 state is about 450 ps. All observed hot bands which involve the ν2 vibration show narrower line widths than the fundamental ν2 band. An explanation of this phenomenon is offered which involves the angular dependence of the dynamical coupling between the inside C–H stretching and the predissociation coordinate.
Phase diagram of a Lennard-Jones solid
Authors: Choi Yumi , Ree Taikyue , Ree Francis H.
Published: July 2002

A phase diagram of a Lennard-Jones solid at kT/ε≥0.8 is constructed by our recent perturbation theory. It shows the stability of the face-centered-cubic phase except within a small pressure and temperature domain, where the hexagonal-close packed phase may occur. The theory predicts anharmonic contributions to the Helmholtz free energy (important to the crystal stability) in good agreement with Monte Carlo data.
Photoproduction and dynamics of oxygen atoms in xenon matrices
Authors: Ryan E. Todd , Weitz Eric
Published: July 2002

The photodissociation of N2O doped in Xe matrices and the subsequent dynamics of atomic oxygen production have been studied. The O atom concentration is monitored via the laser-induced fluorescence of XeO exciplexes produced by the 193 nm excitation of Xe/O pairs. The O atom photoproduction cross section for 193 nm irradiation of N2O is 6.4±1.0×10−20 cm2 at 27 K, comparable to the gas phase value of 1.1×10−19 cm2. Dissociation of XeO exciplexes generates kinetically hot O atoms which are mobile. This photoinduced mobility can lead to O atom loss by recombination. The extent of O atom production as a function of laser irradiation is governed by a competition between the rates of photoproduction and photoinduced loss. The effects of temperature, concentration, and laser fluence on the production of O atoms are considered. The efficiency of photoinduced O atom loss increases significantly with increasing temperature. An ultraviolet absorption spectrum of XeO has been obtained with an absorption cross section of 1.9±0.4×10−16 cm2 at 248 nm.
The effect of surface passivation on rotationally inelastic scattering: N2 scattered from W(110), W(110)–(2×2)N, W(110)–(1×1)H, and Pt(111)
Authors: Hanisco Thomas F. , Kummel Andrew C.
Published: July 2002

The effects of surface reactivity on rotationally inelastic scattering has been examined by comparing the scattering of N2 from the reactive W(110) surface and from the passivated N and H reconstructions, W(110)–(2×2)N and W(110)–(1×1)H, as well as the nonreactive Pt(111) surface. The translational energy, rotational state population distributions, and angular momentum alignment of N2 scattered from these surfaces have been measured as a function of incident energy Ei, incident angle θi, and exit angle θf. The trends in the final translational and rotational energy correlate well with the average atomic mass of each surface. However, some of the greater rotational excitation for N2 scattered from W(110) compared to W(110)–(1×1)H can be attributed to a more anisotropic potential with respect to initial molecular orientation for N2–W(110) compared to N2–W(110)–(1×1)H or N2–Pt(111). The rotational distributions indicate that parallel momentum is not conserved for N2 scattered from W(110) or from W(110)–(2×2)N, but that parallel momentum is partially conserved for N2 scattered from W(110)–(1×1)H. Furthermore, measurements of angular momentum alignment show that significant in-plane forces are present, and that these forces are due to the repulsive part of the N2–surface potential of each system. The results are consistent with the N2–W(110) potential being highly corrugated with respect to both initial molecular orientation and impact parameter and with this corrugation being reduced dramatically by passivating the surface with the adsorption of hydrogen.
Thermionic emission from free, photoexcited tungsten clusters
Authors: Leisner Thomas , Athanassenas Kalliopi , Kreisle Dietmar , Recknagel Ekkehard , Echt Olof
Published: July 2002

We report on delayed electron emission from free tungsten clusters, excited by light from a Q-switched YAG laser. Using a novel ion extraction lens, electron emission can be analyzed over a time range of 50 ns–5 μs after the laser pulse without interference from prompt ions. All clusters of size 5≤n≤40 exhibit delayed emission on this time scale, while delayed emission from smaller clusters does not occur. We analyze the time dependence and size dependence of the emission rate for different wavelengths and fluences. The yield of delayed ions may exceed the yield of prompt ions for intermediate laser fluences. A statistical model is proposed which is based on the assumption that energy randomization in the electronically excited clusters proceeds much faster than in 50 ns, i.e., that the observed phenomenon is the (cluster) analog of thermionic emission. Good agreement with all our experimental findings is achieved, although the model invokes only one adjustable parameter. We argue that other delayed deexcitation channels, namely, emission of atoms or photons, are not significant under our experimental conditions.