bluuebunny/crossref_metadata_2025 huggingface dataset:
Dielectric friction effects on rotational reorientation of three cyanine dyes in n-alcohol solutions
Authors: Laitinen Eira , Korppi-Tommola Jouko , Linnanto Juha
Published: July 2002

We have estimated the effect of dielectric friction on the rotational correlation times of three cationic cyanine dyes. Dielectric corrections were evaluated by using the Stokes–Einstein-Debye hydrodynamic continuum model including the dielectric friction for DiIC2, DiIC6, and DiIC14 in different n-alcohol solutions at room temperature. The dielectric corrections were done to cis and trans conformations of the cyanine dyes. For the trans conformations, which were found more stable than cis conformations, the dielectric model seemed to be more properly suited. The ground and excited state dipole moments for the calculations were evaluated from ab initio molecular orbital calculations and for the excited state experimentally from the Lippert–Mataga plots.
Dual pathway of carbon cluster formation in the laser vaporization
Authors: Moriwaki T. , Kobayashi K. , Osaka M. , Ohara M. , Shiromaru H. , Achiba Y.
Published: July 2002

The existence of two groups of carbon clusters in laser vaporization was confirmed by the covariance analysis and laser fluence dependence study. One is the clusters produced when the higher laser fluence is used, which most likely causes an atomization of graphite directly or very rapid-and-intense fragmentation of large carbon particles. And the vaporized atoms or small clusters grow to larger clusters by the confinement and clustering in the buffer gas. These clusters showed smooth ion distribution both for the cations and anions in the size region up to about a hundred. The other is produced when the laser fluence is lowered. These clusters showed the characteristic ion distribution with magic sizes at C60+,C70+ cations, and C10−,C12−,C16−,C18−,… anions. The origin of the clusters was suggested as the large hot clusters which lead to the fragmentation into these smaller clusters observed.
Collisional energy transfer in H2(vab,jab)+H2(vcd,jcd)
Authors: Mandy M. E. , Martin P. G. , Keogh W. J.
Published: July 2002

Quasiclassical trajectory studies were carried out on the H2(vab,jab) + H2(vcd,jcd) system to survey the effects of initial vibrational, rotational, and translational energy on the collisional energy transfer behavior at total energies of 0.1 and 0.25 Eh. When the internal energy of the collider is low, repulsive interactions dominate and the energy transfer in the target molecule resembles the behavior observed for the He+H2 system, in which interconversion of vibrational and rotational energy is strongly correlated. By contrast, an internally excited collider can yield energy transfer behavior with more randomly distributed vibrational and rotational changes, similar to that observed when a H atom is the collider. Implications for master equation calculations are discussed.
New basis set superposition error free ab initio MO-VB interaction potential: Molecular-dynamics simulation of water at critical and supercritical conditions
Authors: Famulari Antonino , Specchio Roberto , Sironi Maurizio , Raimondi Mario
Published: July 2002

Recently, a controversy has come to light in literature regarding the structure of water in nonambient conditions. Disagreement is evident between the site–site pair correlation functions of water derived from neutron diffraction and those obtained by computer simulations which employ effective pairwise potentials to express the intermolecular interactions. In this paper the SCFMI method (self-consistent field for molecular interaction) followed by nonorthogonal CI (configuration interaction) calculations was used to determine a new water–water interaction potential, which is BSSE (basis set superposition error) free in an a priori fashion. Extensive calculations were performed on water dimer and trimer and a new parametrization of a NCC-like (Niesar–Corongiu–Clementi) potential was accomplished. This was employed in the molecular-dynamics simulation of water. The effect of temperature and density variations was examined. Acceptable agreement between site–site correlation functions derived from neutron diffraction data and from computer simulation was reached. In particular, a weakening of the hydrogen bonded structure was observed on approaching the critical point, which reproduces the experimental behavior. The simulations were performed using the MOTECC (modern techniques in computational chemistry) suite of programs. The present results show the importance of BSSE-free nonorthogonal orbitals in an accurate description of the intermolecular potential of water.
Comment on “Modified nonequilibrium molecular dynamics for fluid flows with energy conservation” [J. Chem. Phys. 106, 5615 (1997)]
Authors: Evans Denis J. , Searles Debra J. , Hoover Wm. G. , Hoover C. G. , Holian Brad Lee , Posch Harald A. , Morriss Gary P.
Published: July 2002

In their recent paper and the associated Response to this Comment, Tuckerman et al. dispute the form of the Liouville equation, as proposed by Liouville in 1838. They go on to introduce a definition of the entropy which is at variance with Boltzmann’s H-function and with Gibbs’ definition of entropy. They argue that their “entropy” is a constant of the motion, equal to its initial equilibrium value regardless of the imposition of external fields. We argue that the analysis of Tuckerman et al. is incorrect and that issues raised by Tuckerman et al. are not at all new but have already been correctly incorporated into nonequilibrium statistical mechanics.
Geometries and energy separations of the electronic states of Ge5+ and Sn5+
Authors: Dai Dingguo , Balasubramanian K.
Published: July 2002

We compute the geometries and energy separations of 13 low-lying electronic states of Ge5+ and Sn5+ with five different structures (edge-capped tetrahedron, C2v; trigonal bipyramid, D3h; tetragonal pyramid, C4v; planar square, D4h; and planar pentagon, D5h). The complete active space multiconfiguration self-consistent-field method (CASSCF) followed by large scale multireference singles+doubles configuration interaction (MRSDCI) computations that included up to 3.77 million configurations are employed. It was found that upon ionization the symmetrical D3h trigonal bipyramidal structures of Ge5 and Sn5 Jahn–Teller distort into nearly degenerate 2B2 and 2A1 electronic states with edge-capped tetrahedral (C2v) geometry. Atomization and dissociation energies of Ge5+ and Sn5+ as well as adiabatic ionization energies of Ge5 and Sn5 are computed.
Dipole polarizabilities of the potassium atom in its ground (4 2S) and excited (4 2P, 5 2S) states, and long-range dispersion interactions between two K atoms
Authors: Mérawa Mohammadou , Bégué Didier
Published: July 2002

The dipole polarizabilities of the potassium atom in its ground (4 2S) and low-lying excited (4 2P, 5 2S) states have been calculated using the time-dependent gauge-invariant variational method [Mérawa, Rérat, and Pouchan, J. Chim. Phys. 91, 1387 (1994)]. The variation of these polarizabilities versus imaginary frequencies [Casimir and Polder, Phys. Rev. 73, 360 (1948)] have been computed, and the long-range dispersion coefficients for the molecular states of K2 dissociating to K(4S)+K(4S), K(4S)+K(4P), K(4P)+K(4P), K(4S)+K(5S), K(5S)+K(4P) and K(5S)+K(5S) have been evaluated. The results obtained for K(4S)+K(4P) interactions are in very good agreement with the recent experimental high-resolution photoassociative data of Wang et al. [J. Chem. Phys. 106, 7899 (1997)]. The long-range dispersion coefficients for the K(4P)+K(4P), K(4P)+K(5S) and K(5S)+K(5S) interactions are new.
Collisional intrashell transitions in alkali Rydberg atoms under zero-electron-kinetic-energy conditions
Authors: Bellomo Paolo , Farrelly David , Uzer T.
Published: July 2002

We present a comprehensive classical model of large-scale angular momentum transfer in alkali Rydberg atoms by collisions with ions near or below the matching velocity, at which the speed of the colliding ion equals the classical, average speed of the Rydberg electron. We model the atomic quantum defect by perturbative methods, and obtain excellent agreement with experiments measuring the total collision cross section and the collisional population of individual high-l states. At the matching velocity, or right below it, the final distribution of angular momenta is peaked along the direction of the ionic beam, which indicates that the population of the ml sublevels is not uniform. We apply our model to intrashell {n,l}→{n,l′} transitions induced by ion-Rydberg collisions under zero-electron-kinetic-energy photoelectron spectroscopy (ZEKE-PES) conditions, and demonstrate that the excitation of ultra-long-lived Rydberg states via ion-Rydberg collisions is a two-step mechanism: First, a collision quenches the quantum defect, but fails to bring about a complete statistical mixing of levels. Subsequent collisions excite preferentially the ultra-long-living, high-l states (ZEKE states) which are mostly responsible for the ZEKE signal.
An investigation of the effects of two level system coupling on single molecule lineshapes in low temperature glasses
Authors: Brown Frank L. H. , Silbey Robert J.
Published: July 2002

A theoretical framework for determining the lineshapes of single molecules in low temperature glasses is presented. Our methods, in contrast to previous efforts, include coupling between two level systems (TLSs). This framework is applied to the physical system consisting of the chromophore terrylene embedded in the amorphous host polystyrene. We analyze the effect of TLS-TLS coupling on both individual lineshapes and linewidth histograms. Our results indicate that, although TLS-TLS coupling is certainly capable of producing noticeable changes in individual spectral lines relative to the uncoupled results, linewidth histograms are relatively unaffected by said coupling. An interpretation of this result is suggested.
Use of Mo/ller-Plesset perturbation theory in molecular calculations: Spectroscopic constants of first row diatomic molecules
Authors: Dunning Thom H. , Peterson Kirk A.
Published: July 2002

The convergence of Mo/ller–Plesset perturbation expansions (MP2–MP4/MP5) for the spectroscopic constants of a selected set of diatomic molecules (BH, CH, HF, N2, CO, and F2) has been investigated. It was found that the second-order perturbation contributions to the spectroscopic constants are strongly dependent on basis set, more so for HF and CO than for BH. The MP5 contributions for HF were essentially zero for the cc-pVDZ basis set, but increased significantly with basis set illustrating the difficulty of using small basis sets as benchmarks for correlated calculations. The convergence behavior of the exact Mo/ller–Plesset perturbation expansions were investigated using estimates of the complete basis set limits obtained using large correlation consistent basis sets. For BH and CH, the perturbation expansions of the spectroscopic constants converge monotonically toward the experimental values, while for HF, N2, CO, and F2, the expansions oscillate about the experimental values. The perturbation expansions are, in general, only slowly converging and, for HF, N2, CO, and F2, appear to be far from convergence at MP4. In fact, for HF, N2, and CO, the errors in the calculated spectroscopic constants for the MP4 method are larger than those for the MP2 method (the only exception is De). The current study, combined with other recent studies, raises serious doubts about the use of Mo/ller–Plesset perturbation theory to describe electron correlation effects in atomic and molecular calculations.
Direct experimental evidence for the thermal nature of delayed electron emission from a superhot C60 molecule
Authors: Bekkerman A. , Tsipinyuk B. , Budrevich A. , Kolodney E.
Published: July 2002

Delayed electron emission from superhot neutral C60 was found to strongly depend on the initial molecular thermal energy. This dependence is clearly demonstrated by measurements of delayed ionization efficiency curves following electron impact for widely different C60 average vibrational energies in the range of 8–14 eV. These measurements provide direct experimental evidence for the thermal nature of delayed ionization of superhot clusters. A simulation based on decay kinetics with thermal rate constants was found to be in good agreement with experiment.
A temperature-dependent Schrödinger equation based on a time-dependent self consistent field approximation
Authors: Peskin Uri , Steinberg Michal
Published: July 2002

A temperature-dependent Schrödinger equation is proposed for the study of quantum dynamics in systems which interact with an external bath. The derivation is based on a time-dependent self consistent field (TDSCF) approximation for a system-bath wave function. As previously shown by Miller, this approximation leads to a quantum mechanical analog to the classical generalized Langevin equation. By replacing the time-evolution of the quantum mechanical bath observables with the corresponding classical trajectories, the fluctuating force and the nonlinear friction kernel in the Langevin-Schrödinger equation become temperature-dependent where the fluctuations intensity is proportional to T. Application of the new equation to the spin-boson model shows agreement with numerically exact path integral calculations for T>0. We relate the success of the TDSCF approximation in this case to the bath-induced noise which diminishes the importance of quantum mechanical system-bath correlations.
Photodissociation dynamics of tert-butyl hydroperoxide at 266 nm: Degenerate four-wave mixing observation of OH state distribution
Authors: Kim Dong-Chan , Lee Keon Woo , Jung Kyung-Hoon , Hahn Jae Won
Published: July 2002

Degenerate four-wave mixing spectroscopy has been used to study the photodissociation dynamics of tert-butyl hydroperoxide (t-BuOOH) at 266 nm, long-wavelength tail of the ultraviolet (UV) absorption continuum. The nascent rotational distributions, spin-orbit state ratio, and Λ-doublet population ratio of the OH (X 2Π) fragments have been extracted. No vibrationally excited OH was observed. The rotational distribution peaked at N″=5. The average rotational energy was estimated to be 4.2% of the available energy. A slight preference for the Π− Λ-doublet component and a statistical distribution of the spin-orbit states were observed. The experimental observations are similar to those found for OH from H2O2 photodissociation and are consistent with dissociation via a repulsive excited state.
The influence of molecular rotation on the direct subsurface absorption of H2 on Pd(111)
Authors: Olsen R. A. , Kroes G. J. , Baerends E. J.
Published: July 2002

Within the generalized gradient approximation (GGA) of density functional theory (DFT) we have calculated a three-dimensional (3D) potential energy surface (PES) including an angular degree of freedom for a H2 molecule interacting with a Pd(111) surface. There is an entrance channel barrier (≈0.09 eV) to both dissociative chemisorption and direct subsurface absorption, but after this barrier is crossed direct subsurface absorption can proceed almost without a barrier. 3D quantum mechanical wave packet calculations incorporating the rotation of H2 in a plane perpendicular to the surface show a large part of the hydrogen going directly subsurface even at low incident kinetic energies. The wave packet calculations also show that in the low energy regime rotation inhibits direct subsurface absorption at low j0 and promotes it at high j0.
Effective Hamiltonians of polymethineimine, polyazine and polyazoethene: A density matrix variation approach
Authors: Chen GuanHua , Su ZhongMin , Shen ZhenWen , Yan YiJing
Published: July 2002

A new variation method is proposed to determine the effective Hamiltonians for conjugated π-electron systems. This method is based on the minimization of the difference between the ground state reduced single electron density matrix calculated from the effective Hamiltonian and its ab initio counterpart under a set of well-defined constraints. Applications are made to various oligomers of polymethineimine (PMI), polyazine (PAZ) and polyazoethene (PAE) at the Hartree–Fock level. Calculated are also the optical gaps of these oligomers. The effective Hamiltonians contain electron–electron Coulomb interactions and are suitable for the study of excited state dynamic processes such as nonlinear optical properties in π-conjugated systems.
Photoelectron spectra of Nbn− clusters: Correlation between electronic structure and hydrogen chemisorption
Authors: Kietzmann H. , Morenzin J. , Bechthold P. S. , Ganteför G. , Eberhardt W.
Published: July 2002

Photoelectron spectra of mass-separated Nbn− clusters reveal an even/odd alternation for n=6–17, indicating a closed electronic shell of the neutral even-numbered clusters. The HOMO–LUMO gap of Nb8, Nb10, and Nb16 is found to be larger than that of the other even-numbered clusters, which correlates with the low H2 reactivities of these species. The spectrum of Nb15− is different from all other clusters in this size range, which might be an indication for a geometric bcc shell closing. The influence of the electronic structure of the clusters on the reactivity is discussed.
Microwave spectrum and molecular structure of the H2NS radical
Authors: Habara Hideta , Yamamoto Satoshi , Saito Shuji
Published: July 2002

The rotational spectral lines of the H2NS and D2NS radicals in the X̃2B1 electronic ground state are observed with a source-modulation microwave spectrometer and a Fourier-transform microwave spectrometer. Molecular constants including hyperfine interaction constants are determined for H2NS and D2NS by a least-squares method. By using the obtained centrifugal distortion constants and inertial defects, the harmonic force field is evaluated, and the frequency of the ν4 vibrational mode (out of plane) is calculated to be 325 cm−1. This value is much smaller than that of the related molecule H2CS, indicating a floppy motion along the out-of-plane mode. The zero-point vibrational average structure was determined as follows; rz(N–H)=1.000(5) Å, rz(N–S)=1.6398(13) Å, and ∠z(HNH)=118.9(7)°, where the numbers in parentheses represent three times the standard deviation.
External field control of condensed phase reactions
Authors: Gross Peter , Schwartz Steven D.
Published: July 2002

Control of tunneling in a symmetric double well coupled to a bath via an external field is demonstrated. Optimal control theory is employed to design a laser field which couples to the reaction coordinate and drives a localized wave packet on the reactant side to the product side at a specified target time τ. Both a very quantumlike system (high barrier) and a low barrier double well are examined; excellent results are obtained for a range of reduced bath viscosities. Analysis of the control fields and corresponding localization dynamics shows that the frequency components of the control field are more or less in resonance with eigenstates of the double well and that the laser field enhances the natural dynamics of the individual wells. Future extension to more complicated models where the field couples to the bath is discussed.
Vibrational echo spectroscopy: Spectral selectivity from vibrational coherence
Authors: Rector K. D. , Zimdars David , Fayer M. D.
Published: July 2002

Theory and experimental data are presented which illustrate a new method for performing two-dimensional vibrational spectroscopy using ultrafast pulsed infrared lasers, called vibrational echo spectroscopy (VES). The VES technique can generate a vibrational spectrum with background suppression using the nonlinear vibrational echo pulse sequence. The vibrational echo pulse sequence is used with the delay between the excitation pulses fixed while the excitation wavelength is varied. A detailed theory of VES is presented which calculates the full third order nonlinear polarization including rephasing and nonrephasing diagrams. Finite width laser pulses are used and the calculations are performed for a model spectrum with two or more peaks. Two mechanisms that can result in background and peak suppression are illustrated. The mechanisms are based on differences in homogeneous dephasing times (T2) or transition dipole matrix element magnitudes. Although the VES line shape differs from the absorption line shape, it is possible to recover the absorption line shape from the VES. The method is demonstrated experimentally on the vibrational mode of CO (center at 1945 cm−1) bound to the active site of the protein myoglobin (Mb-CO). The protein and solvent produce a large absorption background while the VES spectrum of Mb-CO is background free. Calculations are able to reproduce the experimental Mb-CO VES line shape.
Rotationally resolved IR spectroscopy of ammonia trapped in cold helium clusters
Authors: Behrens Michael , Buck Udo , Fröchtenicht Ralf , Hartmann Matthias , Huisken Friedrich , Rohmund Frank
Published: July 2002

Infrared molecular beam depletion spectroscopy of ammonia molecules embedded in large He clusters has been studied in the spectral region of the ammonia umbrella mode from 940 to 1060 cm−1. The transitions between the lowest rotational energy levels could be resolved. By fitting the spectrum with the Hamiltonian of a free rotor, we observe a large shift of the vibrational frequency of 17.4 cm−1 to the blue, an effective decrease of the rotational constant B by 24%, and a reduction of the tunneling splitting by 31% in the excited and 75% in the ground state. This implies an unusually large interaction between the host cluster and the umbrella motion of the ammonia molecule, an intermediate influence on the rotation and also a large perturbation of the inversion by the helium environment.
Emission spectroscopy of jet-cooled CS2 upon excitation of the Σg+1→1B2(1Σu+) transition in the 48 500–51 000 cm−1 region
Authors: Arendt Michael F. , Butler Laurie J.
Published: July 2002

The jet-cooled emission spectroscopy of gaseous carbon disulfide (CS2) was performed in the 48 500–51 000 cm−1 region in order to investigate changes in the early time dynamics of CS2 in the predissociative B21 (1Σu+) state. The spectra show emission into progressions of both the symmetric stretching (ν1) and bending (ν2) motions and combinations bands of these two motions. Analysis shows that the contribution of symmetric stretching and bending motion varies over the range of excitation wavelengths and the progression of ν2 decreases in intensity relative to the progression in ν1 as one scans through the energetic region near the barrier to linearity of this bent excited state molecule. We estimate the barrier height to be approximately 3400 cm−1 above the zero-point level of the B21 (1Σu+) state.
Monte Carlo methods for short polypeptides
Authors: Schofield Jeremy , Ratner Mark A.
Published: July 2002

Nonphysical sampling Monte Carlo techniques that enable average structural properties of short in vacuo polypeptide chains to be calculated accurately are discussed. Updating algorithms developed for Monte Carlo studies of flexible polymer chains are modified and adapted for polypeptide chain systems to improve conformational sampling. Utilizing these methods, the effect of bond angle and bond length constraints in Monte Carlo simulations are examined and it is demonstrated that angle constraints bias structural averages without greatly reducing the computational work.
Negative ion formation in near grazing surface scattering of hyperthermal neutral C60: Image charge effects
Authors: Bekkerman A. , Tsipinyuk B. , Verkhoturov S. , Kolodney E.
Published: July 2002

Formation of negatively charged C60− was observed in the near-grazing scattering of hyperthermal (10–50 eV) neutral C600 from a carbon-covered (monolayer graphite) nickel surface. Under these conditions the normal energy component of the scattered particle (0.3–1.5 eV) is of the same order of magnitude as the attractive potential well. We have measured both energy and angle distributions of the scattered C60− and studied the electron transfer process by comparing the energy and angular distributions of the scattered negatively charged and neutral C60 as a function of primary energy. The shifts observed between the angular and energy distributions maxima of the neutral and negative ion could be analyzed and explained in terms of image charge effects on the outgoing trajectory (deflection) and exit energy (retardation) of the C60−. The angular deflection analysis yield image charge barrier of 0.28±0.02 eV corresponding to a rather large “ion formation” critical distance of 12.9±0.9 Å. We believe that this is the first direct observation of image charge effects in molecule/cluster-surface scattering.
Reaction dynamics of atomic chlorine with methane: Importance of methane bending and torsional excitation in controlling reactivity
Authors: Kandel S. Alex , Zare Richard N.
Published: July 2002

The reactions of atomic chlorine with CH4 and CD4 were studied at five collision energies ranging from 0.13 to 0.29 eV using resonance-enhanced multiphoton ionization of the CH3 and CD3 products. Core-extracted ion arrival profiles were used to determine methyl radical product speed distributions. The distributions contain products that are moving anomalously fast which energetically cannot result from the reaction of ground-state chlorine with ground-state methane. We attribute these products to reaction of ground-state chlorine with methane vibrationally excited in trace quantities into low-energy bending and torsional modes. Measurements of product spatial anisotropy are used to confirm this interpretation and to indicate that the possible reaction of spin–orbit excited chlorine is less important. These low-energy vibrations create large enhancements in reactivity over ground-state molecules, and consequently, vibrationally excited reagents dominate reactivity at low collision energies and contribute substantially at the highest collision energies studied. It is suggested that vibrationally excited reagents play an important role in the thermal kinetics of the reaction of chlorine with methane and may contribute significantly to explain the observed deviation from Arrhenius equation behavior. Scattering distributions of the products of both ground-state and vibrationally excited reactions are reported, and additional measurements of the internal state distributions of the CH3 and CD3 products reveal that the methyl radicals contain very little energy in rotation or vibration.
Configurations of liquid crystalline polymers in nematic solvents
Authors: Carri Gustavo A. , Muthukumar M.
Published: July 2002

We have considered the effect of a nematic solvent on the configurations of liquid crystalline polymers (LCPs) modeled as semiflexible chains. The model for the chain is a generalization of the Kratky–Porod model satisfying the global inextensibility of the chain for all values of coupling with the nematic field. We have calculated analytically the components of the mean-square end-to-end distance, persistence length and radius of gyration parallel and perpendicular to the nematic director, and the asymptotic behaviors of the structure factor. We also discuss the effect of architecture on the configurations of the polymer. We conclude that the configurations of main-chain and side-on side-chain LCPs are more sensitive to the nematic solvent than those of end-on side-chain LCPs.
Fragment recoil anisotropies in the photoinitiated decomposition of HNCO
Authors: Zyrianov M. , Droz-Georget Th. , Reisler H.
Published: July 2002

The photofragment ion imaging technique is used to determine product recoil anisotropy parameters, β, and correlated state distributions in the S1(1A″)←S0(1A′) photoinitiated decomposition of HNCO into three competing channels: (1) N3H+CO, (2) H+NCO, and (3) N1H+CO [where NH3 and NH1 denote NH(X 3Σ−) and NH(a 1Δ), respectively]. In particular, the region in the vicinity of the N1H+CO threshold is investigated. The measured recoil anisotropies fall into two distinct groups corresponding to time scales of <1 ps (β<−0.6), and >5–10 ps (β≅0.0). With 230.1 nm photolysis, CO(J=0–14) originating in channel (3) is produced with β=−0.8±0.05 via direct dissociation on S1 above a barrier of 470±60 cm−1. CO at low J-states appears with most of the available energy in the translational degree of freedom and is correlated with NH1 in its lowest rotational states. A small contribution to channel (3) from S0 dissociation (observed mainly for J=14,15) gives rise to an isotropic recoil distribution, and a hotter correlated NH1 rotational distribution. At the same wavelength, CO correlated with NH3 is identified by its high translational energy and exhibits an isotropic angular distribution. We propose that the pathway leading to its formation is S1→S0→T1. H-atom signals from channel (2) have isotropic angular distributions at photolysis wavelengths 243−215 nm; this places a lower limit of 8140 cm−1 on the barrier to direct dissociation on S1 to channel (2). The >5 ps time scale for the appearance of channel (2) implies dissociation on S0 following internal conversion. The mechanism described here for the one-photon decomposition of HNCO in the wavelength region 260-230 nm is in accord with other available experimental and theoretical findings.
High resolution vibronic spectra of the amino acids tryptophan and tyrosine in 0.38 K cold helium droplets
Authors: Lindinger Albrecht , Toennies J. Peter , Vilesov Andrei F.
Published: July 2002

High resolution vibronic absorption spectra are reported for the amino acids tryptophan and tyrosine embedded in large (N≃103–104) helium droplets. The spectra exhibit a large number of sharp (δν≈0.5 cm−1) 000 band origins which are attributed to different conformers. The significant differences to previous spectra of the isolated molecules from seeded beam expansions are interpreted in terms of the lower temperatures and the suppression of certain conformers in the liquid helium environment.
A coupled-cluster solvent reaction field method
Authors: Christiansen Ove , Mikkelsen Kurt V.
Published: July 2002

A method for introducing the coupling between a molecule and an outer solvent in molecular coupled-cluster calculations is presented. The method is a coupled-cluster self-consistent reaction field (CCSCRF) method. The outer solvent is represented as a dielectric medium which is characterized by the dielectric constants of the solvent. The interaction between solute and solvent is introduced into the coupled-cluster Lagrangian and a self-consistent solution is obtained. Sample calculations are presented for total energies and structures for H2O, H2S, and CO. Furthermore, the rotation barrier of 1,2-Dichloroethane is determined in vacuum and different solvents and the results are compared to experimental data.
Melting behavior of heterogenous atomic clusters: Gapless coexisting phases in (Ar–Xe)13
Authors: Mehra Vishal , Prasad Awadhesh , Ramaswamy Ramakrishna
Published: July 2002

We study the structural and dynamical aspects of 13–atom binary rare-gas clusters of Ar and Xe using constant–energy molecular dynamics simulations. The ground state geometry for ArnXe13−n, n=1−12, remains near-icosahedral, with an Ar atom occupying the central position. The thermodynamic properties of these clusters are significantly different from the pure 13-atom Ar or Xe clusters and for Xe–dominated compositions, melting is preceded by a surface–melting stage. Slow oscillations of the short-time-averaged (STA) temperature are observed both for surface–melting and complete melting stage, suggesting dynamical coexistence between different phases. At the complete melting stage, the oscillations in the STA temperature and the species of the central atom are correlated.
Vibrational spectra of molecular ions isolated in solid neon. XV. Infrared spectroscopic evidence for NeHF+ and HFFH+
Authors: Lugez Catherine L. , Jacox Marilyn E. , Johnson Russell D.
Published: July 2002

When a Ne:HF mixture is subjected to Penning ionization and/or photoionization by neon atoms in their first excited states, between 16.6 and 16.85 eV, and the products are rapidly frozen at approximately 5 K, the infrared spectrum of the resulting deposit includes absorptions assigned to NeHF+ and HFFH+. Evidence is presented suggesting that neon-matrix shifts may be anomalously large for protonated molecules which are derived from species with proton affinities near or below 550 kJ/mol. The results of earlier ab initio studies, as well as those of the present experimental and ab initio studies, indicate that the NeHF+ complex is quite strongly bound. The ν3 absorptions of NeHF+ and NeDF+ and the ν1+ν3 absorption of NeHF+ are tentatively identified. An absorption at 3064.7 cm−1 has isotopic substitution behavior appropriate for its assignment to HFFH+. Density functional calculations were performed in order to estimate the positions of the harmonic vibrational fundamentals of HFFH+-dn, and a more elaborate two-dimensional calculation on the HF-stretching potential surface was conducted in an attempt to take into account the large anharmonic contribution. Processes which occur on ultraviolet irradiation of the deposit are considered.
Vibrational structure of the chloromethane series, CH4−nCln, studied by core photoelectron spectroscopy and ab initio calculations
Authors: Sundin S. , Saethre L. J. , Sorensen S. L. , Ausmees A. , Svensson S.
Published: July 2002

Vibrationally resolved C1s photoelectron spectra of the chloromethane series, CH4−nCln, are reported. The spectra are compared with results from ab initio calculations at the MP2 level using the Z+1 equivalent-cores approximation. Good agreement is generally obtained, and a detailed evaluation of the contributing vibrational modes and the resulting geometry changes upon core ionization is possible. We find that, for all of these molecules, bending modes contribute very little to the vibrational structure. Surprisingly, for both C-H and C-Cl bonds, the bond shortening upon core-ionization decreases with addition of chlorine atoms. For this reason, an intensity model based on the linear-coupling model, which has been successfully applied in earlier studies of simple hydrocarbons, is found to be of limited applicability in predicting the vibrational structure of the chloromethane series. Vertical and adiabatic C1s binding energies have been extracted and compared to the number of chlorine atoms. A steady, but nonlinear increase in binding energy with addition of chlorine atoms is observed.
Density functional theory of molecular structure for thin diblock copolymer films on chemically heterogeneous surfaces
Authors: Nath Shyamal K. , Nealey Paul F. , de Pablo Juan J.
Published: July 2002

A density functional theoretical formalism is applied to study the ordering of block copolymers near patterned surfaces. This formalism is capable of resolving with molecular-level detail the structure of the system in the directions parallel and perpendicular to an interface. The results of theoretical predictions are in good agreement with those of molecular simulations. This formalism permits a systematic study of the effects of temperature, chain architecture, surface–polymer interaction energy, and pattern shape and size on the surface-induced ordering that occurs in systems consisting of block copolymers deposited on chemically heterogeneous surfaces.
Quantum scattering calculations on the SN2 reaction Cl−+CH3Br→ClCH3+Br−
Authors: Schmatz Stefan , Clary David C.
Published: July 2002

The gas-phase SN2 reaction Cl−+CH3Br(v,k)→ClCH3(v′k′)+Br− has been studied using reduced dimensionality time independent quantum scattering theory. The C–Br and C–Cl stretching degrees of freedom (quantum numbers v and v′) and the azimuthal angle (rotation of the CH3 group; quantum numbers k and k′) are treated explicitly. An infinite order sudden approximation and Radau coordinates for the stretching modes are used. The scattering problem is formulated in hyperspherical coordinates. A potential energy surface of Wang, Zhu, and Hase is used. It is found that this surface can reproduce the experimentally observed independence of the rate constant on the internal temperature of CH3Br only if it is scaled to enable the transition state geometry to agree with high level ab initio data. The reaction cross sections show the propensity rule Δk=0 for the azimuthal rotation.
Monte Carlo simulations of smectic phase transitions in flexible–rigid–flexible molecules
Authors: Casey Alison , Harrowell Peter
Published: July 2002

The effect on phase behavior of adding flexible terminal chains to a rigid core is examined with a simple model particle. It is discovered that two types of smectic phase appear in the phase diagram, a smectic A phase and a smectic C phase. The detailed nature of these phases is examined, as is the type of transition that takes place and the particle properties that drive the transition.
Electric field induced lateral instability in a simple autocatalytic front
Authors: Horváth Dezsö , Tóth Ágota , Yoshikawa Kenichi
Published: July 2002

The effect of ionic drift caused by small constant electric field on autocatalytic reaction fronts of ionic species is studied both theoretically and numerically. Besides varying the velocity of propagation, the electric field parallel to the direction of propagation may induce lateral instability in planar fronts resulting in the emergence of cellular structures. The difference in the diffusivities at the onset of instability are lowered when the electric field tends to separate the species spatially. The predictions of the linear stability analysis based on a thin-front approximation are confirmed by the numerical integration of the full two-dimensional system.
Molecular dynamics simulation of the rheological and dynamical properties of a model alkane fluid under confinement
Authors: Cui S. T. , Cummings P. T. , Cochran H. D.
Published: July 2002

We study the effect of wall–fluid interactions on the state conditions and the effective properties of a model dodecane fluid confined between parallel solid walls. A significant increase in the effective density of the confined fluid is observed with increasing strength of the wall–fluid interaction. The effect of the wall–fluid interaction on the rotational relaxation and diffusional relaxation of the fluid is seen in the significant slowing down of the relaxation with increasing wall–fluid interaction strength. The difference between the confined fluid and the three-dimensional bulk fluid is demonstrated by the strong anisotropy of the dynamical properties, the molecular rotation, and self-diffusion. The viscosity of the confined fluid shows a large difference between weak and strong wall–fluid interactions, and a significant difference from bulk fluid at low shear rate.
First detection of an ultraviolet transition in an ionized polycyclic aromatic hydrocarbon
Authors: Chillier Xavier D. F. , Stone Bradley M. , Salama Farid , Allamandola Louis J.
Published: July 2002

We report the first measurement of a polycyclic aromatic hydrocarbon cation electronic transition in the ultraviolet spectral region. The UV excitation spectrum of the Dn–XD0 transition of the perylene cation (C20H12+) isolated in an argon matrix was pumped with tunable, coherent radiation and monitored using the vibronic bands of the D1(2B3g)→(2Au)D0 system. The two component band peaks at 244 and 250.2 nm, close to the S2(1B3u)←(1Ag)S0 transition in neutral perylene.
Spin-orbit relaxation and recombination dynamics in I2−(CO2)n and I2−(OCS)n cluster ions: A new type of photofragment caging reaction
Authors: Sanov Andrei , Sanford Todd , Nandi Sreela , Lineberger W. Carl
Published: July 2002

We report a new type of photofragment caging reaction that is only possible because of the strong solvent-induced perturbation of the inherent electronic structure of the chromophore. The photoexcitation of I2− at 395 nm promotes it to a dissociative state correlating with I−+I*(2P1/2), the only near-ultraviolet dissociation channel for unsolvated I2−. In I2−(CO2)n and I2−(OCS)n clusters, interaction with the solvent is observed to result in extremely fast spin-orbit relaxation. In general, we detect three reaction pathways: (1) direct dissociation of the chromophore to I−+I*(2P1/2); (2) the I2−→I−+I* dissociation, followed by spin-orbit quenching leading to I−+I(2P3/2) products; and (3) the I2−→I−+I* dissociation, followed by spin-orbit quenching and I−+I(2P3/2)→I2− recombination and vibrational relaxation. We present experimental evidence of the spin-orbit relaxation and caging and discuss possible mechanisms. The results include: the measured translational energy release in 395 nm photodissociation of unsolvated I2−, indicating that solvation-free dissociation proceeds exclusively via the I−+I* channel; ionic product distributions in the photodissociation of size-selected I2−(CO2)n and I2−(OCS)n clusters at the same wavelength, indicating the above three reaction channels; and ultrafast pump-probe measurements of absorption recovery, indicating picosecond time scales of the caging reaction. We rule out the mechanisms of spin-orbit quenching relying on I*-solvent interactions without explicitly considering the perturbed electronic structure of I2−. Instead, as described by Delaney et al. (companion paper), the spin-orbit relaxation occurs by electron transfer from I− to I*(2P1/2), giving I(2P3/2)+I−. The 0.93 eV gap between the initial and final states in this transition is bridged by differential solvation due to solvent asymmetry. Favorable comparison of our experimental results and the theoretical simulations of Delaney et al. yield confidence in the mechanism and provide understanding of the role of cluster structure in spin-orbit relaxation and recombination dynamics.
Homogeneous nucleation rates of n-pentanol measured in an upward thermal diffusion cloud chamber
Authors: Rudek Markus M. , Katz Joseph L. , Vidensky Igor V. , Ždı́mal Vladimir , Smolı́k Jiri
Published: July 2002

Homogeneous nucleation rates of n-pentanol as functions of both supersaturation and temperature were measured in two different upward thermal diffusion cloud chambers, by research groups in Prague and Baltimore. The measurements were made at temperatures between 280 K and 320 K. The nucleation rates obtained are compared to the rates measured by Luijten et al. [J. Chem. Phys. 106, 4152 (1997)], by Hrubý et al. [J. Chem. Phys. 104, 5181 (1996)], and by Strey et al. [J. Chem. Phys. 84, 2325 (1986)]. Fair agreement between our data and the data obtained by the other authors also was found.
Anharmonic phonon-polariton excitation through impulsive stimulated Raman scattering and detection through wave vector overtone spectroscopy: Theory and comparison to experiments on lithium tantalate
Authors: Romero-Rochı́n Vı́ctor , Koehl Richard M. , Brennan Ciaran J. , Nelson Keith A.
Published: July 2002

We study theoretically the generation of coherent, anharmonic phonon-polariton responses through impulsive stimulated Raman scattering with intense, crossed ultrafast excitation pulses. We find that the refractive index appears modulated at the stimulated scattering wave vector and the corresponding phonon-polariton frequency, and, due to anharmonicity, at stimulated scattering wave vector overtones and their corresponding frequencies. A realistic model of the soft lattice vibrational mode of the ferroelectric crystal lithium tantalate is considered in detail. Specific predictions for the magnitudes of different wave vector overtone contributions to the lattice displacement are made compared to experimental observations of anharmonic lattice responses.
The effect of a strong external field on the electronic dephasing of a solute that is strongly coupled to a solvent
Authors: Cukier R. I. , Denk C. , Morillo M.
Published: July 2002

A recent theory of strong field spectroscopy (SFS) [R. I. Cukier and M. Morillo, Phys. Rev. B 57, 6972 (1998), M. Morillo and R. I. Cukier, J. Chem. Phys. (110, 7966 (1999)] is generalized to apply to strong solute–solvent coupling. In SFS, a strong external field is used to connect, with the transition dipole, two electronic states of a solute immersed in a medium. In contrast to weak fields, z̄(t), the average population difference of the solute electronic states is changing significantly. For resonant, strong fields, z̄(t) and the average absorbed power, P̄(t), exhibit oscillatory decays in time that reflect the changing z̄(t) and the dissipation arising from the coupling to the medium. When the solute–solvent coupling is relatively weak, the time evolution of the solvent only depends on the initial solute state (autonomous behavior). In this work, appropriate to strong coupling, we derive an equation of motion for the solvent dynamics that depends on the solute’s instantaneous state (nonautonomous behavior). The consequences to z̄(t) and P̄(t) are explored. We find that instead of equalizing the solute populations at long times, now the population is inverted relative to its initial state. We also find that the degree of long-time population inversion can be controlled by turning off the external field before the system has fully relaxed.
A theoretical investigation of the nature of the π-H interaction in ethene–H2O, benzene–H2O, and benzene–(H2O)2
Authors: Tarakeshwar P. , Choi Hyuk Soon , Lee Sang Joo , Lee Jin Yong , Kim Kwang S. , Ha Tae-Kyu , Jang Jee Hwan , Lee Jung Goo , Lee Hosull
Published: July 2002

We have carried out a detailed investigation of the nature of the π-H interaction in the ethene–H2O, benzene–H2O, and benzene–(H2O)2 complexes using large basis sets (ranging from 6-31+G* to TZ2P++) and high levels of theory. The minimum geometries, and hence the vibrational frequencies, of all the complexes have been obtained at the second order Mo/ller–Plesset (MP2) level of theory. The binding energy of the ethene–H2O complex is only about 1 kcal/mol lower than that of the benzene–H2O complex. In the benzene–(H2O)2 complex, the interaction of benzene with the π-bonded water to that with the second water is nearly equivalent. In order to explain the above interesting facets of the interaction of water with benzene and ethene, the interaction energies were decomposed into the individual interaction energy components using the recently developed symmetry adapted perturbation theory (SAPT) program. The SAPT results indicate that the repulsive exchange energies play a crucial role in governing the energies and geometric preferences of these complexes. A detailed analysis of the vibrational frequencies of these complexes has also been done to examine the changes in the frequencies of the monomers upon complexation. It is found that changes in the out-of-plane bending frequencies of benzene and ethene can be correlated to the interaction energies of these complexes, in particular the exchange energy.
Monte Carlo simulation study of the high-temperature phase diagram of model C60 molecules
Authors: Hasegawa M. , Ohno K.
Published: July 2002

The constant-NVT Monte Carlo simulation is performed for model C60 molecules interacting via the Girifalco potential and a full free-energy analysis is made to predict the high-temperature phase diagram. The repulsive part of the C60 potential is very steep and the attractive part is relatively short-ranged. For such a system accurate computations of the virial pressure are difficult in simulations and it is argued that the discrepancies among the previous results for the phase diagram of C60 can partly be attributed to the uncertainties of the virial pressure involved in simulations. To avoid this difficulty we take the energy route to calculate equation of state (EOS), in which the absolute (Helmholtz) free energy is obtained by performing isochoric integration of the excess internal energy. A difficulty of the energy route in the high-temperature limit is resolved by the aid of an analytic method. The exact second and third virial coefficients are also used in the analysis of the fluid EOS. The pressure route is taken to calculate the EOS of the solid phase, in which the virial pressure is numerically more stable than in the fluid phase. The resulting high-temperature phase diagram of C60 is quite systematic and free from uncertainties, and the liquid–vapor critical point is found at Tc=1980 K and ρc=0.44 nm−3, whereas the triple point at Tt=1880 and ρt=0.74 nm−3, confirming the existence of a stable liquid phase over the range of ∼100 K.