bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral hole burning in polymorphic systems: Single site pressure phenomena and glassy behavior
Authors: Schellenberg P. , Friedrich J. , Kikas J.
Published: July 2002

We measured the pressure-induced shift and broadening of spectral holes for 14 different impurity sites in a polycrystalline solid solution of chlorin (17,18-dihydro-21H,23H-porphine) in benzophenone. In addition, we investigated the glassy phase of the same system in a frequency range which covered the entire inhomogeneously broadened band. The temperature range of the experiments was between 1.5 and 4.2 K. Pressure levels up to 10 MPa were applied. In the glassy phase, we observed a linear frequency dependence of the shift per pressure. However, in the crystalline phase, the general trend of the data clearly indicated a nonlinear frequency dependence. Yet, the individual sites, especially those with lower frequencies, showed a kind of stochastic scattering around the general trend behavior. The results were interpreted within the frame of existing models. The experiments demonstrate that matrix isolation combined with hole burning spectroscopy of large molecular impurities in polymorphic host materials under varying pressure conditions is a useful technique for determining molecular as well as bulk parameters, for examining problems of microelasticity and microplasticity, and for investigating the fundamental differences between glasses and crystals.
Predissociation in the Ã 2A′ state of HNF (DNF): NH (ND) Photofragment excitation spectroscopy
Authors: Chen Jing , Dagdigian Paul J.
Published: July 2002

Photofragment excitation spectra of room-temperature HNF and DNF have been measured by monitoring laser fluorescence of the NH(X 3Σ−) and ND(X 3Σ−) fragments, respectively. These fragments were observed upon the excitation of the following parent molecule Ã 2A′ vibrational levels: HNF (0,v2′,0), with v2′ = 3–6; and DNF (0,v2′,0), with v2′ = 5–7, and (0,v2′,1), with v2′ = 3 and 4. With the exception of the highest levels, the photofragment excitation spectra were found to have complicated structures which were drastically different and more complicated than the K structure in the parent molecule absorption and fluorescence excitation spectra. The internal energy content of the fragments was also measured. For the lower HNF (DNF) levels the fragments possess very little rotational energy; for higher parent molecule levels the fragment rotational energy is correlated with the parent energy. The fragmentation of excited HNF (DNF) is complicated by the fact that there are three close-lying possible dissociation channels, to NH(X 3Σ−)+F(2P), NF(X 3Σ−)+H(2S), and HF(X 1Σ+)+N(2D). These results on the photofragmentation of excited HNF (DNF) and other observations from this laboratory on the parent laser fluorescence spectrum and measured decay lifetimes are discussed.
First and second energy derivative analyses of the vinylidene and acetylene triplet state potential energy hypersurfaces
Authors: Yamaguchi Yukio , Vacek George , Thomas J. Russell , DeLeeuw Bradley J. , Schaefer Henry F.
Published: July 2002

First and second derivatives of the orbital, electronic, nuclear, and total energies for the self-consistent-field (SCF) wave function have been used to study the triplet state of the CCH2 and HCCH molecules. The diagonal elements of the Lagrangian matrix for the general open-shell SCF wave function are used as the ‘‘orbital’’ energies. The first and second derivatives of the orbital energies in terms of the normal coordinates are determined by the finite difference method, while the corresponding derivatives of the electronic, nuclear, and total SCF energies are obtained via analytic derivative techniques. It is demonstrated that the derivatives of the energies with respect to the normal coordinates provide useful chemical information for the triplet state potential energy hypersurfaces of the CCH2 and HCCH molecules. Specifically, the isomerization reactions from ã 3B2 vinylidene to b̃ 3Bu acetylene and from ã 3B2 acetylene to b̃ 3Bu acetylene are found to be electronically stable processes and their negative force constants (or imaginary vibrational frequencies) are seen to be controlled by the negative nuclear contributions. Energy derivative analysis may give strong quantitative support for arguments based on the conventional qualitative molecular orbital (MO) theory.
Spectral diffusion and 14N quadrupole splittings in absorption detected magnetic resonance hole burning spectra of photosynthetic reaction centers
Authors: Greis J. W. , Angerhofer A. , Norris J. R. , Scheer H. , Struck A. , von Schütz J. U.
Published: July 2002

Zero field absorption detected magnetic resonance hole burning measurements were performed on photosynthetic reaction centers of the bacteria Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis. Extrapolation to zero microwave power yielded pseudohomogeneous linewidths of 2.0 MHz for Rhodopseudomonas viridis, 1.0 and 0.9 MHz for the protonated forms of Rhodobacter sphaeroides R26 with and without monomer bacteriochlorophyll exchanged, and 0.25 MHz as an upper limit for fully deuterated reaction centers of Rhodobacter sphaeroides R26. The measured linewidths were interpreted as being due to unresolved hyperfine interaction between the nuclear spins and the triplet electron spin, the line shape being determined by spectral diffusion among the nuclei. The difference in linewidths between Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis is then explained by triplet delocalization on the special pair in the former, and localization on one dimer half on the latter. In the fully deuterated sample, four quadrupole satellites were observed in the hole spectra arising from the eight 14N nitrogens in the special pair. The quadrupole parameters seem to be very similar for all nitrogens and were determined to κ=1.25±0.1 MHz and η=0.9±0.1 MHz.
Strong resonance enhancement of the CN two-photon absorption B 2Σ+←X 2Σ+(3,0) by the A 2Πi, v′=4 level
Authors: Guthrie John A. , Anderson William R. , Kotlar Anthony J. , Huang Yuhui , Halpern Joshua B.
Published: July 2002

We have observed a strong two-photon absorption in the B 2Σ+←X 2Σ+(3,0) band of CN by means of a resonant enhancement through the A 2Πi, v′=4 level. Many lines are seen in the two-photon spectrum due to multiple single-photon near resonances in the A 2Πi←X 2Σ+(4,0) band. The detuning of the laser from these resonances varies from less than one to hundreds of wave numbers, producing unusually large intensity variations in the two-photon spectrum. This effect is not observed in two-photon transitions far from resonance. Resonant enhancement is observed over a range from N=5 to 20. We know of no other molecular two-photon transition in which a near resonance produces such dramatically varying intensities over a short range of rotational levels. A calculation of the line strengths for these transitions reproduces the major features of the spectrum.
Guided-ion beam measurements of N++H2O charge-transfer and chemical reaction channels
Authors: Dressler Rainer A. , Murad Edmond
Published: July 2002

Guided-ion beam measurements of N++H2O charge-transfer and chemically reactive channels are presented for collision energies ranging from 0.1 eV to 20 eV c.m. The charge-transfer reaction is not as efficient as expected considering the favorable Franck–Condon overlap at resonance. The analysis of product-ion time-of-flight spectra demonstrates that energy transfer is efficient in orbiting collisions at all investigated collision energies. At low collision energies, formation of NO+, and HNO+ and/or NOH+, is observed. The chemically reactive channels exhibit a near 1/ET dependence at low collision energies. Above 0.5 eV, the HNO+/NOH+ cross section drops abruptly below the instrumental sensitivity. The NO+ cross section exhibits a minimum at 1.2 eV, followed by an increase in cross section with energy reaching a maximum of 4×10−16 cm2 at 7 eV before decreasing again. The decline at higher energies is partly attributed to a dissociative loss of the product ions. O+ formation is observed at and above the thermodynamic threshold associated with the O++N+2H asymptotic limit.
Experimental and theoretical studies of isomeric CH3S2 and CH3S+2
Authors: Ma Z.-X. , Liao C. L. , Ng C. Y. , Cheung Yu-San , Li Wai-Kee , Baer Tomas
Published: July 2002

By combining molecular beam photodissociation and photoionization measurements with ab initio Gaussian-2 (G2) calculations on the CH3S2 and CH3S+2 systems, we have shown that CH3SS is the dominant isomer formed in the photodissociation process, CH3SSCH3+hν(193 nm) →CH3S2+CH3. The experimental ionization energy for CH3SS (8.97±0.02 eV) and the heat of formation at 0 K for CH3SS+ (217.7±1.2 kcal/mol) are in excellent agreement with the G2 results. The photoionization efficiency spectrum observed for CH3SS is also consistent with the theoretical prediction that the Franck–Condon factor for the photoionization process, CH3SS+hν →CH3SS++e−, is not favorable. Based on the statistical modeling of experimental rates obtained previously for HS loss in the unimolecular decomposition of CH3SSCH+3 and the comparison with G2 ab initio predictions, we conclude that CH2SSH+ is most likely the isomer structure formed near the experimental appearance energy (11.07 eV) observed for the photodissociative ionization process, CH3SSCH3+hν→CH3S+2+CH3+e−.
CO(v,J) product state distributions from the reaction O(3P)+OCS→CO+SO
Authors: Nickolaisen Scott L. , Veney David W. , Cartland Harry E.
Published: July 2002

The title reaction was studied by probing CO[v,J] product state distributions with a tunable infrared diode laser. Only the fastest O(3P) atoms formed by 355 nm photolysis of NO2, i.e., those corresponding to NO[2Π1/2, v=0], have sufficient energy to surmount the activation barrier. Thus these experiments were performed under monoenergetic, near-threshold conditions. The CO[v,J] distributions were extracted from the relative slopes of the transient infrared absorption signals. Little vibrational excitation is observed. The vibrational branching ratio is [v=1]/[v=0]≊0.05; CO[v≥2] was not detected. In contrast, rotational degrees of freedom are much hotter. The nascent CO[v=0] rotational distribution is well characterized by a temperature of TR=4400±390 K. Total CO internal excitation, on the average 3150 cm−1, accounts for 16% of the energy available to products. Surprisal analyses based on several models of energy disposal show that the nascent distributions are nonstatistically cold. The results are consistent with direct attack at the S atom to form an OSCO intermediate, followed by rapid C–S bond scission which exerts a torque on the CO product.
The density of reactive levels in NO2 unimolecular decomposition
Authors: Ionov S. I. , Davis H. F. , Mikhaylichenko K. , Valachovic L. , Beaudet R. A. , Wittig C.
Published: July 2002

Laser induced fluorescence spectra of expansion-cooled NO2/Ne samples (1 and 2 K) are reported for transitions that originate from the lowest rovibronic levels and terminate on levels near D0. At 1 K, nearly all transitions originate from N″=0. With the present resolution of 0.02 cm−1, the 1 K spectra are resolved rather well. The high density of transitions is due to couplings between rovibronic levels with different N and K quantum numbers and with electronic characters that borrow oscillator strength from bright B2 vibronic species of the mixed 2A1/2B2 electronic system. Just above reaction threshold, such rovibronic species comprise the manifold of levels sampled by optically prepared wave packets. However, at higher energies we argue that the density of B2 vibronic species is a more relevant parameter to describe the nature of unimolecular reactions. Nuances of the optical excitation process are discussed.
The stability and structure of the hyperlithiated molecules Li3S and Li4S: An experimental and ab initio study
Authors: Kudo Hiroshi , Yokoyama Keiichi , Wu C. H.
Published: July 2002

The thermodynamical properties of the hyperlithiated molecules Li3S and Li4S were investigated by means of Knudsen effusion mass spectrometry and their stability and structure were studied through ab initio molecular orbital calculations. The Li3S and Li4S molecules were found to be stable toward loss of one or two lithium atoms, respectively, to form the octet molecule Li2S. The experimental dissociation energies were D00(Li2S–Li)=33.1±1.6 and D00(Li2S–2Li)=83.9±2.7 kcal/mol. The atomization energies were determined as D00(Li3S)=161.3±3.8 and D00(Li4S)=211.9±4.2 kcal/mol. The ionization potential observed for Li3S was 4.4±0.2 eV. From the theoretical calculations, the occupancies of nine valence electrons in Li3S (C3v) and ten valence electrons in Li4S (C2v) were seen as (5a1)2(3e)4(6a1)2(7a1)1 and (6a1)2(3b1)2(7a1)2(3b2)2(8a1)2, respectively. The singly occupied 7a1 orbital of Li3S and the highest occupied 8a1 orbital of Li4S were found to be involved in the formation of a lithium ‘‘cage,’’ which should contribute to the thermodynamic stability of these molecules.
Ab initio study of the potential energy surfaces of doublet valence and Rydberg states of FCO
Authors: Krossner Th. , Zülicke L. , Vetter R. , Perić M. , Peyerimhoff S. D.
Published: July 2002

Multireference configuration interaction calculations (MRD-CI) were carried out in order to obtain the bending and stretching potential energy curves of several low-lying electronic states of the fluoroformyl radical FCO. The study of interactions between valence and Rydberg states as well as that of the dissociative behavior is included in the investigation.
Dipole moment function and equilibrium structure of methane in an analytical, anharmonic nine-dimensional potential surface related to experimental rotational constants and transition moments by quantum Monte Carlo calculations
Authors: Hollenstein Hans , Marquardt Roberto R. , Quack Martin , Suhm Martin A.
Published: July 2002

The pure rotational spectrum in the far-infrared and its absolute intensity in the vibrational ground state of CHD3 and CH3D, and the integrated band strength of the N=5 CH-stretching overtone of CHD3 in the near infrared to visible were measured by high-resolution interferometric Fourier transform techniques. The far-infrared data result in permanent electric dipole moments (‖μz0‖=(5.69±0.14)×10−3 D for CHD3, ‖μz0‖=(5.57±0.10)×10−3 D for CH3D), consistent with previous experimental data. The integrated N=5 overtone cross section is found to be (0.828±0.068) fm2. The overtone data are used, together with previous data, to derive a new, nine-dimensional, isotopically invariant dipole moment function for CH4 within the chromophore model for the CH chromophore in CHD3. With this function, the experimental data can be reproduced to an averaged factor of 1.2, in the best case. In the vibrational ground state, a nine-dimensional calculation of expectation values on a new, fully anharmonic potential surface was performed using the solution of the rovibrational Schrödinger equation by diffusion quantum Monte Carlo methods. The results for the rotational constants of several isotopomers, which include significant contributions from rovibrational interactions, indicate that the equilibrium CH bond length of methane is re=108.6 pm. The calculated value for the vibrationally averaged permanent dipole moment from these nine-dimensional vibrational quantum calculations, using the dipole moment function consistent with the analysis of the overtone bands, is μz0=−(6.6±0.4)×10−3 D for CHD3 (with positive z coordinate for the H atom) and μz0=(6.8±0.5)×10−3 D for CH3D (with positive z coordinate for the D atom) in essential agreement with the far-infrared rotational intensities. The sign could be determined unambiguously by comparison with ab initio data. We predict the permanent dipole moment of several further methane isotopomers. The polarity of the CH bond in methane is C−–H+, within our simple bond dipole model, but is discussed to be a model dependent (not purely experimental) quantity.
Structure of two-component clusters
Authors: Clarke A. S. , Kapral R. , Patey G. N.
Published: July 2002

Phase separation in binary liquid Lennard-Jones clusters is investigated employing computer simulation methods. Clusters ranging in size from 50 to 240 particles are considered with special emphasis on systems with equal numbers of A and B particles. Cluster morphology is systematically explored by varying the ratios, α=εAB/εAA, β=εBB/εAA, Γ=σAB/σAA, and Δ=σBB/σAA, where σ and ε are the Lennard-Jones size and energy parameters. A detailed α, β ‘‘phase diagram’’ is presented for the case Γ=Δ=1. Stable phase separated clusters are shown to fall into two general classes: elongated clusters of cylindrical or dumbbell shape, the ends of which are A-rich and B-rich phases, and spherical coated clusters consisting of a core of one species coated by the other. More quantitative structural information is given in the form of interfacial density profiles. We also propose two theoretical models for phase separation in binary clusters. One is a simple macroscopiclike droplet approach and the other is a mean field lattice model. Both simple models capture many of the important physical features observed in the computer simulations. Together they provide insight into the nature of phase separation in small systems.
Parametric light scattering
Authors: Verbiest Thierry , Kauranen Martti , Persoons André
Published: July 2002

We observe a nonlinear light-scattering process in which incident photons at two different frequencies give rise to scattered radiation at the sum frequency in an isotropic medium. We utilize this process to determine the ratio β322/β333 of the dominant components of the molecular hyperpolarizability tensor β for para-nitroaniline.
Ab initio study of torsional potentials in 2,2′-bithiophene and 3,4′- and 3,3′-dimethyl-2,2′-bithiophene as models of the backbone flexibility in polythiophene and poly(3-methylthiophene)
Authors: Hernandez V. , Lopez Navarrete J. T.
Published: July 2002

Ab initio molecular orbital theory is employed to calculate the gas-phase barrier to internal rotation in 2,2′-bithiophene. Ground state geometries are fully optimized at the restricted Hartree–Fock level of theory using the 3-21G* and 6-31G** basis sets. Methylation in β-positions modulates the geometry, the inter-ring twist angle and the conformational properties of thiophene dimers. These methyl substitution effects have been assessed by calculations on the 3,4′ and 3,3′-dimethyl derivatives in a number of selected conformations. A meaningful picture of the molecular relaxation on rotation is attained by allowing for full geometry optimization at both levels of calculation.
Local vibrational densities of states in glasses: Numerical study of a model of amorphous silicon
Authors: Ishibashi Kiyoshi , Tsumuraya Kazuo , Nakata Shinji
Published: July 2002

We analyze for the first time the relation between the shape of the local vibrational densities of states (LDOSs) and the local environments of atoms in glasses. We calculated the LDOSs in model silicon glasses by use of a recursion method. The glasses are produced by a constant pressure molecular dynamics method with a three-body potential by Stillinger and Weber. The transverse optic (TO) peaks reduce in height with an increase of the number of the coordinated atoms. The heights of the TO peaks also decrease with the increase of the number of the coordinated atoms that have (2,3,0) Voronoi signature. The heights of the transverse acoustic peak are insensitive to the number of bonds. The origin of the decrease of the TO peak is discussed. The slopes of the low frequency side of the LDOSs are insensitive to the local structure of the glasses. The long-range force field in the present glass is the same as that in the crystal state.
Heat flux induced by an external force in a strongly shearing dilute gas
Authors: Garzó V.
Published: July 2002

Coupling between energy and momentum transport in a dilute gas subject to uniform shear flow is analyzed. Heat flux is created in the system by the action of a nonconservative external force. The results are obtained by using the Bhatnagar–Gross–Krook kinetic model. The moments of the velocity distribution function are expressed in terms of a perturbation expansion in powers of the heat field strength, the coefficients being highly nonlinear functions of the shear rate. In particular, the thermal conductivity tensor and the shear viscosity coefficient up to second-order approximation are explicitly evaluated. It is shown that the usual choice of the heat field proposed in computer simulations leads to a thermal conductivity tensor different from the one obtained in the thermal gradient problem, confirming previous results. In order to avoid this discrepancy, an alternative external force is proposed.
Poisson–Boltzmann calculations of ions in charged capillaries
Authors: Lo Wai Yin , Chan Kwong-Yu
Published: July 2002

The Poisson–Boltzmann (PB) equation of ions inside an infinitely long charged pore is solved using two different boundary conditions of the electric field due to the pore wall. The results show that the boundary condition generally adopted in the literature leads to a violation of electroneutrality in the ion–pore system.
Competition between continuous etching and surface passivation for Cl2 chemisorption onto GaAs(100) c(8×2), GaAs(100) c(2×8), and GaAs(110) (1×1) surfaces
Authors: Sullivan Daniel J. D. , Flaum Harris C. , Kummel Andrew C.
Published: July 2002

The effects of surface temperature (Ts) and surface structure upon the passivation and etching of GaAs(100) and GaAs(110) surfaces by Cl2 have been studied. The Ga-rich GaAs(100) Ga-c(8×2) and the stoichiometric GaAs(110) (1×1) surfaces form stable ordered monochloride overlayers when exposed to Cl2 at 300 K. The ordered overlayers formed are AsCl on the GaAs(110) (1×1) surface and GaCl on the GaAs(100) Ga-c(8×2) surface. In contrast, the As-rich GaAs(100) As-c(2×8) surface undergoes continuous uptake of Cl2 under equivalent conditions. All three surfaces exhibit continuous uptake of Cl2 at surface temperatures above 600 K, where continuous etching is known to occur. In this paper, the continuous uptake of Cl2 is shown to result from etching of the surface. In addition it is shown that the Ga-rich GaAs(100) Ga-c(8×2) and the stoichiometric GaAs(110) (1×1) surfaces can be induced to undergo continuous etching at surface temperatures of 300 K, if they are first exposed to Cl2 with surface temperatures above 600 K. This slight pre-etch disrupts the surface order and exposes multiple dangling bonds. The disrupted surface does not allow for a monochloride passivation layer to form; instead, volatile trichloride products form, leading to continuous etching. Therefore, the dissociative adsorption of molecular chlorine on GaAs(100) and GaAs(110) surfaces involves a competition between etching and passivation in which monochloride passivation is favored only on the well-ordered surfaces.
Dynamic Monte Carlo description of thermal desorption processes
Authors: Weinketz Sieghard
Published: July 2002

The applicability of the dynamic Monte Carlo method of Fichthorn and Weinberg, in which the time evolution of a system is described in terms of the absolute number of different microscopic possible events and their associated transition rates, is discussed for the case of thermal desorption simulations. It is shown that the definition of the time increment at each successful event leads naturally to the macroscopic differential equation of desorption, in the case of simple first- and second-order processes in which the only possible events are desorption and diffusion. This equivalence is numerically demonstrated for a second-order case. In the sequence, the equivalence of this method with the Monte Carlo method of Sales and Zgrablich for more complex desorption processes, allowing for lateral interactions between adsorbates, is shown, even though the dynamic Monte Carlo method does not bear their limitation of a rapid surface diffusion condition, thus being able to describe a more complex ‘‘kinetics’’ of surface reactive processes, and therefore be applied to a wider class of phenomena, such as surface catalysis.
Metal/conjugated polymer interfaces: Sodium, magnesium, aluminum, and calcium on trans-polyacetylene
Authors: Fredriksson C. , Stafström S.
Published: July 2002

Quantum chemical calculations are performed to theoretically study the interaction between low work function metal atoms and trans-tetradecaheptaene, an extended model molecule for π-conjugated polymers. We present characterizations of metal/polyene complexes obtained by the local spin density method. The results show that charge transfer reactions occur for Na and Ca and the formation of a covalent bond for Al. Mg atoms, however, are found not to react significantly with π-conjugated systems. Differences in the bonding mechanism between the metals are discussed and the theoretical density of valence states are compared to experimental photoelectron spectroscopy (UPS) data. The results demonstrate that the local spin density method can be successfully used to interpret valence UPS spectra from early stages of metal/π-conjugated interface formation.
A comparison of classical trajectory and statistical unimolecular rate theory calculations of Al3 decomposition
Authors: Peslherbe Gilles H. , Hase William L.
Published: July 2002

Classical trajectories are used to simulate the dissociation of microcanonical ensembles of Al3 clusters with 0.25–2.00 kcal/mol energy in excess of the dissociation threshold. Unimolecular lifetime distributions for the ensembles are in accord with the random lifetime prescription of Rice–Ramsperger–Kassel–Marcus (RRKM) theory and Al3 is identified as an intrinsic RRKM molecule. Unimolecular rate constants determined from the trajectories are compared with the predictions of variational RRKM theory with harmonic vibrator and flexible transition state models, phase space theory (PST), and the orbiting transition state model of phase space theory (OTS/PST). The flexible RRKM model, PST, and OTS/PST give Al2- - -Al transition state sums of state which agree to within 1%. The harmonic vibrator RRKM model gives a sum of states which varies from only 40% to 10% larger as the excess energy is increased from 0.25 to 2.0 kcal/mol. Adiabatic switching and direct integration of the phase integral are used to determine the reactant Al3 anharmonic density of states. For the energies considered in this work, anharmonicity determined by adiabatic switching increases the Al3 density of states from the harmonic value by a factor of 2.5–2.9. The anharmonic density of states determined by solving the phase integral depends on where the reactant/product phase space boundary is positioned. Placing it at the vibrator transition state gives an anharmonic density of states nearly the same as that determined by adiabatic switching, while placing it at the looser flexible transition state gives a density of states 35% larger at the highest energy. Using this latter anharmonic density of states gives OTS/PST and flexible RRKM unimolecular rate constants which agree with the trajectory results to within 25% at the lowest energy and 10% at the highest energy. Product energy partitioning distributions determined from the trajectories are compared with the predictions of PST, OTS/PST, and the Engelking model. Both OTS/PST and PST give product energy partitioning distributions which are in near quantitative agreement with the trajectory results.
A self-consistent approach to a density functional for homogeneous fluids
Authors: Rickayzen G. , Kalpaxis P. , Chacon E.
Published: July 2002

A density functional, originally proposed by Rickayzen and Augousti for the study of the inhomogeneous hard sphere fluid, is generalized and applied to investigate the properties of the homogeneous hard sphere fluid. In principle, it is possible to determine simultaneously and self-consistently the two-particle direct correlation function, the equation of state and the strength of the excess free energy. In practice, it was found that, with the original form of excess free energy, convergence could not be achieved. With the generalized functional, however, it is possible to derive self-consistently the direct correlation function and, at the same time, obtain agreement between the virial pressure, the functional pressure, and the compressibility. Moreover, good agreement is obtained between the resulting pair distribution function and direct correlation function and the corresponding quantities obtained from computer simulation. At the largest reduced density studied, 0.90, there are small discrepancies which are most marked in the values of the direct correlation function near the origin.
Evaluation of resonance contributions to thermal reaction rates using quantum flux correlation functions
Authors: Thachuk Mark , Schatz George C.
Published: July 2002

We consider the evaluation of thermal rate constants using quantum flux correlation functions for chemical reactions in which metastable states (resonances) play a significant role in the reaction dynamics. The evaluation of rate constants is hindered in this case because of slowly decaying oscillations in the correlation functions but we show that it is possible to remove these oscillations by projecting the resonant states from the wave packets used to calculate flux correlation functions. These projected states do contribute to the reactive flux, but it is not difficult to include for this using the resonance widths. The resulting theory thus uses a combination of short time wave packet propagation for the direct contribution, and bound-state methods for the resonant contribution, thereby achieving a balance between the strengths of time dependent and time independent methods. We illustrate this theory through an application to a simple one-dimensional potential.
Further analysis of solutions to the time-independent wave packet equations for quantum dynamics: General initial wave packets
Authors: Hoffman David K. , Huang Youhong , Zhu Wei , Kouri Donald J.
Published: July 2002

In this paper we reexamine, and analyze solutions to, the recently derived time-independent wave packet-Schrödinger (TIWS) and time-independent wave packet-Lippmann–Schwinger (TIWLS) equations. These equations are so named because they are inhomogeneous, with the inhomogeneity being the initial L2 wave packet from an underlying time-dependent treatment of the dynamics. We explicitly show that a particular solution of the homogeneous Schrödinger equation can be constructed out of two particular solutions of the inhomogeneous TIW equation satisfying causal and anticausal boundary conditions. The structure of this solution of the homogeneous equation is shown to depend sensitively on the nature of the initial wave packet inhomogeneity, but, as we demonstrate, correct scattering information can be obtained even when the initial wave packet is nonzero only in the target region. It thus becomes possible to carry out quantum scattering calculations in which one need not propagate the wave packet from the noninteracting to the interacting region. The method is illustrated by calculations for two 1-D scattering problems, namely the transmission of an electron through a single barrier and through a double barrier. The latter is especially challenging because of the occurrence of long-lived resonances with the electron trapped inside the double barrier. In addition, we show that the method can still make use of absorbing potentials to decrease grid size, and we show how the formalism can be used to treat inelastic and reactive scattering, and radial scattering variables, thereby constituting a general approach to time-independent wave packet quantum scattering.
Ultrafast liquid dynamics studied by third and fifth order three pulse photon echoes
Authors: Joo Taiha , Jia Yiwei , Fleming Graham R.
Published: July 2002

We report a new spectroscopic measurement based on fifth order nonlinear response from a molecule in a room temperature liquid. In conjunction with conventional three pulse stimulated photon echo, detailed information on solvent–solute dynamics for a wide range of time scales has been obtained. Both the ultrafast inertial components and much slower dynamics of the solvent are found to make important contributions to the solvent relaxation. The potential of the fifth order measurements is discussed via numerical calculations using model solvent spectral densities.
Ultrafast electron dynamics at the liquid–metal interface: Femtosecond studies using surface plasmons in aqueous silver colloid
Authors: Roberti T. W. , Smith B. A. , Zhang J. Z.
Published: July 2002

We report direct femtosecond measurements of the dynamics of photoinduced electrons at the liquid–metal interface produced by exciting the surface plasmon band of aqueous silver colloidal particles. The electron plasma resonance created initially dephases into individually excited electrons in less than 150 fs. This is followed by a large component, fast exponential decay with a time constant of 2 ps, which is attributed to electronic energy relaxation through electron–phonon coupling. A slower 40 ps decay is also observed and attributed to subsequent cooling of the excited phonons due to phonon–solvent interaction. The decay dynamics, especially the 2 ps decay, are relatively insensitive to the solvent environment, indicating that the early time decay is dominated by the properties of the silver particles. The solvent molecules play an important role in the phonon cooling process following the electronic energy relaxation. The results show that the majority of the electrons created through photoexcitation of the plasmon band decay in less than 50 ps.
Multiconfiguration self-consistent-field treatment of H2 desorption from Si(100)-2×1H
Authors: Jing Ze , Whitten Jerry L.
Published: July 2002

The present work studies the symmetric and asymmetric pathways of the recombinative desorption of molecular hydrogen from the monohydride Si(100)-2×1H surface by the cluster method. The transition states are searched at the self-consistent-field and multiconfiguration self-consistent-field levels of theory and the energies are calculated at the configuration interaction level of theory. For the asymmetric transition state, the structure determined at the self-consistent-field level is similar to that determined at the multiconfiguration self-consistent-field level. However, for the symmetric transition state, electron correlation is found to play such a crucial role that multiconfiguration self-consistent-field theory has to be used to obtain the saddle point. At the configuration interaction level of theory, the activation energies for the symmetric and asymmetric pathways are computed to be 86.3 and 85.0 kcal/mol, respectively. Compared with the experimental values of 45-66 kcal/mol, the large barriers suggest that the direct desorption mechanism is not applicable. A multistep desorption mechanism which involves a delocalized process in the formation of dihydride SiH2 and a localized desorption of H2 has been proposed to explain the experimental observations.
Calculated properties of P2, P4, and of closed-shell clusters up to P18
Authors: Häser Marco , Treutler Oliver
Published: July 2002

The thermodynamic stabilities of the most stable even-membered and electrically neutral molecules (clusters) of phosphorus P2 through P18 are calculated in high accuracy. The larger clusters Pn, n≳4, turn out to be unstable at elevated temperatures, but are thermodynamically stable with respect to decay into P4 at low temperatures. To aid an experimental identification of such clusters, calculated vibrational frequencies, ionization potentials, and electronic excitation energies are supplied. As a side result we find the equilibrium bond length in P4 to be substantially shorter (below 220 pm) than the established experimental value r0=222.28±0.05 pm.
Study of I−(CO2)n, Br−(CO2)n, and I−(N2O)n clusters by anion photoelectron spectroscopy
Authors: Arnold Don W. , Bradforth Stephen E. , Kim Eun H. , Neumark Daniel M.
Published: July 2002

Photoelectron spectra of the I−(CO2)n=1–13, I−(N2O)n=1–12, and Br−(CO2)n=1–11 clusters are presented. The spectra provide information about the stepwise solvation of the bromide and iodide anions and about the size of the first solvation shells in these clusters. The data suggest that significantly different solute–solvent interactions exist in the three sets of clusters studied here. The X−(CO2)n spectra exhibit resolved progressions which are assigned to in-phase CO2 solvent bending vibrations in the neutral clusters. These vibrations are excited by photodetachment of anion clusters in which the CO2 molecules are distorted from linearity by a charge-quadrupole interaction. The I−(N2O)n spectra do not show any vibrational structure, presumably because the weaker ion–solvent interactions are insufficient to distort the N2O molecules.
Ab initio prediction of the structure, harmonic vibrational frequencies, and dissociation energy of the H2–GeH+3–H2 cluster ion
Authors: Archibong Edet F. , Schreiner Peter F. , Leszczyński Jerzy , Schleyer Paul von Rague , Schaefer Henry F. , Sullivan Richard
Published: July 2002

Ab initio predictions of the molecular geometry, harmonic vibrational frequencies, and dissociation energies are reported for the germanium hydride cluster ion GeH+7. Seven stationary points were located on the potential energy surface (PES) of GeH+7 using the self-consistent field (SCF), configuration interaction including single and double excitations (CISD), and coupled cluster including single, double, and perturbatively included triple excitations [CCSD(T)] methods in conjunction with a double-ζ plus polarization (DZP) and a triple-ζ plus polarization TZ(3d1f,1p) quality basis set. The most stable structure has a C2 symmetry with the two H2 subunits rotating freely about the symmetry axis of the GeH+3 fragment. Our best estimate of the dissociation energy for GeH+7, taking into account the zero point vibrational energy (ZPVE) is 3.10 kcal/mol compared to 4.6 and 1.2 kcal/mol obtained, respectively, for the SiH+7 and CH+7 cluster ions.
How many configurations are needed in a time-dependent Hartree treatment of the photodissociation of ICN?
Authors: Liu Li , Fang Jian-Yun , Guo Hua
Published: July 2002

In this work, we explore the applicability of a multiconfiguration version of the time-dependent Hartree approximation in photodissociation dynamics. A two-dimensional model system based on the A-band photodissociation of ICN is used. The propagation of the wave packet, which is expanded on grids, is carried out using pseudospectral methods and a predictor–corrector integrator. It is found that the quantum state resolved quantities, such as the final CN rotational distribution, require many more configurations than averaged quantities like the absorption spectrum. The calculated results from this work agree well with previous quantum calculations.
Dynamical solvent effect in electron transfer reactions occurring in a mixture of two polar solvents
Authors: Zusman Leonid
Published: July 2002

The theory of the dynamical solvent effect for electron transfer reactions occurring in a mixture of two polar solvents is developed. A simple formula is obtained which connects the rate of electron transfer with the rotational and translational diffusion coefficients of the polar components of the mixture and with the binary correlation functions of the mixture.
Vibrational mode and collision energy effects on a highly constrained reaction: OCS+(ν)+OCS→CS+2+CO2 and S+2+2 CO
Authors: Chiu Yu-hui , Yang Baorui , Fu Hungshin , Anderson Scott L.
Published: July 2002

We report the effects of collision energy and OCS+ vibrational state (ν1, ν2, and ν3) on the reaction of OCS+ with OCS. Production of CS+2+CO2 is exoergic and the cross section shows no evidence of an activation barrier. Nonetheless, the cross section is only ∼0.1% of the collision cross section, even at low collision energies where formation of an intermediate complex is facile. There appears to be a severe phase-space (steric) bottleneck for this rearrangement reaction. CS+2 production is weakly inhibited by collision energy, and enhanced by all three modes of OCS+ reactant vibrational excitation. Production of S+2 is endoergic and is enhanced by collision energy and by ν2 (bend) and ν3 (CS stretch) excitation. Excitation of ν1 (CO stretch) does not enhance this channel, even though it is the highest energy mode. At high collision energies, S+2 production becomes relatively efficient, suggesting that the reaction mechanism for this channel is direct with no significant bottleneck.
Barriers for hydrogen atom diffusion on the Si(100)-2×1 surface
Authors: Nachtigall P. , Jordan K. D.
Published: July 2002

Density functional theory is used in conjunction with cluster models to calculate activation energies for diffusion of H atoms on the Si(100)-2×1 surface. The activation energy for diffusion along the surface dimer rows is calculated to be 52 kcal/mol, and that for diffusion perpendicular to the dimer rows is even higher, leading us to conclude that H-atom diffusion on the Si(100)-2×1 surface probably does not play an important role in the H2 desorption process. The activation energies obtained in the present study are appreciably higher than those obtained from prior calculations using density functional theory. A major factor causing these differences is our use of the Becke3-LYP functional rather than the Becke–Perdew functional, used previously.
Orientational processes in liquid nitromethane studied by depolarized light scattering and transient optical Kerr effect
Authors: Giorgini M. G. , Foggi P. , Cataliotti R. S. , Distefano M. R. , Morresi A. , Mariani L.
Published: July 2002

The dynamics of nitromethane in the liquid phase is investigated by steady-state (Raman and Rayleigh) and time-resolved (optical Kerr effect) spectroscopic experiments performed at variable temperature. Both experiments show that the entire relaxation process is completed in a few picoseconds and that the dynamics can be described by assuming a biexponential function for the molecular response. The prominent dynamical feature at longer times appears to be that of a diffusionally reorienting symmetric top. The time dependence for orientational correlation at the shorter times observed in the coherent optical processes (Rayleigh and OKE) suggests that perturbative phenomena in the subpicosecond time scale (collisional and cage effects) are operative prior to the onset of the diffusional regime. Reorientational times from steady-state coherent and incoherent light scattering experiments enable us to exclude that pair-particle orientational correlation is effective. The same orientational activation energy was estimated from all the experiments. The reorientation times closely follow the η/T (η shear viscosity) linear dependence, conforming to the predictions of slip rather than stick Stokes–Einstein–Debye hydrodynamic theory.
Deuterium isotope effect on the solvation dynamics
Authors: Pal Haridas , Nagasawa Yutaka , Tominaga Keisuke , Kumazaki Shigeichi , Yoshihara Keitaro
Published: July 2002

Deuterium isotope effect on the solvation dynamics is observed in the system of aniline (AN) as a solvent for the first time by the dynamic Stokes shift method. Perdeuterated AN (AN–d7) or amino deuterated AN (AN–d2) shows slower solvation dynamics than normal AN. Deuterium effect on the solvation of N,N–dimethylaniline (DMA) is also studied and there is no isotope effect on the solvation process. The differences between AN and DMA are proposed to be related to the presence and absence of the intermolecular hydrogen bondings.
A simple technique to estimate partition functions and equilibrium constants from Monte Carlo simulations
Authors: Vieth Michal , Kolinski Andrzej , Skolnick Jeffrey
Published: July 2002

A combined Monte Carlo (MC) simulation-statistical mechanical treatment is proposed to calculate the internal partition function and equilibrium constant. The method has been applied to a number of one and multidimensional analytical functions. When sampling is incomplete, various factorization approximations for estimating the partition function are discussed. The resulting errors are smaller when the ratios of the partition functions are calculated (as in the determination of equilibrium constants) as opposed to the partition function itself.
Fused hard-sphere chain molecules: Comparison between Monte Carlo simulation for the bulk pressure and generalized Flory theories
Authors: Costa Lorenza A. , Zhou Yaoqi , Hall Carol K. , Carrà Sergio
Published: July 2002

We report Monte Carlo simulation results for the bulk pressure of fused-hard-sphere (FHS) chain fluids with bond-length-to-bead-diameter ratios ≊ 0.4 at chain lengths n=4, 8 and 16. We also report density profiles for FHS chain fluids at a hard wall. The results for the compressibility factor are compared to results from extensions of the Generalized Flory (GF) and Generalized Flory Dimer (GFD) theories proposed by Yethiraj et al. and by us. Our new GF theory, GF-AB, significantly improves the prediction of the bulk pressure of fused-hard-sphere chains over the GFD theories proposed by Yethiraj et al. and by us although the GFD theories give slightly better low-density results. The GFD-A theory, the GFD-B theory and the new theories (GF-AB, GFD-AB, and GFD-AC) satisfy the exact zero-bonding-length limit. All theories considered recover the GF or GFD theories at the tangent hard-sphere chain limit.
Phase transition and entropy of amorphous ices
Authors: Johari G. P.
Published: July 2002

Mishima’s recent data [J. Chem. Phys. 100, 5910 (1994)], showing that the isothermal interconversion of low- and high-density forms of pressure amorphized ice is of first order thermodynamically, have been analyzed in terms of the configurational entropy. At the equilibrium pressure of 2 kbar and 135 K, the entropy of transformation is zero or negligible so that the configurational entropy of the two forms is about the same. This distinguishes the low-density form (whose entropy is about 3J/mol/K higher than that of hexagonal ice) from the hyperquenched glassy water. Comparison of calorimetric and relaxation behaviors of the pressure-amorphized low density and hyperquenched glassy water also indicates that their structures differ. Reasons for this difference are given and the low magnitude of the residual entropy of glassy water is discussed. It is argued that model-based calculations of residual entropy do not yield a plausible value for liquids.
Potential energy surfaces for Pt3+H2 and Pd3+H2 systems
Authors: Dai Dingguo , Liao D. W. , Balasubramanian K.
Published: July 2002

Potential energy surfaces of Pt3+H2 and Pd3+H2 systems are computed for different modes of approach using the complete active space multiconfiguration self-consistent field theory (CAS-MCSCF). The equilibrium geometries and the dissociation limits were computed using multireference singles+doubles configuration method which included up to 1.54 million configurations. For both in-plane and out-of-plane approaches barriers arise from the crossing of the Pt3+H+H (or Pd3+H+H) surface and the Pt3+H2 (or Pd3+H2) surface. Three potential minima were found in the in-plane potential energy surface in the post-barrier region. One of these minima exhibited two Pt–H–Pt (or Pd–H–Pd) bridge bonds. For Pd3H2 this structure was found to be stable while for Pt3H2 two nearly degenerate structures were found as candidates for the ground state. The barrier for H2 dissociation in the out-of-plane approach was found to be larger and the out-of-plane minimum was higher in energy compared to the in-plane minimum for both Pt3H2 and Pd3H2. The barrier for H2 dissociation was found to be considerably smaller for Pt3 compared to Pd3 and thus Pt3 is more reactive towards H2 compared to Pd3, a finding consistent with experiment. The dissociation energy of Pd3H2 was found to be larger than Pt3H2. These trends, the nature of bonding, Mulliken populations, and avoided crossings in the potential energy surfaces are discussed.