bluuebunny/crossref_metadata_2025 huggingface dataset:
The photodissociation of ClNO through excitation in the T1 state: An ab initio study
Authors: Sölter Dirk , Werner Hans-Joachim , Dirke Michael von , Untch Agathe , Vegiri Aliki , Schinke Reinhard
Published: July 2002

An ab initio study of the photodissociation of ClNO following excitation of the T1 state is presented. The corresponding three-dimensional potential-energy surface has been calculated using extensive complete-active-space self-consistent-field (CASSCF) and multireference-averaged coupled pair functional (MR-ACPF) wave functions and large basis sets. The nuclear dynamics has been treated by solving the time-dependent Schrödinger equation in three dimensions. The steepness of the potential with respect to the dissociation coordinate in the Franck–Condon region is found to be very crucial for a realistic description of the dissociation dynamics. It controls directly the lifetime of the ClNO(T1) complex and therefore the widths of the vibrational structures in the absorption spectrum. Due to very large dynamical electron correlation effects, the CASSCF potential is found to be much too steep. Even with extended MR-ACPF wave functions an empirical scaling of the correlation energy is necessary in order to obtain a potential which is sufficiently flat in the transition region to allow for the splitting of each vibrational band into three bending peaks. Only the absorption spectrum calculated with the scaled MR-ACPF potential is in very good agreement with the measured spectrum. The dissociation proceeds adiabatically as far as the vibrational degree of freedom of NO is concerned with the result that excitation of ClNO(T1) within vibrational band n* yields almost exclusively products NO(n*). The degree of rotational excitation of the NO fragment is relatively low, i.e., the distributions peak at low rotational quantum numbers. The calculations reproduce the intriguing relation between the bending state in the ClNO(T1) complex (k*) and the multimodal structures in the final rotational-state distributions, first observed experimentally by Reisler and co-workers [J. Chem. Phys. 89, 6547 (1988)].
The impact of vibrationally excited levels of the transition state on CO (v=0,J ) distributions resulting from dissociation of H2CO
Authors: van Zee Roger D. , Pibel Charles D. , Butenhoff Thomas J. , Moore C. Bradley
Published: July 2002

The dissociation of formaldehyde into molecular products is known to proceed through vibrationally excited, quantum ergodic levels of the ground electronic state (H2CO*) and then a transition state (H2CO‡) according to the following mechanism: H2CO(S0)+hν→H2CO(S1)→H2CO*(S0) →H2CO‡→H2+CO. This paper reports the distribution of rotational energy in the CO (v=0) fragment following excitation to single rotational levels of the S1 2141 band of formaldehyde. The maxima of the rotational distributions resulting from photolysis on different rotational lines are observed to vary between J=38 and 49. Variations in the widths of the distributions are also observed. These variations show no systematic trends in the quantum indices or radiative lifetime of the parent molecule. Distributions calculated using an infinite-order-sudden approximation-based algorithm show fluctuations similar to those observed experimentally when each of the vibrational levels of the transition state through which the molecule can decay is randomly weighted. Thus it is understood that the ergodic character of each H2CO* S0 eigenstate uniquely projects onto the vibrational levels of the transition state, and since different levels of the transition state give rise to differing product distributions, decay through different S0 levels produces differing rotational distributions.
Intersystem crossing and intramultiplet mixing of excited Zn atoms by Xe
Authors: Umemoto Hironobu , Ohnuma Toshiharu , Ikeda Hiroyuki , Tsunashima Shigeru , Kuwahara Kazuya , Misaizu Fuminori , Fuke Kiyokazu
Published: July 2002

The intersystem crossing of Zn(4 1P1) and the intramultiplet mixing of Zn(4 3PJ) by Xe were examined by using pulsed laser techniques. The following thermally averaged cross sections were obtained: Zn(4 1P1)+Xe→Zn(4 3P2)+Xe: 3.4, Zn(4 1P1)+Xe→Zn(4 3P1)+Xe: 0.4, Zn(4 1P1)+Xe→Zn(4 3P0)+Xe: <0.01, Zn(4 3P1)+Xe→Zn(4 3P2)+Xe: 12.0×10−3, Zn(4 3P1)+Xe→Zn(4 3P0)+Xe: 6.3×10−3, in units of 10−16 cm2. These experimental results could well be reproduced by quantum close-coupling calculations by assuming suitable potential energy curves. The intersystem crossing by He was found to be inefficient.
Deexcitation cross sections of He (2 3S, 2 1S, and 2 1P) by molecules containing group-IV elements
Authors: Yoshida Hiroaki , Ukai Masatoshi , Kawamura Hideo , Kouchi Noriyuki , Hatano Yoshihiko
Published: July 2002

Deexcitation cross sections of He(2 3S), He(2 1S), and He(2 1P) by C2H6, Si2H6, CF4, SiF4, and SiCl4 have been measured at a mean collisional energy corresponding to room temperature (295 K) and compared systematically with those by CH4, SiH4, and GeH4 (previously reported) to understand general features of the cross section values dependent on both the electronic states of excited helium atoms and the target molecules containing group-IV elements and to substantiate further the deexcitation mechanism. In the de-excitation of the metastable atoms, He(2 3S) and He(2 1S), the magnitude of the cross sections has mainly been ascribed to a short-range electron exchange interaction. The obtained cross section values for He(2 3S) are compared with a semiempirical formula and it is concluded that the number of the equivalent electrons which can be ionized with the excitation energy of He(2 3S) and the lowest ionization potential of the target molecule are of great importance in determination of the cross section values. Systematic comparisons of the deexcitation cross section values of the resonant atom, He(2 1P), with theoretical ones have shown an importance of not only deexcitation transition mainly ascribed to a long-range dipole–dipole interaction between He(2 1P) and a target molecule but also a bent trajectory caused by an attractive interaction potential between them to determine the cross section values.
Constant-ionic-state spectroscopy of high-v Rydberg states of molecular hydrogen
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

The (X 2Σ+g)np Rydberg states converging to the X 2Σ+g, v+≥9 ionization thresholds of molecular hydrogen were excited by double-resonance excitation via the E,F 1Σ+g, v = 6 level. The energy region of interest (139 500–140 000 cm−1) included the X 2Σ+g, v+=9, N+=0–3 ionization thresholds as well as the energetic threshold for ion pair formation. The decay of the autoionizing Rydberg states was studied by both conventional and constant-ionic-state photoelectron spectrometry. The results support the Δv=minimum propensity rule for vibrational autoionization of Rydberg states with a high level of vibrational excitation. However, the relative intensities of the (X 2Σ+g)np, v′≥ 9 Rydberg series converging to different rotational levels of the ion are dramatically different from those for the analogous Rydberg series with v′≤2. In addition, the two-color photoelectron spectrum via the E,F 1Σg+, v=6 level shows considerable intensity in the v+=7 and 8 photoelectron bands both on and off resonance, in marked contrast with the single-color photoelectron spectrum obtained for the same intermediate levels. These observations may be due to the dependence of the transition dipole matrix elements on the internuclear distance and to the presence of doubly excited electronic states that cross the H2+ X 2Σ+g potential curve at a total energy close to that accessed by the two-color excitation scheme.
Direct indication of the liquid–gas region in Lennard-Jones systems by conventional molecular dynamics calculations
Authors: Hoheisel C.
Published: July 2002

We demonstrate the occurrence of coexisting gas and liquid phases in the Lennard-Jones (LJ) system by direct evidence using conventional molecular dynamics calculations (MDC) for the NVEp ensemble. The computations require only evaluation of nearest neighbor distributions in the N-particle system. The results are practically independent of the potential cutoff and the number of particles used for the MDC. While the coexistence region of the LJ system can be detected reliably for densities outside the neighborhood of the critical point, reliable determination of the critical point poses problems. However, this disadvantage cannot be avoided by any other method using model calculations with a finite number of particles.
Surface tension of dilute polymer solutions. I. A renormalization group approach
Authors: Myers Kurt R. , Nemirovsky A. M. , Freed Karl F.
Published: July 2002

The surface tension increment is evaluated for dilute polymer solutions. The first virial coefficient is calculated to first order in excluded volume near two limiting boundary conditions: repulsive (Dirichlet) and reflecting (von Neumann). An interpolation function extends the calculations to intermediate values of the polymer–surface interaction strength and provides the surface pressure as a function of both polymer–polymer and polymer–surface interactions. Comparison with experiments for polystyrene in toluene suggest the importance of nonuniversal contributions to the surface tension increment.
Electronic structure of Cu6, Ag6, Au6, and their positive ions
Authors: Liao Dai-Wei , Balasubramanian K.
Published: July 2002

The ground state energies of the group IB hexamer (Cu6, Ag6, and Au6) for four possible structural models, namely, 1A1 with a capped pentagonal structure (C5v), 1A1 with a tripyramidal structure (C2v), 1A1g with a circular ring structure (D6h), and 3T2g with an octahedral structure, as well as the positive ions, Cu+6, Ag+6, and Au+6 were computed using complete active space multiconfiguration self-consistent field followed by first, second order configuration interaction and multireference singles+doubles configuration interaction including d correlation methods. We considered excited states of Cu+6 also. The results indicated that the capped pentagonal structure is the most favorable structure in energy among the four structures for all three hexamers. The computed ionization potentials (IPs) of Cu6, Ag6, and Au6 follow the order IP(Au6)>IP(Cu6)>IP(Ag6).
Simulation of the band structure of liquids: A correction and some further developments
Authors: Ganguly Kishor , Stratt Richard M.
Published: July 2002

A recent paper of ours compared an analytical, liquid-theory-based set of predictions for electronic band structure in liquids with the results of explicit computer simulation. In the interim, we discovered both an error in one of the figures from this paper and some new ways of using solid-state physics perspectives to remove finite-size simulation artifacts. The results that one obtains after correcting the error, and after applying the new lessons, demonstrate that the liquid-theory predictions are even more accurate than our previous work had indicated, especially for p bands.
Density dependence of excess electronic ground-state energies in simple atomic fluids
Authors: Space B. , Coker D. F. , Liu Z. H. , Berne B. J. , Martyna G.
Published: July 2002

The ground-state energies of an excess electron E0 as a function of solvent density are computed using model electron–atom pseudopotentials in fluid helium, argon, and xenon. E0 is a lower bound to the experimentally measurable threshold to photoconductivity, V0. The nonuniqueness of the pseudopotential description of electron–molecule interactions is demonstrated. We find that when many-body polarization effects are included, our calculated E0 results are in close agreement with experimental V0 values indicating that the conduction-band energy lies close to the ground-state energy across a broad range of densities in these polarizable fluids. If the many-body nature of the polarization interaction is ignored the ground-state energies deviate significantly from the V0 results highlighting the importance of accurate treatment of many-body polarization interactions. It is shown that a mean-field theory of polarization gives substantial agreement with full many-body calculations. This allows us to introduce a mean-field, density-dependent pair potential which greatly simplifies such many-body calculations. In the more polarizable systems, it is found that the spatial extent of the ground-state wave function as a function of solvent density is correlated with the density dependence of both V0 and the electron mobility, and it becomes uniformly spread throughout our simulation cell as the electron mobility goes through its maximum value at intermediate solvent densities.
A comparison of classical and quantum analyses of electron localization sites in liquid water
Authors: Motakabbir Kazi A. , Schnitker Jurgen , Rossky Peter J.
Published: July 2002

The results of an earlier study [Schnitker, Rossky, and Kenney-Wallace, J. Chem. Phys. 85, 2986 (1986)] in which likely sites for electron localization in pure liquid water were identified and characterized via a physically motivated purely classical analysis are statistically compared to a corresponding fully quantum mechanical treatment of the excess electronic ground state. It is shown that the most energetically favorable localization sites identified by the classical treatment correspond reasonably to the quantum mechanical result both energetically and spatially. It is found that the existence and location of a physically localized ground state can be determined from the classical results if both the minimum of the estimated absolute total electronic energy and the difference between this minimum and the alternative local minima identified within a solvent configuration are considered. Further, the results confirm that the concentration of such effective sites is relatively high in the liquid (∼0.01 M). Hence, the classical approach has merit as a qualitative tool for the analysis of the electronic states supported by the preexisting configurational order in a liquid.
Pulse radiolysis studies of ion–electron recombination in helium. Pressure and temperature effects
Authors: van Sonsbeek Roger J. , Cooper Ronald , Bhave R. N.
Published: July 2002

Using pulse radiolysis techniques, rates of recombination of electrons with He+2 ions in the presence of helium over a wide pressure range (40–900 Torr) and at temperatures of 200, 235, 275, and 295 K were measured. Two- and three-body recombination processes were resolved, and a temperature dependence for the three-body recombination rate constant of T(−2.9±1.2)gas observed. This result agrees well with theoretical predictions of a temperature exponent of −2.5 by Bates and Khare and Pitaevskii, and remarkably well with recent work by Cao and Johnsen which gave a temperature exponent of −2.9 for the rate constant for three-body recombination of electrons with simple molecular ions in the presence of helium.
Photoionization mass spectrometric studies of the isomeric transient species CH2SH and CH3S
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

The products of the reaction of F atoms with CH3SH are studied by photoionization mass spectrometry. Two thresholds, at 7.536±0.003 eV and 9.262±0.005 eV, are observed at mass 47, and identified with the adiabatic ionization potentials of CH2SH and CH3S, respectively. These conclusions are confirmed by additional experiments with CD3SH, where an adiabatic ionization potential of 7.522±0.003 eV is obtained at mass 49 (CD2SH+) and a value of 9.268±0.005 eV is found at mass 50 (CD3S+). From an earlier appearance potential for CH2SH+ (CH3SH) and the present ionization potential of CH2SH, an upper limit for Dd0d (H–CH2SH ) of ≤93.97±0.13 kcal/mol is deduced. By giving weight to the measured proton affinity of CH2S, one obtains D0(H–CH2SH )=92.4±2.0 kcal/mol. The corresponding S–H bond energy in CH3SH derived from a recent kinetics-based value for ΔH0f0(CH3S) is 86.1±0.6 kcal/mol. A discrepancy in the difference of ΔH0f0(CH3S+) and ΔH0f0(CH2SH+) arising from recent theoretical and experimental determinations is resolved in favor of the ab initio values.
Deexcitation electron spectroscopy of core-excited NO
Authors: Carroll T. X. , Thomas T. D.
Published: July 2002

The electron–electron coincidence method has been used to measure the deexcitation electron spectra of core-excited nitric oxide. Comparison of the deexcitation spectra with Auger spectra reveals the importance of participator and spectator decay in each case. Term splittings in the core-excited molecule and selection rules for Auger decay play important roles in simplifying the deexcitation spectra. Analysis of the spectra reveals that the core hole valence interaction is larger for nitrogen than for oxygen. As a result, the level orderings in the two core-excited molecules, N*O and NO*, are different.
Angular momentum dependent classical chaos in HD2+
Authors: Fahrer Norman , Schlier Christoph
Published: July 2002

We determine Ec(J), the angular momentum dependent energy of onset of classical chaos in HD2+, from the divergence of pairs of trajectories started from microcanonical initial conditions. Ec(J) drops from 0.66 eV at J=0 to 0.03 eV at about J=30, and has a secondary maximum at J=50.
Interfacial tensions of phase-separated polymer solutions
Authors: Xia K.-Q. , Franck Carl , Widom B.
Published: July 2002

Measurements of the capillary length in phase-separated solutions of polymethylmethacrylate in 3-octanone (PMMA/3-OCT) for each of five different molecular weights of PMMA, and in phase-separated solutions of polystyrene in methylcyclohexane (PS/MCH) for a single molecular weight of polymer, are reported. Measurements on PMMA/3-OCT were by the sessile-drop method; those on PS/MCH were by both the sessile-drop and capillary-rise methods. Interfacial tensions are estimated from estimated density differences and the measured capillary lengths. A theoretically predicted scaling law for the dependence of capillary length on polymer molecular weight and temperature is tested and at least semiquantitatively verified. As by-products of the sample preparation, the molecular-weight dependences of the critical composition and critical solution temperature of PMMA/3-OCT were also determined. The volume fraction of polymer at the critical point, φc, is found to be proportional to the −0.37±0.01 power of the molecular weight, in agreement with values of this exponent found by others for polystyrene solutions.
Interconversion of diborane (4) isomers
Authors: Stanton John F. , Gauss Jürgen , Bartlett Rodney J. , Helgaker Trygve , Jo/rgensen Poul , Jensen Hans Jorgen Aa. , Taylor Peter R.
Published: July 2002

Highly correlated electronic structure calculations using many-body perturbation theory (MBPT) and coupled-cluster (CC) gradient techniques are applied to a study of a reaction pathway which links the two forms [C2v and D2d ] of diborane(4) [B2H4]. A reaction coordinate which preserves C2 symmetry is studied, as this mechanism is allowed by orbital symmetry rules. However, calculations show that the minimum energy path does not conform to this idealized mechanism. Rather, the reaction coordinate bifurcates, and the transition state contains no nontrivial elements of symmetry. At the level of partial fourth-order many-body perturbation theory [SDQ-MBPT(4)] with a large triple-zeta plus double polarization basis set, differences in distances between the hydrogen atoms corresponding to the bridge atoms in the C2v form and the two boron atoms [δr(B–H)=‖r(B1–H)−r(B2–H)—] are 0.14 and 0.81 Å, reflecting the pronounced asymmetry of the transition state structure. We find that the C2v isomer should be the thermodynamically favored form of B2H4. At the coupled-cluster singles and doubles level with a noniterative treatment of triple excitation effects, the barrier to isomerization is found to be 6.3 kcal/mole using a large generally contracted basis set. This result, along with statistical arguments concerning the rate of reaction, suggest that equilibrium may be established relatively rapidly, and that rigid molecule treatments of B2H4 spectra may not be appropriate.
X-ray and neutron scattering studies of the structure of hyperquenched glassy water
Authors: Bellissent-Funel M. C. , Bosio L. , Hallbrucker A. , Mayer E. , Sridi-Dorbez R.
Published: July 2002

X-ray and neutron diffraction measurements were performed on glassy water prepared by rapid cooling of water droplets on a cryoplate. Structure factors and radial distribution functions were found to be nearly identical to those obtained from amorphous ice formed either by vapor deposition onto substrates cooled at 77 K or after heating the high-density amorphous ice.
Theoretical investigations of the aluminum/polythiophene interface
Authors: Boman M. , Stafström S. , Brédas J. L.
Published: July 2002

Interactions at the interface between aluminum and polythiophene are studied theoretically using ab initio Hartree–Fock and Mo/ller–Plesset perturbation theories as well as semiempirical techniques. Aluminum is found to interact strongly with the polythiophene chain. The fundamental interaction unit corresponds to an aluminum dimer (Al2) bound to a single thiophene ring. This type of interaction strongly modifies the geometrical structure of the polymer chain and induces localization of the π-electron system. The calculated charge transfer between aluminum and the thiophene system is found to be in good agreement with the charge transfer deduced from experimental core level spectra of the aluminum/polythiophene interface. A detailed analysis of the electronic structure, including the correlation effects, of a model system (Al2 interacting with α-trithienylene) is performed. Higher concentrations of aluminum atoms on a polythiophene chain results in the formation of additional Al2–thiophene complexes. Within the first monolayer, clustering of aluminum on polythiophene is shown to be energetically unfavorable.
Detection and characterization of gas-phase GaCl using resonance enhanced multiphoton ionization
Authors: Dearden David V. , Johnson Russell D. , Hudgens Jeffrey W.
Published: July 2002

We report resonance enhanced multiphoton ionization (REMPI) spectra of gas-phase gallium monochloride (GaCl) produced with laser light tuned between 330 and 430 nm. The spectra originate from one, two, and three photon resonances with electronic states that reside between 29 500 and 80 000 cm−1. We have assigned five Rydberg series comprised of 20 new electronic states. Each state exhibits a short vibrational progression. A least-squares fit of the unperturbed Rydberg state origins yields the adiabatic ionization potential, IPa(GaCl)=80 540±10 cm−1. We have also identified bands that originate from one-photon resonances with the previously known a 3Π state. We conservatively estimate the REMPI detection sensitivity for GaCl to be 109 molecules cm−3 (laser pulse)−1.
Photodissociation spectroscopy of Mg+–H2O and Mg+–D2O
Authors: Willey K. F. , Yeh C. S. , Robbins D. L. , Pilgrim J. S. , Duncan M. A.
Published: July 2002

Mg+–H2O ion–molecule complexes are produced in a pulsed supersonic nozzle cluster source. These complexes are mass selected and studied with laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer system. An electronic transition assigned as 2B2←X 2A1 is observed with an origin at 28 396 cm−1. The spectrum has a prominent progression in the metal-H2O stretching mode with a frequency (ω′e) of 518.0 cm−1. An extrapolation of this progression fixes the excited state dissociation energy (D′0) at 15 787 cm−1. The corresponding ground state value (D″0) is 8514 cm−1 (24.3 kcal/mol). The solvated bending mode, and symmetric and asymmetric stretching modes of water are also active in the complex, as are the magnesium bending modes. A second electronic transition assigned as 2B1←X 2A1 is observed with an origin at 30 267 cm−1 and a metal stretch frequency for Mg+–H2O of 488.5 cm−1 (ΔG1/2). Spectra of both excited states are also observed for Mg+–D2O. Partially resolved rotational structure is analyzed for both isotopes, leading to the conclusion that the complex has a structure with C2v symmetry. This study was guided by ab initio calculations by Bauschlicher and co-workers, which provide accurate predictions of the electronic transition energies, vibrational constants, and dissociation energies.
Electrostatic adsorption of colloidal particles on the walls of a planar slit: Simulation versus theory
Authors: González-Mozuelos P. , Alejandre J. , Medina-Noyola M.
Published: July 2002

A simple theoretical scheme is employed to calculate the equilibrium local concentration profile of a model suspension of highly charged colloidal particles inside a planar slit in the regime corresponding to attractive and weakly repulsive wall–particle interactions. In addition, we report the results of Monte Carlo simulations for the same system and conditions. As expected from previous theoretical work involving a single confining wall, we find that in this regime, theory and simulations predict the existence of an electrostatically adsorbed monolayer of colloidal particles on each of the two confining walls of the slit, and that this is independent of the wall-to-wall separation. Furthermore, we find that this monolayer ‘‘renormalizes’’ the charge of the corresponding wall, thus producing the same structure of the nonadsorbed particles, as that produced by highly repulsive walls. This idea is shown to be quantitatively accurate according to both, the simulation and the theoretical results.
Generalized Langevin dynamics simulation of activated processes in solution: Ion pair interconversion in water
Authors: Rey R. , Guàrdia E. , Padró J. A.
Published: July 2002

The association–dissociation processes for Na+–Cl−, Na+–Na+, and Cl−–Cl− ion pairs in water are studied by generalized Langevin dynamics (GLD) simulation. The required solvent averaged interionic potentials and time-dependent friction coefficients were obtained previously from molecular dynamics (MD) calculations explicitly including the solvent particles. The reliability of the method is checked by comparing the GLD results for the Na+–Cl− ion pair with those obtained from full deterministic MD. The reactive flux method has been employed to compute the transmission coefficients and to study dynamic properties of activated trajectories. The results have been compared with theoretical predictions. The influence of the friction kernel has been investigated considering different models for the time-dependent friction coefficients. The mean first passage time for the dissociation process has also been calculated from GLD simulations.
Multiconfigurational spin tensor electron propagator electron affinities for F, BO, CN, OH, and NH2
Authors: Yeager Danny L. , Nichols Jeffrey A. , Golab Joseph T.
Published: July 2002

We applied the multiconfigurational spin tensor electron propagator method (MCSTEP) to the systems F, OH, NH2, BO, and CN for the determination of vertical and adiabatic electron affinities (EAs). These are the first MCSTEP EA calculations for systems that are not pseudo two-electron systems and the first time MCSTEP is used for EAs of molecules. Using standard Dunning core-valence basis sets supplemented with diffuse functions and with relatively small complete active spaces, MCSTEP results are in very good to excellent agreement with experiment. Comparisons with EAs determined by other methods using exactly the same basis sets show that MCSTEP is generally more consistent and reliable.
Analytic ROHF–MBPT(2) second derivatives
Authors: Gauss Jürgen , Stanton John F. , Bartlett Rodney J.
Published: July 2002

The theory and a first implementation of analytic second derivatives for the second-order energy within many-body perturbation theory [MBPT(2)] based on a restricted open-shell Hartree–Fock (ROHF) reference function are presented. The applicability of the method is demonstrated by calculating harmonic frequency and infrared intensities for the 2B1 state of the CH2N molecule, which suffers from appreciable spin contamination.
Locally enhanced sampling in free energy calculations: Application of mean field approximation to accurate calculation of free energy differences
Authors: Verkhivker Gennady , Elber Ron , Nowak Wieslaw
Published: July 2002

Mean field approximation is employed for accurate calculation of free energy differences. Significantly enhanced sampling is obtained for local changes. In an example for the mutation of a residue in a protein, the increase in the sampling yielded converged results at a significantly lower computational cost than the usual approach.
Time-dependent Hartree approximation applied to the photodissociation of ICN
Authors: Lee Soo-Y.
Published: July 2002

A time-dependent, quantum-mechanical calculation of wave packet dynamics in the Hartree approximation is applied to the nonadiabatic transition between the linear and bent excited state surfaces in ICN. The model of ICN photodissociation in the Ã continuum given by Goldfield et al. is used. Both the probability and the energy of the propagated Hartree wave packets are conserved. Starting with the ground wave packet on the linear excited state surface, there is a final transfer of about 24% probability to the bent excited state surface. The probability transfer is almost complete in 12 fs. The autocorrelation function for the absorption spectrum is significant for less than 5 fs and there is good agreement between the absorption spectrum calculated by the Fourier transform of the autocorrelation function and by the projection onto asymptotic states after 100 fs. Such an agreement suggests that the time-dependent Hartree approximation is a valid and good approximation for the problem. The absorption spectrum is resolved into I(2P3/2) and I*(2P1/2) components and these do not agree with the experimental results of Pitts and Baronavski, which supports previous conclusions that the Goldfield et al. ICN potentials are inadequate. Calculations of the CN rotational distribution as a function of the photolysis wavelength for both surfaces are also presented, and the results are interpreted using the wave packet picture. There is only qualitative agreement with the semiclassical results of Goldfield et al. and the recent time-independent, quantum coupled-channel calculations of Guo et al.
Theory of thermal unimolecular reactions at low pressures. III. Superposition of weak and strong collisions
Authors: Troe J.
Published: July 2002

Biexponential models for collisional energy transfer are formulated representing superimposed weak and strong collisions. Analytical solutions of the master equation for low pressure thermal unimolecular reactions at steady state are derived for such models. Nonequilibrium populations of excited states as well as collision efficiencies βc in the low pressure rate constant differ considerably from the corresponding quantities for simple exponential collision models. The results are analyzed in terms of general expressions of the collision efficiency βc as a function of the average energy transfer quantities 〈ΔE〉 and 〈ΔE2〉.
Periodic perturbations of the oscillatory CO oxidation on Pt(110): Model calculations
Authors: Krischer K. , Eiswirth M. , Ertl G.
Published: July 2002

The periodically perturbed oscillations in the isothermal CO oxidation on a Pt(110) surface at low pressure were modeled using the recently developed reconstruction model of kinetic oscillations, in which the usual Langmuir–Hinshelwood mechanism is coupled with the CO-driven 1×2⇄1×1 phase transition of the surface. The experimental findings (entrainment bands, quasiperiodicity, critical slowing down) could be well reproduced with this model. The calculations revealed a complicated bifurcation fine structure of the Arnol’d tongues. While the skeleton structure turned out to be the same as in other studies, some additional features were found, and several details (mainly in the p/2 tongues) were different from forced systems examined previously. A co-dimension-3 bifurcation (in which a second-order resonance coincides with a degenerate period doubling) is discussed in this context.
Spectroscopy and relaxation dynamics of I2Arn clusters. Geminate recombination and cluster fragmentation
Authors: Fei Suli , Zheng Xiaonan , Heaven Michael C. , Tellinghuisen Joel
Published: July 2002

I2Arn clusters yield visible and near-UV emissions when excited to the states which correlate with the first ion-pair manifold of I2. These states may be accessed by 193 nm excitation of ground-state clusters, or near-UV excitation of electronically metastable I2(A′ or A)Arm. Comparisons of the cluster and I2/Ar matrix spectra suggest that such excitations result in fragmentation and ‘‘melting’’ of the clusters prior to emission. 532 nm photodissociation of I2 within the clusters is followed by geminate recombination which populates the A′, A, and X states. The probability for ejection of the recombined I2 from the clusters during the relaxation process appears to be size dependent. These results represent tentative steps towards the study of I(2P3/2)+I(2P3/2) recombination in Ar clusters. They also indicate that dissociation, recombination, and fragmentation processes may be used as a general method for generating metastable species in supersonic expansions.
Spinodal decomposition in a polymer mixture
Authors: Kuwahara N. , Sato H. , Kubota K.
Published: July 2002

Light scattering was used to study spinodal decomposition in a critical mixture of polystyrene and polymethylphenylsiloxane. Time dependence of the wave number of the maximum scattering intensity was expressed by power-law relations. Those exponent values are consistent with the values obtained for the metallic alloys.
Intramolecular dynamics. III. Theoretical studies of the CH overtone spectra for benzene
Authors: Zhang Yongfeng , Marcus R. A.
Published: July 2002

The CH overtone spectra for vCH=1, 2, and 3 for an intermediate sized molecule, benzene, and the related intramolecular vibrational dynamics are treated theoretically. For this purpose, an artificial intelligence (AI) search technique is employed, using the evaluation function developed in Part II. The curvilinear local-normal mode coordinate system discussed in Part I is also used. The main features of the theoretical spectra are in reasonable accord with those of the experimental ones. Results reflecting the important role of the symmetry of molecules even in their high energy states are described.
Comment on: Exact perturbation treatment of the basis set superposition correction
Authors: Mayer I.
Published: July 2002

It is shown that Sadlej’s recent proposition1 for an ‘‘exact’’ exclusion of the basis set superposition (BSS) error is erroneous, because it leads to meaningless results for the interaction of a large and small molecule. After a very brief summary of our recent method for solving the BSS problem, a reply is also given to Sadlej’s criticism1 about it.
Molecular-dynamics simulation of polymethylene chain confined in cylindrical potentials. I. Nature of the conformational defects
Authors: Yamamoto Takashi , Kimikawa Yuichi
Published: July 2002

The conformational motion of a polymethylene molecule constrained by a cylindrical potential is simulated up to 100 ps. The molecule consists of 60 CH2 groups and has variable bond lengths, bond angles, and dihedral angles. Our main concern here is the excitation and the dynamics of the conformational defects: kinks, jogs, etc. Under weaker constraint a number of gauche bonds are excited; they mostly form pairs such as gtḡ kinks or gtttḡ jogs. These conformational defects show no continuous drift in space. Instead they often annihilate and then recreate at different sites showing apparently random positional changes. The conformational defects produce characteristic strain fields around them. It seems that the conformational defects interact attractively through these strain fields. This is evidenced by remarkably correlated spatial distributions of the gauche bonds.
Experimental probes of dissociative states: Fano profiles in the state-specific photodissociation of FNO
Authors: Brandon J. T. , Reid S. A. , Robie D. C. , Reisler H.
Published: July 2002

We have observed strongly asymmetric features in the S1←S0 state-specific photofragment yield spectrum of jet-cooled FNO. We interpret these as Fano profiles, arising from interference between direct and indirect dissociation following excitation to a region near the top of a barrier on the S1 potential energy surface.
Spectroscopic study of Λ-doublet, spin, and rotational relaxation in the NH(A 3Π,v=0) state
Authors: Kaes A. , Stuhl F.
Published: July 2002

Electronically excited NH(A 3Π) radicals in single N′, J′, e/f states were investigated by pumping on isolated NH(A←X) lines of the (0,0) band. Collision-induced transitions among the different Λ-doublet, spin, and rotational states were monitored by fluorescence spectra. In collisions with NH3, a propensity for conservation of spin is observed. In the original spin unit, rotational relaxation occurs preferably to the neighboring rotational levels. The efficiency of spin-unit changes decrease with increasing ΔΩ. For NH3 collisions inducing a fine-structure change, the rotational distribution is found to be thermal and no memory of the original rotational level is left. In collisions with Ar, spin is not conserved. Generally, relaxation into states of the same Λ-doublet component occurs with approximately the same probability as into the other component. Relaxation processes induced by Ar are less efficient than those caused by collisions with NH3.
Spectra and relaxation paths of Hg(3P1) in rare gas matrices
Authors: Crépin C. , Tramer A.
Published: July 2002

Absorption, fluorescence-excitation and fluorescence spectra, and lifetimes have been measured for 3P1↔1S0 and 3P0→1S0 transitions in Hg atoms isolated in Ar, Kr, and Xe matrices. In all systems, the 3P1 state decays mainly by fluorescence emission; the 3P1■3P0 relaxation, inducing the long-lived 3P0→1S0 emission, being very inefficient. The 3P1→1S0 emission in Ar and Kr is a mirror image of the absorption, while a strong redshift in Xe corresponds to the formation of Hg*Xe exciplexes. The results are discussed in a model assuming the additivity of interatomic potentials deduced from the spectroscopic studies of jet-cooled Hg–rare-gas complexes.
Nonadiabatic tunneling in one-dimensional finite periodic potential system
Authors: Nakamura Hiroki
Published: July 2002

Nonadiabatic tunneling in an ideal one-dimensional finite periodic potential system is studied. Resonant transmission, complete reflection which does not exist in the case of ordinary adiabatic tunneling, and acceleration of transmission are analyzed. Some peculiar features resulting from the complete reflection are found and discussed. A system with an impurity is also investigated with an emphasis on the role of complete reflection. The analyses are illustrated by numerical examples.
A self-consistent-field integral equation theory for nonuniform polymer fluids
Authors: Woodward C. E.
Published: July 2002

An integral equation theory is developed for nonuniform polymer fluids. It is based on an exact formulation of the density functional theory for polymers, which is essentially identical in form to the popular self-consistent-field approximations. A nonuniform site–site Ornstein–Zernike equation is obtained. As well, the Triezenberg–Zwanzig–Wertheim–Lovett– Mou–Buff equations are generalized to polymer fluids. The Percus–Yevick and mean spherical approximations are suggested as plausible closures to these equations.
The 2 1A
g state of trans,trans-1,3,5,7-octatetraene in free jet expansions
Authors: Petek Hrvoje , Bell Andrew J. , Choi Young S. , Yoshihara Keitaro , Tounge Brett A. , Christensen Ronald L.
Published: July 2002

One- and two-photon fluorescence excitation and emission spectra of the S1↔S0 transition of trans,trans-1,3,5,7-octatetraene have been measured for the first time in free jet expansions. The one-photon excitation spectrum is the same, with the exception of significant differences in the intensities of a few lines, as the two-color, resonance-enhanced, two-photon ionization spectrum, previously assigned to the 2 1A′←1 1A′ transition of cis,trans-1,3,5,7-octatetraene. However, comparison of the one- and two-photon fluorescence excitation spectra shows clearly that the carrier of the spectrum has inversion symmetry, as expected for trans,trans-1,3,5,7-octatetraene. The one-photon spectrum is built on bu Herzberg–Teller promoting modes, which are origins of progressions in ag modes, while the two-photon spectrum is due to a single progression in ag modes starting from the 2 1Ag←1 1Ag electronic origin. The appearance of out-of-plane vibrations, possibly including torsions of the polyene framework, suggests large differences in force constants and perhaps in the geometries of the 2 1Ag and 1 1Ag potential surfaces. For 2 1Ag vibronic levels with energies ≤1000 cm−1, the fluorescence lifetimes vary between 170 and 450 ns due to the dependence of radiative and nonradiative decay rates on the vibronic state. An abrupt increase in the nonradiative decay rates at ∼2100 cm−1 excess energy is tentatively ascribed to trans→cis isomerization. This work demonstrates that the one- and two-photon cross sections of the 2 1Ag←1 1Ag transitions of all-trans linear polyenes are sufficiently large to allow the study of 2 1Ag states under isolated, unperturbed conditions.
Enhancement of sorption kinetics by an oscillatory electric field
Authors: Raudino Antonio , Larter Raima
Published: July 2002

We have developed a theoretical model to investigate the effect of an oscillatory electric field on the sorption kinetics of a charged reactant onto a reactive colloid particle. The model assumes a diffusion-controlled kinetics under the effect of a local electric field induced at the electrolyte solution–colloid particle interface. The analytical results predict a quadratic relationship between the sorption rate and the applied field intensity and a complex dependence of the kinetics on the field frequency which, in concentrated electrolyte solutions, may show a maximum at certain critical frequencies. In physical terms, these results mean that the oscillating field enhances the sorption rate, making the reaction faster than it is in the absence of a time-dependent field.
Surface modes on a polymer gel of finite thickness
Authors: Kumaran V.
Published: July 2002

The fluctuations in the height of the free surface of a polymer gel of finite thickness are analyzed in the limit where the period of elastic oscillations is small compared to the viscous relaxation time. In this limit, the dominant forces in the momentum conservation equation are the elastic and surface tension forces; the viscous forces enter as a subdominant correction. Zero stress boundary conditions are applied at the free surface, while two different types of boundary conditions are considered at the other surface— for ‘‘grafted’’ gels zero displacement conditions are applied, while for ‘‘adsorbed’’ gels the displacement normal to the surface is zero but the surface is permitted to move in the lateral direction. There are multiple frequencies of oscillation, all of which are consistent with the boundary conditions, and it is found that the frequency and the decay rates of the oscillations are lower for the adsorbed gels. The static structure factor is calculated from the energy storage due to the elastic strain, surface deformation, and kinetic energy of motion. The structure factor has a peak at a nonzero value of the wave number for a grafted gel, while the maximum occurs at zero wave number for an adsorbed gel. An increase in the surface tension reduces the magnitude of the peak, and shifts it to lower values of the wave number.
Scattering from mixtures of flexible and stiff polymers
Authors: Hammouda Boualem
Published: July 2002

The random phase approximation (RPA) is used to calculate the scattering function for multicomponent mixtures of flexible and stiff polymers. These calculations will be useful for the qualitative interpretation of small angle neutron scattering (SANS) data from binary mixtures of polymer liquid crystals and flexible polymers in the isotropic phase region. They can predict the spinodal line as well as the isotropic-to-nematic phase transition line. Depending on the volume fraction of the rigid polymer and on the relative values of the Flory–Huggins and Maier–Saupe interaction parameters, either the spinodal line or the isotropic-to-nematic phase transition is reached first.
Dipolar interaction assisted effects on second harmonic generation
Authors: Guan H. W. , Wang C. H.
Published: July 2002

Measurements of the second order nonlinear susceptibility by second harmonic generation (SHG) signal were made on the 2-methyl 4-nitroaniline/poly(methyl methacrylate) guest/host system as a function of the nonlinear optical (NLO) chromophore concentration and the poling field strength. The polar orientational order parameters (POP) L33 and L31 (defined in the text) were calculated from the SHG signals. The POPs were found to depend on the NLO chromophore concentration. The concentration dependence of POP is described in terms of Kirkwood’s g factor that describes the orientational pair correlation. The ratio of polar order parameters (L33/L31), measured in the weak poling field condition, is 3, independent of the NLO chromophore concentration; this result is in agreement with the result of a recent calculation. The usual Onsager local field factor, derived without considering orientational pair correlations, was shown to be inadequate. The static dielectric constant of the guest/host system is extracted from the order parameter, using the Kirkwood–Frölich theory of dielectrics.
Comment on molecular dynamics simulations of monolayers of fluorinated amphiphiles
Authors: Shin Seokmin , Collazo Nancy , Rice Stuart A.
Published: July 2002

We report the results of new molecular dynamics simulations of liquid-supported monolayers of perfluorinated and partially fluorinated amphiphiles such as F(CF2)11COOH and F(CF2)10CH2COOH. The new simulations include a representation of the superhelical structure of the perfluoroalkane portion of the amphiphile chain in the intramolecular potential energy; in addition, the calculation of the collective tilt angle of the monolayer is improved to include the effect of the azimuthal distribution of individual molecular tilt angles. The results of the simulations are in agreement with the available experimental data. In particular, the packing structure and the observed breakup of the homogeneous ordered monolayer into ordered islands with the same collective tilt of the molecules are correctly predicted as are the very small collective tilt angles. These new results remove the discrepancy between predicted and observed collective tilt angles reported in our previous papers [J. Chem. Phys. 96, 1352, 4735 (1992)].
Quantum adiabatic switching
Authors: Kohen Daniela , Tannor David J.
Published: July 2002

The quantum adiabatic theorem is explored as a potentially useful tool for obtaining highly excited eigenstates without requiring the calculation of all lower states. Starting in an eigenstate of some H0, the Hamiltonian is deformed adiabatically to the final H; the state of the system at the final time is an eigenstate of the final H which correlates with the eigenstates of H0. The method is free from the difficulties which are present in classical adiabatic switching, i.e., separatrix crossing (tunneling of both the coordinate space and dynamical type) presents no problem, isolated avoided crossings are accurately reproduced, and final states of the correct symmetry are obtained perforce by starting out with properly symmetrized states of the H0. The key issue concerning the possible utility of the technique is the ability to take large time steps in the propagation. The physical motivation for large time steps is that the spatial change in the wave function over a single period of motion is minor. The time steps possible using either the short iterative Lanczos (SIL) or the split operator (SPO) propagation methods are indeed larger than for a conventional propagation, but not sufficiently large as to make the method practical in the general case without further modifications.