bluuebunny/crossref_metadata_2025 huggingface dataset:
Diffusive dynamics on potential energy surfaces: Nonequilibrium CO binding to heme proteins
Authors: Agmon Noam , Rabinovich Savely
Published: July 2002

Theory and practice of reaction dynamics on two-dimensional potential energy surfaces is investigated. Nonequilibrium multidimensional barrier crossing, occurring when the initial density is located near the ridgeline separating reactants and products, is treated by solving the time-dependent Smoluchowski equation as a function of diffusion anisotropy. For a locally separable potential, and slow relaxation in the perpendicular mode, the problem reduces to a one-dimensional Smoluchowski equation with a sink term. It may be further approximated as a first-order reaction with a time-dependent rate coefficient. These approximations are compared with exact two-dimensional propagations on a potential surface representing CO binding to α heme. The intermediate-time power-law decay of the survival probability is analyzed with the aid of the above approximations. The power also shows some kind of critical behavior near the isotropic diffusion limit, where these approximations are no longer valid. For fast relaxation, a nonmonotonic survival probability is observed. The long time decay of the survival probability is governed by the equilibrium rate coefficient. We calculate its anisotropy dependence and compare it with two asymptotic expansions for the lowest eigenvalue of the Smoluchowski operator−for the one-dimensional sink-Smoluchowski operator and the fully two-dimensional operator. The difference between the fast relaxation limit of these one- and two-dimensional equations may provide a quantitative explanation for previous problems in extrapolating to high temperatures using the Agmon–Hopfield model. The implications of these results to heme protein dynamics are discussed and new experiments are proposed.
Three-dimensional doorway-state theory for analyses of absorption bands of many-oscillator systems
Authors: Torii Hajime , Tasumi Mitsuo
Published: July 2002

A three-dimensional doorway-state theory is worked out for the purpose of analyzing absorption bands of many-oscillator systems, such as the amide-I infrared (ir) bands of globular proteins. An appropriate basis set of the doorway-state subspace is defined, so that the complication in interpretation of computational results, which may arise from the interference among the transition dipoles of the basis states, is avoided. As an example of application of the formulation, calculations of the doorway states are given for the ir band of a model system consisting of 400 oscillators. It is shown that the doorway-state formalism provides band assignments based on the origin of ir intensity, even in the case where the molecule has an irregular structure or irregular force constants and the molecular-eigenstate picture is not appropriate to describe vibrational motions responsible for the ir intensity. Possibility of applying this formalism to problems in other energy regions (such as electronic absorption of molecular aggregates) is also pointed out.
Calculation of the hyperfine splitting constants for the ground and excited states of NH2 radical
Authors: Nakatsuji H. , Izawa M.
Published: July 2002

Hyperfine splitting constants of the ground 2B1 and excited 2A1 states of NH2 radical are calculated by the symmetry adapted cluster-configuration interaction method with the use of the basis set which satisfies the cusp condition at the position of the nucleus. The calculated values compare well with the experimental values when the vibrational effects are considered for the 2A1 state. The nature of the vibrational wave functions for the double well 2A1 potential is discussed in some detail.
Reactions of Ar+ with halocarbons and of I+ with CF3I
Authors: Morris Robert A. , Van Doren Jane M. , Viggiano A. A. , Paulson John F.
Published: July 2002

The gas phase reactions of Ar+ with the halocarbons CF3Cl, CF3Br, CF3I, CF4, C2F6, and C2F4 have been studied using a variable temperature-selected ion flow tube (VT-SIFT) instrument operated at 298 and 500 K. Rate constants and product branching percentages were measured at both temperatures. Ar+ reacts at the collisional rate with all of the above neutrals at both 298 and 500 K. The reactions with CF3X yield CF+3 and CF2X+ for all X (the reaction with CF4 produces only CF+3 ). For X=I, there is an additional channel leading to the ionic product I+. The reaction of Ar+ with C2F6 produces both CF+3 and C2F+5. The reaction of Ar+ with C2F4 forms a rich product spectrum consisting of the ions CF+, CF+2, CF+3, C2F+3, and C2F+4. The reaction product distributions are compared with results from ionization experiments such as photoion–photoelectron coincidence (PIPECO) and electron impact mass spectrometry, and in some cases excellent agreement is found. The reaction of I+ with CF3I, which is a secondary reaction in the Ar+/CF3I system, was investigated at 298 K in separate experiments. This reaction is rapid and forms four product ions: CF+3, CF2I+, CF3I+, and I+2. The results are compared with previously published information.
High-resolution stimulated Brillouin gain spectroscopy of liquid carbon disulfide: Dependence of the linewidths and peak gain on scattering angle
Authors: Grubbs W. Tandy , MacPhail Richard A.
Published: July 2002

We have used narrow linewidth continuous-wave lasers to obtain high-resolution stimulated Brillouin gain (SBG) spectra of liquid carbon disulfide at various scattering angles. For back-scattering angles, the linewidths of the SBG peaks reflect the natural lifetimes of the stimulated acoustic waves, whereas for forward-scattering angles the linewidths are dominated by laser jitter and by the uncertainty in the scattering wave vector that results from focusing the laser beams in the interaction region. The peak gain for the forward-scattering SBG spectra is an order of magnitude smaller than that observed at a back-scattering angle of 178°. The dependence of the peak gain on scattering angle can be accounted for quantitatively by considering the angle dependencies of the Brillouin frequency, linewidth, and laser beam crossing efficiency.
Ab initio calculation of the local geometry of Cu+:NaF and Cu+:NaCl
Authors: Luaña Víctor , Flórez Manuel
Published: July 2002

A new description of the nature and scope of the impurity centers Cu:NaF and Cu:NaCl emerges when the local equilibrium geometry and wave function are obtained from cluster-in-the-lattice calculations involving clusters from 7 to 33 ions. The numerical results reveal the importance of simulating the defects by means of clusters having, at least, a boundary shell of fixed ions whose wave functions can follow the geometrical changes of the cluster inner shells. Inward relaxations of 0.089 Å for Cu:NaF and 0.085 Å for Cu:NaCl are deduced from the best calculations, in agreement with recent measurements on the last system.
Nonperturbative wave packet dynamics of the photodissociation of H2+ in ultrashort laser pulses
Authors: Abou-Rachid Hakima , Nguyen-Dang T. Tung , Chaudhury Rajat K. , He Xin
Published: July 2002

The wave packet dynamics of the photodissociation of H2+ under excitation by laser pulses of short durations at 329.7 nm are studied. The photodissociation process involves essentially two coupled channels, and the detailed mechanism for the formation of fragment kinetic energy spectra is examined by following the evolution of structures in the coupled-channel wave functions in momentum space. These structures appear in the channels’ momentum wave functions at P≠0, as the v=0 ground vibrational state is promoted to the dissociative channel then accelerated. The variations of these structures reflect the interplay between local laser-induced transitions and the accelerating–decelerating action of intrinsic molecular forces. The wave packet dynamics are studied for rectangular and Gaussian pulses of varying durations and peak intensities. In addition, two forms of channel couplings were considered corresponding to two different choices of the gauge: the electric-field (EF) gauge, in which the matter–field interaction is of the length form and the radiation-field (RF) gauge, in which it is of the velocity form.
Infrared spectroscopy of CO2–D(H)Br: Molecular structure and its reliability
Authors: Zeng Y. P. , Sharpe S. W. , Shin S. K. , Wittig C. , Beaudet R. A.
Published: July 2002

A high resolution rovibrational absorption spectrum of the weakly bonded CO2–DBr complex has been recorded in the 2350 cm−1 region by exciting the CO2 asymmetric stretch vibration with a tunable diode laser. The CO2–DBr band origin associated with this mode is 2348.2710 cm−1, red-shifted by 0.87 cm−1 from uncomplexed CO2. The position of the hydrogen atom is determined from differences in moments-of-inertia between CO2–DBr and CO2–HBr, i.e., by using the Kraitchman method. From this, we conclude that ground state CO2–H(D)Br has an average geometry that is planar and inertially T-shaped, with essentially parallel HBr and CO2 axes. Average values of intermolecular parameters are: Rcm=3.58 Å, θBrCO=79.8°, and θHBrC=93.1°. The validity of using the Kraitchman method, which was designed for use with rigid molecules, with a floppy complex like CO2–HBr is discussed. The experimental structure is corroborated qualitatively by results from Mo/ller–Plesset second-order perturbation calculations, corrected for basis set superposition errors. The theoretical equilibrium geometry for the inertially T-shaped complex is planar with structural parameters: RCBr=3.62 Å, θBrCO=89°, and θHBrC=86°. A number of cuts on the four dimensional intermolecular potential surface confirm large zero-point amplitudes, which are known to be characteristic of such systems, and these cuts are used to estimate tunneling splittings. Tunneling is shown to occur by out-of-plane rotation of the H atom, in accord with the experimental observations of Rice et al. There is no significant in-plane tunneling. A quasilinear hingelike isomer (OCO–HBr) with ROH=2.35 Å at equilibrium is calculated to be as stable as the T-shaped complex; however, this species has yet to be observed experimentally. Photoinitiated reactions in CO2–HX complexes are discussed.
On the coupled-electron-pair approximation based on a multireference state
Authors: Fulde Peter , Stoll Hermann
Published: July 2002

Starting from a multiconfiguration self-consistent-field ground state, correlations are included by means of a coupled-electron-pair approximation (CEPA). Different sets of equations are derived with the help of a projection method, which correspond to CEPA-0, CEPA-1, and CEPA-2 schemes, respectively.
NH3 physisorption on MgO(100) substrate. Potential calculations revisited
Authors: Picaud S. , Lakhlifi A. , Girardet C.
Published: July 2002

On the basis of new experimental results on the adsorption–desorption–diffusion mechanisms of NH3 on the MgO(100) surface, the adequacy of interaction potentials based on the physisorption hypothesis is discussed. It is shown that the calculated adsorption energy is consistent with the experimental one when the effective charge of the Mg and O substrate atoms is equal to 1; this latter value is very close to that determined elsewhere using a self-consistent tight binding method. The calculations interpret the low value for the saturation coverage in terms of repulsive lateral interaction between ammonia molecules. The influence of charge transfer between NH3 and Mg on the behavior of the adsorption energy with the NH3 coverage is also quantitatively estimated.
Metastable, ordered solids of rod-like molecules quenched from their liquids at far-from-equilibrium conditions: Phonon spectra of 4-n-octyl- and 4-n-dodecyl-4′-cyanobiphenyl
Authors: Rothschild Walter G. , Perrot Michel , De Zen Jean-Marc
Published: July 2002

We report low-frequency Raman spectra of the title compounds in their metastable solid state quenched rapidly from the high-temperature liquid. The phonon-like spectra indicate crystal-like order superposed on a vibrational density-of-states background. The results imply ‘‘order through fluctuation’’ effects via autocatalytic and inhibitory growth.
Thermal and percolative transitions and the need for independent symmetry breakings in branched polymers on a Bethe lattice
Authors: Gujrati P. D.
Published: July 2002

We consider a very general model of equilibrium polymerization of branched polymers. Our model contains, as a special case, the ‘‘a priori equal probability’’ model considered by Flory and Stockmayer. In this limit, the model exhibits only percolation transition. We solve our general model in the interior of a Bethe lattice. There are thermal as well as percolation transitions in the model. Each of the two transitions requires an independent spontaneous symmetry breaking; neither implies the other. Without spontaneous symmetry breaking, the transitions do not manifest themselves. Thermal transitions correspond to singularities in the equation of state. Percolation transitions, on the other hand, do not correspond to any singularity in the equation of state. We also discuss the failure of a topological identity, valid for any finite Cayley tree, in the interior of the Bethe lattice. We consider various different cases to show the usefulness of our model. In particular, we argue that one must distinguish between the ‘‘tree approximation’’ of Flory on a general lattice and our exact solution on the Bethe lattice. The former, in general, allows for loop formation, whereas there are no loops allowed in the latter solution.
Relaxation dynamics in the B(1/2) and C(3/2) charge transfer states of XeF in solid Ar
Authors: Hoffman G. J. , Imre Dan G. , Zadoyan R. , Schwentner N. , Apkarian V. A.
Published: July 2002

Dispersed laser induced fluorescence, and time domain measurements using the optical Kerr effect are applied to study the relaxation dynamics of Xe+F− (B 2Σ1/2 and C 2Π3/2) charge transfer states in solid Ar. Very fast vibrational relaxation is observed in the C emitting site: excitation near v=20 leads to population of v=0 of the C state in 13(±2) ps. In the B emitting site, the lower vibrational states relax sequentially. Relaxation times of 800(±30) ps for 1→0 and 250(±30) ps for 2→1, are measured directly; and 150(±30) ps for 3→2 and <30 ps for 4→3 are estimated from spectral intensities. A new, much faster relaxation channel, which leads to B(v=1, and v=0) is open to states above v=3 in the B emitting site. This fast channel has a relaxation time of 7(±1) ps and must involve multiple internal conversions among the nested electronic states in the ionic manifold. Under intense pumping, the excited population relaxes by stimulated emission. Stimulated radiative relaxation rates larger than 1.5×1011 s−1 are observed for B(v=0).
Rotational energy transfer in the Na2 b 3Πu state: Propensity rules for transitions between hyperfine components
Authors: Li Li , Zhu Qingshi , Lyyra A. M. , Whang T.-J. , Stwalley W. C. , Field R. W. , Alexander M. H.
Published: July 2002

The changes in the hyperfine quantum number (F′) that accompany collision-induced ΔJ, ΔΩ, and Δ(e/f-parity) transitions in the Na2 b 3Πu-state have been studied by sub-Doppler, cw, perturbation-facilitated optical–optical double resonance (PFOODR) spectroscopy. The Na2 is contained in a heat pipe oven at ∼1 Torr and the primary collision partner is Na(3s 2S). The PUMP laser selectively excites a single b 3Π0u v′=12, J′=43e, s or 44e, a rotational level, the parent level. All F′ hyperfine components of the parent J′ level are directly populated by the PUMP laser, but with different velocity projections relative to the laser propagation direction. Thus each parent hf component is labeled by its longitudinal velocity. As the PROBE laser is scanned through various 2 3ΠΩg v=2, J←b 3ΠΩ′u v′=12, J′ transitions, sub-Doppler hyperfine structure can be resolved on each parent and daughter rotational line in the PFOODR fluorescence excitation spectrum. The collisional propensity rule ΔF=ΔJ is obeyed for (s, a permutation symmetry conserving) ΔJ′=0, ΔΩ′=0, +1, and +2 and ΔJ′=±1, ±2, ΔΩ′=0 collision-induced transitions. No systematic exploration of the parent-J′, Ω′ dependence of the ΔF propensity was undertaken; in particular, the present study was restricted to the high-J limit where the Na2/Na collisions are not sudden relative to the rotational (half) period and where J≫I. The ΔF=ΔJ hyperfine propensity rule observed for high-J levels of the Na2 b 3Πu state is consistent with previous theoretical predictions of a ΔF=0 propensity for collision-induced ΔJ=0 transitions between Λ-doublet components of the OH X 2Π state and a ΔF=ΔJ propensity for collision-induced transitions between CaBr X 2Σ+ rotational levels.
Electron–phonon coupling in conjugated polymers: Reference force field and transferable coupling constants for polyacetylene
Authors: Girlando Alberto , Painelli Anna , Soos Z. G.
Published: July 2002

A Herzberg–Teller expansion for π electrons in the ground state of conjugated polymers identifies quadratic electron–phonon (e–ph) contributions and suggests a σ+π reference force field F0 based on butadiene. Linear response theory then fixes linear e–ph coupling constants for the Raman shifts in polyacetylene (PA) due to π-electron fluctuations. The same coupling constants and the known isotopic dependences of the reference accurately give the Raman frequencies of (CD)x and (13CH)x; with a different susceptibility, F0 and the coupling constants also account for Raman modes of polyenes and for photo- or dopant-induced infrared modes of PA and its isotopes. We develop a general phenomenological approach for identifying modes coupled to π-electron fluctuations and show that both CCC and CCH bends are weakly active in PA. Linear and quadratic e–ph coupling constants and various susceptibilities are related to π-electron Hamiltonians, primarily to Hückel models and to Coulomb interactions in the Pariser–Parr–Pople model. We also relate our approach to previous PA models with linear e–ph coupling or modified force fields.
Monte Carlo simulation of the adsorption of diblock copolymers from a nonselective solvent. II. Structure of adsorbed layer
Authors: Zhan Yongjian , Mattice Wayne L. , Napper Donald H.
Published: July 2002

The adsorption of a diblock copolymer from a nonselective solvent is studied by using a Monte Carlo method. Simulation results for the structure of the adsorbed layer, segment density profiles, and the conformational properties are presented for adsorption energy per segment varying from −0.25 to −1.25, in units of kT, and for different volume fractions and different block sizes. It is found that the bound fraction for the anchoring block A increases as the surface affinity increases, decreases as the length of the anchoring block increases, and decreases as the volume fraction of copolymer increases. The probability of end adsorption decreases, while the probability for flattened chains increases as the surface affinity increases. The probability of adsorption for the middle beads is larger than that for the end beads. The segment density profiles show the maximum of the adsorption for the anchoring block is at the first layer near the surface. The surface coverage for block A at the first layer increases as the concentration of copolymer increases and as the surface affinity increases. The height and position of the adsorption peak for the nonanchoring blocks are determined by the length of the nonanchoring block and the surface affinity. The data of the end-to-end distance of adsorbed chains show that the nonanchoring block is stretched.
Symmetry-adapted cluster–configuration interaction method applied to high-spin multiplicity
Authors: Nakatsuji Hiroshi , Ehara Masahiro
Published: July 2002

The SAC–CI (symmetry-adapted cluster–configuration interaction) method is extended to high-spin multiplicity and applied to the quartet, quintet, sixtet, and septet states of N2, N2+, OH, and m-phenylenebis(methylene) (m-PBM) molecules. The results show good agreement with those of the full-CI reference calculations, though the dimensions of the calculations are much smaller than those of the full-CI method. The smallness of the calculational efforts and the excellent agreement with the full-CI results assure that the SAC–CI method is useful and accurate not only for singlet, doublet, and triplet states but also for high-spin multiplet states.
Dynamics of vibrationally excited van der Waals complex: Stimulated-emission-pumping laser-induced-fluorescence spectroscopy of the anisole⋅Ar complex
Authors: Takayanagi Masao , Hanazaki Ichiro
Published: July 2002

The stimulated-emission-pumping laser-induced-fluorescence technique was applied to investigate the dynamics of vibrationally excited states of van der Waals complexes, anisole-d0⋅Ar and anisole-d3⋅Ar, produced in the supersonic expansion. In this technique, the complex excited to a specific vibrational state by stimulated-emission pumping is detected by measuring the laser-induced-fluorescence excitation spectrum with an appropriate delay to probe the vibrational relaxation or predissociation. Five vibrational states, 18b1, 6a1, 121, 16a2, and 11, of the complexes were investigated. Main results obtained in this study are (1) The vibrational excitation of one of 11, 16a2, and 121 gives hot bands in the laser-induced-fluorescence excitation spectrum due to the anisole monomer produced by vibrational predissociation. (2) Excitation of 18b1, whose energy is below the dissociation threshold, does not lead to vibrational relaxation of the complexes. (3) Excitation of 6a1 causes the predissociation into vibrationally cold monomers. (4) The predissociation is completed within 300 ns, the shortest available delay time between the stimulated-emission pumping and probe pulse.
A new implementation of stationary perturbation theory for separable quantum-mechanical problems
Authors: Fernández Francisco M. , Castro E. A.
Published: July 2002

We improve a recently developed implementation of perturbation theory for separable problems. On choosing a new ansatz for the perturbed wave function that takes into account the singularities of the eigenvalue equation explicitly, the resulting perturbation equations become simpler. As an illustrative example, we consider the Stark shift of the microwave spectra of a polarizable symmetric-top molecule.
The relation between macroscopic quantities and the solid–fluid interfacial structure
Authors: Liu Xiang-Yang , Bennema P.
Published: July 2002

The relation between thermodynamic quantities and the interfacial structure is described on the basis of a layer model (or an inhomogeneous cell model). By introducing the surface characteristic scaling factor C *l (or C l0) and the surface concentration (or density) distribution constant Kl, thermodynamic quantities of the interfacial phase are correlated with the corresponding quantities of the bulk phase. Relevant expressions are derived from basic principles of statistic thermodynamics. In order to clarify the relevant consequences, two systems, the hard-sphere–hard-wall system and the crystal–solution system of aliphatic compounds, are discussed. It is shown that within the framework of our model, experimental data are explained in a satisfactory way. The character of solid–fluid interfaces is discussed, in terms of wetting conditions. As a consequence of this approach, solid–fluid interfaces are classified into three typical cases: the equivalent wetting case (C *l or C l0=1), the more than equivalent wetting case (C *l or C l0≤1), and the less than equivalent wetting case (C *l or C l0≥1). It turns out that except the equivalent wetting case, the concentration of solute (or the density of fluid units) in the interfacial phase will differ from that in the bulk fluid phase at equilibrium.
Influence of Si surface structure on reaction mechanism: Atomic hydrogen+adsorbed Br
Authors: Koleske D. D. , Gates S. M.
Published: July 2002

The reaction of atomic hydrogen with adsorbed Br is compared on Si(100) and Si(111) surfaces from 50 °C to 300 °C. On both surfaces, Br removal rate is first order in atomic hydrogen flux, first-order in Br coverage, and exhibits a near zero activation energy. On Si(111), this rate also depends on surface hydrogen coverage, indicating that different mechanisms occur on these surfaces.
Interatomic potentials and the phase diagram of Xe/Pt(111)
Authors: Rejto Paul A. , Andersen Hans C.
Published: July 2002

We present a microscopic model for the Xe/Pt(111) system that is consistent with the experimental desorption energy, the experimental vibrational frequency of the Xe atom in the direction normal to the Pt(111) surface, and salient features of the experimental phase diagram of Xe/Pt(111). The interatomic potentials in this model were obtained using a simple technique that we have developed for generalizing the typical pairwise-additive atom–atom central potentials used in modeling physisorption systems to make them noncentral and more flexible in their functional form. We applied this technique to the Lennard-Jones pair potential and fit the parameters to reproduce the experimental binding energy, the frequency for vibration of the adsorbate normal to the surface, and a reasonable choice of the binding distance. We adjusted the corrugation of the potential ΔV, defined as the energy barrier for motion of an adsorbate atom from one binding site to another, in order to fit as much of the phase diagram as possible. Our model for the Xe–Pt interaction was constructed on the basis of the assumption that the binding site is located in the threefold site of Pt(111). When the Xe–Xe interaction was represented by the form appropriate for atoms in the gas phase, we were unable to find a stable commensurate phase for any choice of the corrugation that predicted a low temperature incommensurate phase. When a substrate mediated contribution to the Xe–Xe interaction was included in the model, we found that the commensurate phase was stable in an intermediate temperature range with an incommensurate phase stable at low temperature for a range of values of the corrugation. For a choice of ΔV=171 K, the striped incommensurate phase is stable at low temperature, there is a phase transition to a √3 ×√3R30° phase at T=65±5 K, and the commensurate phase melts to become a liquid at T≊120 K. These results are in reasonably good agreement with the experiments.
Comment on: Reversible multiple time scale molecular dynamics
Authors: Toxvaerd S.
Published: July 2002

The algorithm published by M. Tuckerman, B. J. Berne, and G. J. Martyna, J. Chem. Phys. 97, 1990 (1992) is shown to be a reformulation of the molecular dynamics Leap-frog algorithm for integration of Hamilton’s equation.
Reply to Comment on: Reversible multiple time scale molecular dynamics
Authors: Tuckerman M. , Berne B. J. , Martyna G. J.
Published: July 2002

In response to the comment by Toxvaerd [J. Chem. Phys. 99, XXX (1993)], it is shown that two simple integrators, position Verlet and velocity Verlet, derived as introductory examples give rise to distinct trajectories. Leap-frog forms are derived for the integrators and using operator algebra the true relationship between the two trajectories are determined. The trajectories do not diverge as a function of time but mirror each other. This was not pointed out in the original paper but it is presented here in response to the comment. The simpler analysis of the Toxvaerd comment does not correctly define this relationship and incorrectly implies that the two methods generate identical trajectories.
Exploration of approximations of the kinetic-exchange-correlation energy
Authors: Sinnott Susan B. , Kelchner Cynthia L. , DePristo Andrew E.
Published: July 2002

We present the results of our work to estimate the many-body kinetic-exchange-correlation energy with a two-body approximation. The approximation does not use any adjustable parameters or any functional form. Results of this approximation used within the corrected effective medium method in place of exact numerical integration are provided for Ni, Cu, Rh, Pd, and Ag. Systems that are studied include diatomics, surfaces and the bulk lattice. Among the quantities calculated are surface energies, energies of formation for various bimetallic solids, and the barriers and predicted mechanism of the diffusion of Cu on Cu(100). Finally, we discuss the validity of approximating a many-body energy with a two-body function.
Determination of potential energy curves for HeNe+ from mobility data, spectroscopic measurements, and theoretical calculations
Authors: Viehland Larry A. , Mason E. A.
Published: July 2002

Potential energy curves for the three lowest states of HeNe+(X 2Σ1/2+,A1 2Π3/2,A2 2Π1/2) are determined from a combination of mobility and spectroscopic measurements, in conjunction with a theoretical expression giving the A1 curve in terms of the X and A2 curves plus the energy splitting between the 2P3/2 and 2P1/2 states of Ne+. The results are also consistent with measurements of the scattering of beams of fast Ne+ ions by He gas. The best available ab initio potentials give mobilities in poor agreement with experiment, largely because of poor predictions of the positions of the potential minima. It is also shown that, contrary to initial expectations, mixture formulas for mobility based on simple momentum-transfer theory are valid for open-shell systems with multiple interaction potentials. Illustrative calculations are given for the mobility of Ne+ in a He+Ne mixture; both HeNe+ and Ne2+ have multiple potentials.
The trans-bending and F–F stretching vibrations of HF dimer in νHF=1: The influence of intermolecular vibrational excitation on the predissociation dynamics
Authors: Bohac E. J. , Miller R. E.
Published: July 2002

The optothermal detection method has been used to obtain near infrared spectra for combination bands of HF dimer associated with the F–F stretching (ν4) and trans-bending (ν5) vibrations, built upon the HF fundamental stretches (ν1 and ν2). These spectroscopic results are used to estimate the intermolecular trans-bend and F–F stretch frequencies, as well as the associated tunneling frequencies, all corresponding to the excited HF stretch vibrational states. Photofragment angular distributions have also been obtained for these excited states. These provide new insights into the effects of intermolecular vibrational excitation on the vibrational predissociation dynamics of this dimer. The initial state dependence of the final rotational state distributions suggests that the intermolecular stretch and bend are decoupled, with the energy in the F–F stretch evolving directly into relative translation of the photofragments, while excitation of the bend has no effect on the average kinetic energy release.
The determination of excited-state geometries by analyzing resonance Raman spectra. Example—1(π−π*) state of o-chloranil
Authors: Bettermann H. , Dasting I. , Rauch W.
Published: July 2002

Resonance Raman spectra of o-chloranil were obtained by exciting the energetically lowest π–π* transition. Raman signals of three totally symmetric stretching modes (C=C, C=O, and C–C) showed pronounced intensity alterations. The intensity changes of these modes were analyzed using a dispersion formula which is based on the Kramers–Heisenberg–Dirac equation. Three-dimensional Franck–Condon integrals were calculated with the algorithm of Kupka and Cribb. The mathematical fitting of the experimentally derived spectra yielded the displacements of the potential curves along the considered normal coordinates and data of the Duschinsky mode mixing. The geometrical shifts along directions of ground-state normal coordinates were transformed to bond-length changes with the use of the eigenvector matrix L of the ground-state vibrations. L was obtained by a complete force field analysis. The excited-state geometry of o-chloranil is characterized by the contraction of the di-keto system accompanied by a weakening of the acrolein unit. The largest changes appear in the butadiene subsystem, where a conversion of bond order occurs between the double bonds and the single bond.
Single crystal electron paramagnetic resonance of γ-irradiated Hofmann-type benzene clathrates: Electron capture by Ni(II) center of a polymeric network
Authors: Murugesan R. , Padiyan D. Pathinettam , Thamaraichelvan A.
Published: July 2002

The electron paramagnetic resonance (EPR) spectra of γ-irradiated single crystals of M(NH3)2Ni(CN)4⋅2C6H6 (M=Cd and Zn) are studied at 77 K. On the basis of the analysis of spin Hamiltonian parameters, one of the radicals is identified to be Ni(I) species formed due to the electron trap on the Ni(II) center of the polymeric network. The other radical is proposed to be the electron loss M(III) center. Radicals originating from the guest benzene molecules are not formed.
De-excitation of H(2p) in a collision with a H2 molecule
Authors: Terazawa Norihisa , Ukai Masatoshi , Kouchi Noriyuki , Kameta Kosei , Hatano Yoshihiko , Tanaka Kenichiro
Published: July 2002

The velocity assigned experiment of the collisional de-excitation of a H(2p) atom by a H2 molecule is demonstrated by measuring the time-dependent intensity of Lyman-α radiation emitted in the photodissociation of H2 using a pulsed synchrotron radiation in the vacuum ultraviolet region. Observed decay rates of Lyman-α radiation have given the rate constant or cross section for the de-excitation of H(2p) by H2. The cross section is about 3×10−14 cm2 in the region of the collisional velocity between 4.6×105 and 12×105 cm/s, which is several times larger than that for the H(2s) de-excitation. It has been concluded that the l-changing collision is quite important in the de-excitation of H(2p) by H2.
Theory of vibrational energy relaxation in liquids: Vibrational–vibrational energy transfer
Authors: Adelman S. A. , Muralidhar R. , Stote R. H.
Published: July 2002

A theoretical treatment of the vibrational–vibrational (VV) contribution to the vibrational energy relaxation time T1 of a solute normal mode in a molecular solvent, which extends a previous treatment [S. A. Adelman, R. H. Stote, and R. Muralidhar, J. Chem. Phys. 99, 1320 (1993), henceforth called Paper I] of the vibrational–translational–rotational (VTR) contribution to T1, is outlined and expressions for this VV contribution, valid for the infinitely dilute diatomic solution, are presented. The treatment is based on the formula T1=β−1(ωl), where β(ω) is the friction kernel of the relaxing solute mode and where ωl is its liquid phase frequency. β(ω) is evaluated as the cosine transform of the autocorrelation function 〈ℱ̃(t)ℱĩ〉0v of the fluctuating generalized force exerted by the vibrating solvent on the solute normal mode coordinate conditional that this coordinate is fixed at its equilibrium value. 〈ℱ̃(t)ℱ̃〉0v is expressed as a superposition of the rigid solvent autocorrelation function 〈ℱ̃(t)ℱ̃〉0 and a correction which accounts for solvent vibrational motion. For diatomic solvents one has 〈ℱ̃(t)ℱ̃〉0v= 〈ℱ̃(t)ℱ̃〉0+NSMD(t) cos ωDt F(ΩD), where NS=number of solvent molecules, MD(t) is the vibrational force gradient autocorrelation function, ωD and ΩD are solvent molecule liquid phase frequencies, and F(Ω)=1/2ℏΩ−1 coth[ℏΩ/2kBT].

The Gaussian model is assumed for 〈ℱ̃(t)ℱ̃〉0 and MD(t) yielding β(ω) as a superposition of a Gaussian centered at ω=0 which mediates VTR processes and a Gaussian centered at ω=ωD which mediates VV processes. Vector integral expressions for MD(t), ωD, and ΩD are presented which are similar to the expressions for ωl and 〈ℱ̃(t)ℱ̃〉0 given in Paper I. These expressions permit the evaluation of the VV contribution to T1 from the atomic masses, bondlengths, vibrational frequencies, and site–site interaction potentials of the solute and solvent molecules and from specified rigid solvent equilibrium site–site pair correlation functions of the liquid solution.
Temperature coefficients for the chain dimensions of polystyrene and polymethylmethacrylate
Authors: Boothroyd A. T. , Rennie A. R. , Wignall G. D.
Published: July 2002

We have made precise measurements of the unperturbed radius of gyration 〈S2〉1/20 of polystyrene and polymethylmethacrylate molecules in the melt over the temperature range 120 °C–240 °C by small-angle neutron scattering. Contrary to most previous reports, based on dilute solution measurements, we find that neither polymer exhibits a significant change in dimensions with temperature, and we deduce temperature coefficients, κ=d ln〈S2〉0/dT, of (0.0±0.1)×10−3 K−1 for polystyrene and (0.1±0.1)×10−3 K−1 for polymethylmethacrylate. These κ values are close to those given by thermoelastic measurements, but are difficult to reconcile with predictions obtained from the rotational isomeric state model.
Correlation effects in hydrogen-bonded polymer blends
Authors: Veytsman Boris , Painter Paul
Published: July 2002

In hydrogen-bonded polymer blends there are long range correlations that are a result of the combined covalent and hydrogen-bond connections. A mean field description of these mixtures is presented which indicates that there should be an infinite correlation length above some percolation threshold. There are various consequences of the compositional heterogeneities or clustering that results from these effects and these are discussed in this paper.
Electronic structure of U4+, Np4+, and Pu4+ doped into ThSiO4 single crystal
Authors: Krupa J. C. , Carnall W. T.
Published: July 2002

The optical spectra of tetravalent ions of early actinides, U4+, Np4+, and Pu4+, were interpreted via a semiempirical Hamiltonian in D2d site symmetry, in terms of electronic transitions within their respective 5fn ground state configurations. Systematic trends in the semiempirical parameters describing the intraconfiguration transitions have suggested that those for U4+ ion are not representative of the light actinide series.
Photoionization and photofragmentation of BxNy clusters produced by laser vaporization of boron nitride
Authors: Roland P. A. , Wynne J. J.
Published: July 2002

Vaporization of hexagonal boron nitride with a 532 nm laser, followed by supersonic expansion cooling, produces a variety of BxNy clusters which are detected by photoionization of the neutral clusters with a 194 nm laser, followed by time-of-flight (TOF) mass spectrometry. At low 194 nm photoionization fluence (<50 mJ/cm2), the mass spectrum shows resolved peaks corresponding to the clusters By+1N+y for y=1–8, as well as an unresolved region corresponding to clusters ranging from atomic mass unit 100–1000. At higher photoionization fluence, resolved peaks, corresponding to B+x clusters for x=2–100, appear and grow with increasing fluence. Concommitantly, the envelope of the unresolved region changes shape and grows with a diminished dependence on fluence, indicating that the unresolved clusters are being photofragmented as well as photoionized by the 194 nm radiation. The products of such photofragmentation are the aforementioned B+x clusters. This process is modeled using an Arrhenius relation to describe the probability of fragmentation.
The 3 1Σ+g ‘‘shelf’’ state of Na2
Authors: Tsai Chin-Chun , Whang Thou-Jen , Bahns John T. , Stwalley William C.
Published: July 2002

The 3 1Σ+g state of Na2 has been extensively studied using an optical–optical double resonance technique and a shielded cylindrical space-charge-limited ionization detector. A wide range of vibrational quantum numbers 0≤v≤141 was observed which covered over 99.9% of the potential well depth. A ‘‘shelf’’ on the potential energy curve was found near v=23. A hybrid potential was constructed based on the Rydberg–Klein–Rees (RKR) potential curve and a local inverse perturbation approach (IPA) for the shelf region. For this potential, there is a shallow second local minimum ∼5 cm−1 in depth at the shelf region. The long range energy also compares well with long range theory.
Thermal electron attachment to C6F5X and C6H5X (X=I, Br, Cl, and F)
Authors: Shimamori Hiroshi , Tatsumi Yoshitsugu , Sunagawa Takeyoshi
Published: July 2002

Rate constants have been measured for thermal electron attachment to C6F5X (X=I, Br, Cl, F, and H) and C6H5X (X=I, Br, Cl, and F) at room temperature in N2 buffer gas (1–100 Torr) using the pulse-radiolysis microwave cavity method. For all the compounds studied, the rate constants are of the two-body type. Unexpectedly, the values for C6F5X except C6F5H are all the same (∼2×10−7 cm3 molecule−1 s−1), which are higher than most of the previous values, while that for C6F5H, measured in Xe and Ar buffer gases, is very low (7×10−12 cm3 molecule−1 s−1). For C6H5X, the value decreases dramatically with varying X from I to Br to Cl as 1.0×10−8 to 6.5×10−12 to 3×10−14 cm3 molecule−1 s−1, and that for C6H5F must be much lower than 10−13 cm3 molecule−1 s−1. These results for the magnitude of the rate constant are rationalized by the variation in the energy of a transient negative-ion state of each molecule, which results from a combination of the electron affinities of constituents (halogen atom X and C6F5 radical) and the strength of the C6F5–X (or C6H5–X) bond.
Alternate schemes for the coherent laser control of chemical reactions
Authors: Gauthier Daniel J.
Published: July 2002

Several schemes are presented for the coherent laser control of chemical reactions. They are based on the principle of interference between quantum mechanical excitation pathways first proposed by Brumer and Shapiro [Acc. Chem. Res. 22, 407 (1989)]. The conclusion that these schemes may by useful for coherent laser control is based on the fact that, for each of the schemes, the quantum mechanical interference effect has been observed previously in nonlinear optical experiments; however, no attempt to control the interference was attempted in these experiments.
Intergranular liquid in solids and premelting of ice
Authors: Johari G. P. , Pascheto W. , Jones S. J.
Published: July 2002

That the equilibrium state of impurity-free polycrystalline ice contains water has been expressed in terms of free-energy changes, and further developed to show that liquid in cavities formed at grain junctions in a microcrystalline solid containing no impurities affects its electrical properties and increases its heat capacity. General formulas for the volume of a liquid contained in the pockets formed at four-grain junctions and veins formed at three-grain junctions in equilibrium in an impurity-free polycrystalline mass are given. The variation of the volume fraction of intergranular water in impurity-free ice and the ice’s total free energy have been calculated for different grain sizes and temperatures. Calculations also confirm that intergranular water or premelting contributes significantly to the pure ice’s heat capacity.
The B←X electronic spectra of N+2–Nen (1≤n≤8)
Authors: Bieske E. J. , Soliva A. M. , Friedmann A. , Maier J. P.
Published: July 2002

Spectra of 14N+2–20Ne, 14N+2–22Ne, and 15N+2–20Ne have been recorded in the region of the B 2Σ+u←X 2Σ+g origin transition of N+2. Measurements are made by mass selecting cooled ionic complexes and photodissociating them whilst monitoring the N+2 fragment ion intensity as the laser wavelength is scanned. Various bands are assigned to transitions involving the stretching and bending motions of the Ne...Ne+2 bond with their structure and spacings consistent with transitions between quasilinear geometries in the X and the B states. Spectra of complexes with up to eight neon atoms attached to a 14N+2 core have also been measured. Evidence from shifts of the band origins and analysis of the vibrational frequencies of N+2–Ne2 and N+2–Ne3 suggest a structure where the Ne ligands are sited at one end of the N+2 chromophore.
Comparative molecular dynamics simulation study of the benzene–graphite and the benzene–1,12-dodecanediol–graphite interface
Authors: Winkler Roland G. , Hentschke Reinhard
Published: July 2002

Using the molecular dynamics simulation technique, we study the effect of a highly ordered physisorbed alkyl monolayer on the surface induced liquid structure near a smooth solid–liquid interface. The system considered here is 1,12-dodecanediol adsorbed at the benzene–graphite interface. We compare the results on the graphite–1,12-dodecanediol–benzene interface to previous simulations on the bare graphite–benzene interface. We find that the surface induced liquid structure near the adsorbate covered interface differs from the structure induced by the bare interface only within the first solvation shell. Beyond the first solvation shell, there is no discernible difference, even though the surface induced structure is still pronounced. In addition, we study the structure of the hydrogen bond stabilized molecular adsorbate network. Here we find that the hydrogen bonds link the 1,12-dodecanediol molecules in a chain-like fashion across the boundaries of the herringbone structure formed by the adsorbate.
A partially restricted Hartree–Fock approach
Authors: Kollmar Christian
Published: July 2002

We present a Hartree–Fock method which gives the molecular orbitals (MOs) of a single determinant wave function consisting of a restricted part (one orbital assigned to each pair of spin-up and spin-down electrons) and an unrestricted part (different orbitals for different spins). The MOs of the restricted part transform as irreducible representations of the point group of the molecule, whereas those of the unrestricted part may be symmetry broken. A modified coupling operator formalism is used for the determination of the MOs which result from different Fock operators for the closed shell orbitals (restricted part) and for the spin-up and spin-down orbitals of the unrestricted part. The method is illustrated by considering the symmetry breaking in the conjugated π system of short-chain radical intermediates formed in the course of the solid state polymerization reaction of diacteylene crystals.
A study of anthracene–Arn (n=0–5) in the ground cationic state by laser threshold photoelectron spectroscopy: Selective ionization of complex isomers formed in the free jet expansion
Authors: Cockett Martin C. R. , Kimura Katsumi
Published: July 2002

The van der Waals complexes formed between anthracene and argon in a free jet expansion are studied using laser resonance enhanced multiphoton ionization (REMPI) threshold photoelectron spectroscopy with the aim of selectively ionizing specific isomers of small- to medium-sized clusters for which discrete absorption peaks exist in the excitation spectrum. Two-color (1+1′) REMPI threshold photoelectron spectra of a number of isomers of anthracene–Arn (n=1–5) have been recorded in addition to that of anthracene itself. The following adiabatic ionization energies (Ia) have been obtained to within ±5 cm−1: 59 872 (n=0), 59 807 and 59 825 (n=1), 59 757 and 59 774 (n=2), 59 695 (n=3), 59 606 and 59 660 (n=4), and 59 565 cm−1 (n=5). For n=1–3, detailed van der Waals cation vibrational structure was observed, showing progressions in both bending and stretching mode vibrations. The resulting vibrational information together with the ionization energy red shifts has helped in assigning bands observed in the threshold photoelectron spectra to particular geometric isomers some of which were not necessarily observed in the excitation spectrum.