bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoselection and the structure of highly excited states: Rotationally resolved spin–orbit autoionization spectrum of HCl
Authors: Haber Kenneth S. , Patsilinakou Evdokia , Jiang Yanan , Grant Edward R.
Published: July 2002

Ultraviolet two-photon photoselection, followed by visible one-photon absorption is applied to HCl to record the first double-resonant spin–orbit autoionization spectrum of a hydrogen halide, and the first rotationally resolved such spectrum of HCl. The J=2 level of the F 1Δ2(v=0) Rydberg state serves as the intermediate two-photon resonance. The ionization-detected absorption spectrum from this initial state, scanned across the 634 cm−1 interval between the lower 2Π3/2 and upper 2Π1/2 thresholds, shows a complex system consisting of hundreds of sharp lines converging to the accessible rotational limits of the upper spin–orbit threshold. The complexity of the spectrum is attributed to the relaxed selection rules associated with dipole transitions from a state in Hund’s case (a) to a manifold approaching Hund’s case (e), in concert with the irregularities expected for angular momentum coupling intermediate between the limits of case (c) and case (e). A simple case (e) fit over the central portion of the spectrum yields tentative assignment of a number of series converging to the J+=1/2, 3/2, and 5/2 levels of the ion. No evidence is found for series converging to higher rotational levels of the ion, even though such series are not excluded by angular momentum selection rules; transitions to Rydberg electronic angular momenta required for access to states of higher J+ are found associated with series observed converging to lower rotational thresholds.
Obtaining long-time dynamics in the spectroscopic spin-boson model via path integration
Authors: Coalson Rob D.
Published: July 2002

The spectroscopic spin-boson model [J. Chem. Phys. 86, 995 (1987)] for quantum dynamics of nuclei on potential surfaces coupled by nonradiative effects is studied via path integral techniques. In particular, long time recurrence dynamics of the electronic absorption correlation function of a multimode system characterized by strong nonradiative coupling is computed by both direct enumeration of spin configurations and by a novel Monte Carlo method based on the extended adiabatic formulation of the spin-boson problem [J. Chem. Phys. 92, 4993 (1990)]. Besides providing a demonstration of the utility of path integration for studying detailed spectroscopic signatures of nuclear motion on excited electronic potential surfaces, it is argued that the basic technical procedures can be applied to a number of problems in which a quantum system is coupled to an environmental oscillator bath.
Energy component analysis of the Jahn–Teller effect in the methane radical cation
Authors: Boyd Russell J. , Darvesh Katherine Valenta , Fricker Paul D.
Published: July 2002

Extensive configuration-interaction calculations with double-zeta plus polarization and near triple-zeta plus polarization basis sets are used to analyze the Jahn–Teller (JT) effect in the methane radical cation. Energy component analysis shows that the Jahn–Teller effect leads to a decrease in the expectation value of the electron-nuclear attraction energy, an increase in the expectation value of the interelectronic repulsion energy, and an increase in the internuclear repulsion energy. These observations are consistent with a contraction of the electron cloud. The dominant factor in the −0.0550 hartree Jahn–Teller distortion (Td→C2v) in CH+4 is the −0.5262 hartree change in the electron–nuclear attraction energy. The differences in all energy components are large in relation to the JT distortion. Interelectronic repulsion plays a dominant role in determining the relative energies of the possible JT distorted structures, but electron correlation effects are relatively unimportant.
The role of symmetry in collisions of N2 with N+2
Authors: Sohlberg Karl , Futrell Jean , Szalewicz Krzysztof
Published: July 2002

Classical trajectory methods have been used to explore the excitation of vibrations in gas-phase collisions of the nitrogen molecular ion with its parent molecule. The near symmetry of the reactants is shown to result in a high probability that the two molecules are excited by an equal amount of energy. This provides a possible explanation of the molecular beam measurements that show that the total number of vibrational energy quanta excited in the collision is, with a high probability, even.
Numerical simulations of homogeneous and inhomogeneous ionic systems: An efficient alternative to the Ewald method
Authors: Caillol J. M. , Levesque D.
Published: July 2002

A new method for the numerical simulation of ionic systems is proposed; it is a very efficient alternative to the well-known Ewald method for the study of homogeneous and inhomogeneous phases of Coulomb systems. Its main feature is the use of a simulation cell which is the three dimensional surface of a four dimensional sphere. When the ionic interaction is the potential solution of the Poisson’s equation in this non-Euclidean space, it is established by simulations that the results of the Ewald method and of the proposed method are identical for an homogeneous phase. The comparison with previous simulations for inhomogeneous systems demonstrates also the reliability and efficiency of the method.
Influence of transition state resonances on integral cross sections and product rovibrational distributions for the Cl+HCl→ClH+Cl reaction
Authors: Schatz George C. , Sokolovski D. , Connor J. N. L.
Published: July 2002

An accurate quantum scattering calculation for the Cl+HCl→ClH+Cl reaction has been performed. In particular, we study the influence of the lowest transition state resonance on the energy dependence of the state-to-state integral cross sections and product rovibrational distributions. The calculations use a recently developed centrifugal sudden hyperspherical (CSH) coordinate reactive scattering code. The Bondi–Connor–Manz–Römelt semiempirical potential energy surface is employed. All 161 partial waves needed for the convergence of the cross sections are included in the calculations. We find that the resonance perturbs certain reagent and product rotational levels of the vibrational ground state (v=0, j=14–16), as well as all open rotational levels (j=0–8) of the first vibrationally excited state (v=1). Transitions from the ground reagent to the ground product vibrational state, such as v=0, j=15→v′=0, j ′=15, show almost no resonance structure in the integral cross sections; rather direct scattering dominates the partial wave sum. On the other hand, transitions between perturbed v=0 rotational states and any v′=1 rotational state, or between any v=1 state and perturbed v′=0 states, or between any v=1 and any v′=1 state, show a novel resonance feature in the integral cross sections. This novel feature is a sudden smooth ‘‘step’’ in the integral cross section, centered at the resonance energy for the partial wave with zero total angular momentum quantum number (J=0). The step has a width equal to the J=0 resonance width. Sometimes this step is superimposed on a slowly varying background which arises from direct scattering. A quantitative description of these resonant steps in the integral cross sections is developed using a J-shift approximation. Because the resonance influences all rotational states for v=v′=1 in a similar way, there is no significant effect on the product rotational distributions due to the resonance. However, the resonance does produce detectable stepping behavior in the product vibrational distribution.
Doppler-free high resolution laser spectroscopy of the Cs2 D 1Σ+u state and the predissociation
Authors: Katô Hajime , Kobayashi Tooru , Chosa Munehiro , Nakahori Tadaki , Iida Tetsuya , Kasahara Shunji , Baba Masaaki
Published: July 2002

A Doppler-free polarization spectrum of the D 1Σ+u(v′,J′)–X 1Σ+g(v″,J″) transition has been observed, and the molecular constants of the D 1Σ+u state were determined. A remarkable variation of the vibrational spacing ΔGv with v′, and large deviations of ΔGv from the fitted ΔGv curve for v′≳35 were analyzed. The potential curve of the perturbing state, which was identified as the bound (2)3Πu state, was estimated. By using an experimental setup where a collimated cesium beam is crossed at right angles by the laser beam, we measured the excitation spectra by monitoring separately the molecular fluorescence and the D2 or D1 atomic emission. We found that a predissociation of the D 1Σ+u state occurred for v′≳20, and the probability of predissociation increased with v′. The measured v′ and J′ dependence of the linewidth Γ showed a significant increase for v′≳25. The predissociation of the D 1Σ+u state is caused by a combination of spin–orbit interaction between the D 1Σ+u(v′J′) and (2)3Π0u(e vJ′) levels, and the L-uncoupling interaction between the bound (2)3Π0u(e vJ′) and dissociative c 3Σ+u(N = J′J′) levels.
Dynamic light scattering study of ionic KNO3–Ca(NO3)2 mixtures
Authors: Pavlatou E. A. , Rizos A. K. , Papatheodorou G. N. , Fytas G.
Published: July 2002

We report spontaneous Brillouin and photon correlation spectroscopic results on the dynamics of the local density fluctuations in the ionic mixed salt xKNO3⋅yCa(NO3)2 with molar ratios 2:1, 3:2, and 1:1. For these mixtures, the density relaxation function g(t) changes from a stretched exponential (β=0.47) near the glass transition temperature Tg, to an exponential (β=1) form at high temperatures, whereas the composition dependence of the relaxation time τ reflects mainly differences between the Tg values. The 3:2 fragile glass (Tg=60 °C) is one of the few examples where g(t) is now known from the glass-like (60–90 °C) to the liquid-like (T>100 °C) region through the application of various dynamic scattering techniques. The τ(T) and the evolution of the width β of the distribution are discussed in terms of the physical pictures implied by current models of the liquid and glass transition phase.
Improvements in the generation and detection of Kr(3P) and Kr(3P2) atoms in a flow reactor: Decay constants in He buffer and total quenching rate constants for Xe, N2, CO, H2, CF4, and CH4
Authors: Sobczynski R. , Setser D. W.
Published: July 2002

The generation and decay of Kr(3P2) and Kr(3P0) atoms in a flowing afterglow reactor at 300 K using He carrier gas have been characterized and compared with Ar carrier gas. The dominant loss for the Kr(3P2) and Kr(3P0) atoms in He is diffusion to and quenching at the wall; the two-body quenching constants are of the order of 1×10−15 cm3 s−1 for each. The Kr(3P0) concentration in He carrier was sufficiently high that the total quenching rate constants were measured without resorting to optical pumping from the lower energy Kr(3P2) state. Room temperature rate constants are reported and the product states are discussed for the Kr(3P2) and Kr(3P0) reactions with Xe, CO, N2, H2, CF4, and CH4; the difference between the Kr(3P0) and Kr(3P2) rate constants for N2 is nearly a factor of 6. Quenching rate constants for 13CO are ∼25% smaller than those for 12CO. The Kr(3P0) reaction with CO gives mainly CO(b 3Σ+) and (e 3Σ−); the CO(b, v′=0) level is much more rotationally excited than CO(b, v′=1). Excitation-transfer to Xe from Kr(3P2) gives mainly Xe(6p[3/2]2 and [3/2]1 ); the Kr(3P0) reaction favors the Xe(7s) states. A systematic study of the best electrode configuration and operating conditions for the dc discharge using He carrier gas is reported for the generation of Xe(3P2), Ne(3P0) and Ne(3P2), as well as Kr(3P2) and Kr(3P0).
Raman spectra of mass-selected vanadium dimers in argon matrices
Authors: Hu Zhendong , Shen Bo , Zhou Qinwei , Deosaran S. , Lombardi J. R. , Lindsay D. M. , Harbich W.
Published: July 2002

We report the first Raman measurements on a matrix sample prepared by the mass-selected ion deposition technique. Vanadium dimer ions, produced by sputtering, were mass selected with a Wien filter and codeposited with Ar and electrons. Raman spectra gave ωe″= 536.9(1.1) and ωexe″ = 4.1(0.1) cm−1, in good agreement with previous measurements.
A rotating bond order formulation of the atom diatom potential energy surface
Authors: Laganà Antonio
Published: July 2002

The advantage of mapping calculated potential energy values onto the space of the bond order coordinates is discussed with special concern for the possibility of designing functional representations of the interaction. A rotating model defined in the bond order space is proposed.
Semiclassical perturbation treatment of molecular charge exchange
Authors: Cross R. J.
Published: July 2002

We develop two semiclassical perturbation approximations to treat the charge exchange between a molecular ion and its neutral counterpart. In one case we use a diabatic vibrational basis set appropriate to the dissociated species. The scattering is then treated using an exponential first-order perturbation theory. In the second case a vibrationally adiabatic basis set is used. Comparisons with exact results for O+2+O2 show that the adiabatic theory is the more accurate at 8 eV, but the two have comparable accuracy at 36 eV.
The gas-phase thermochemistry of TiH
Authors: Chen Yu-Min , Clemmer D. E. , Armentrout P. B.
Published: July 2002

The homolytic bond dissociation energy of the titanium neutral hydride D0(Ti–H) is determined experimentally for the first time by using guided ion beam tandem mass spectrometry to measure the kinetic energy dependence of the endothermic hydride abstraction reactions of Ti+ with methylamine, dimethylamine, and trimethylamine. From the thresholds of these reactions, the value of D0(Ti–H)=2.12±0.09 eV (48.9±2.1 kcal/mol) at 298 K is derived. Other 298 K thermodynamic values obtained are D0(Ti+–H−)=8.19±0.09 eV (188.8±2.1 kcal/mol), I.E.(TiH)=6.59±0.14 eV, P.A.(Ti−)=15.64±0.09 eV (360.6±2.1 kcal/mol), and ΔfH(TiH)=116.4±2.3 kcal/mol. This thermochemistry is compared with theoretical values and its relationship to hydride bond energies for the other first row transition metals is discussed.
Approximate constants of motion and energy transfer pathways in highly excited acetylene
Authors: Kellman Michael E. , Chen Gengxin
Published: July 2002

Approximate constants of motion of acetylene (C2H2), analyzed previously below 10 000 cm−1, are determined from analysis of a nonlinear least squares fit of the highly excited vibrational absorption spectrum. Although there are at least ten distinct Fermi resonance couplings in the measured spectrum up to 24 000 cm−1, there is one, and quite possibly two, good constants of motion. These constants are pointed out to be equivalent to a preferred energy transfer pathway discussed by Smith and Winn. It is suggested that these constants may also apply to ‘‘unassignable’’ stimulated emission pumping spectra, which sample a different region of phase space.
Nonlocal contributions to the thermodynamics of inhomogeneous homopolymer-solvent systems
Authors: McMullen William E.
Published: July 2002

We derive several recent approximation schemes for the thermodynamics of inhomogeneous homopolymer solutions by functionally expanding the nonideal part of the free energy about the local density. The functional expansion contains all orders in the gradients of the monomer density even when truncated at low order. When applied to systems with slowly varying densities, it reduces to a widely used square-gradient theory of homopolymers and blends. We also recover the so-called Roe functional by considering infinitely long Gaussian chains. However, this result only applies to a small region near the center of the interface. We employ a one-parameter, exponential decay trial function and the variational theorem to obtain estimates of the surface tension between two coexisting homopolymer-solvent phases. This particular trial function facilitates a comparison of the numerically evaluated nonlocal free energy functional, the analytic summation of the full gradient expansion, the square-gradient approximation, and the Roe functional. Within the scope of a Gaussian-chain model and a Flory–Huggins local free energy, we compare the various approximations with regards to their predictions for the surface tension, the nonlocal free energy density, and the interfacial profiles. The full gradient expansion does not converge to a useful result near the center of the interface of moderately segregated phases or when the interface becomes narrow as for strongly segregated homopolymer solutions. The square-gradient theory overestimates the free energy density in the wings of the interface but underestimates this quantity near the interface’s center. These two errors partially cancel, and the square-gradient theory provides an accurate estimate of the surface tension of moderately segregated homopolymer solutions. Unlike many other theories of interfaces which derive from considerations of two-body correlations in a homogeneous reference phase, the lowest nontrivial contribution of the functional expansion for Gaussian chains derives from three-body correlations. This feature is in qualitative accord with recent lattice theories of interfaces that treat chain biases in the interfacial region and thus implicitly contain three-body correlations.
Quadrupole moments of CuH, AgH, and AuH. A study of the electron correlation and relativistic effects
Authors: Kellö Vladimir , Sadlej Andrzej J.
Published: July 2002

High-level correlated calculations of the quadrupole moment of the group-Ib hydrides have been carried out with large Gaussian-type-orbital/contracted-Gaussian-type-orbital basis sets. The nonrelativistic results obtained for CuH, AgH, and AuH at the self-consistent-field Hartree–Fock level of approximation and for the origin at the heavy atom are −2.16, −3.17, and −1.94 a.u., respectively. The electron correlation contribution computed by using the coupled-cluster approaches reduces those values to −0.25, −1.14, and −0.18 a.u., respectively. The relativistic corrections have been obtained from the recently developed quasirelativistic method. They have been found to be of increasing importance for the investigated series of hydrides. The final relativistically corrected values of molecular quadrupole moments are found to be small for CuH (+0.05 a.u.) and AgH (−0.36 a.u.) while the corresponding value for AuH is large and positive (+2.00 a.u.).
Recombination of N+4 ions with electrons
Authors: Cao Y. S. , Johnsen R.
Published: July 2002

Spectroscopic observations of N2 second positive band emissions from photoionized afterglow plasmas indicate that dissociative recombination of electrons with N+4 ions results in the formation of N2 molecules in the C 3Πu (v=0,1) state. The measured recombination rate coefficient of (2.6±0.3)×10−6 cm3/s at T=300 K is considerably larger than that found in earlier work.
A magnetic-field-induced change of dynamics from the small molecule behavior to the intermediate case in the S1 state of pyrimidine vapor
Authors: Ohta Nobuhiro , Takemura Takeshi
Published: July 2002

Fluorescence intensity and decay of pyrimidine vapor have been measured as a function of external magnetic field (H) with excitation at the individual rotational lines belonging to the 6a20 and 1220 bands of the S0→S1 transition. On excitation into very low rotational levels of 6a2 or 122, dynamics at zero field is characterized by the small molecule limit, where fluorescence exhibits a nearly single exponential decay superimposed by the quantum beats, but the fluorescence decay at the initial stage of time becomes faster with increasing H and the decay profile becomes biexponential at high fields: A magnetic-field-induced change of dynamics from the small molecule behavior to the intermediate case occurs in the S1 state. A field strength required for the change becomes smaller with increasing the excess vibrational energy above the S1 origin (ΔE) and with increasing the rotational quantum number of the excited level (J′). On excitation into higher rotational levels, on the other hand, dynamics at zero field is characterized by the intermediate case, where fluorescence exhibits a biexponential decay, and only the slow component is efficiently quenched by H. Magnetic quenching of fluorescence is confirmed to become more efficient with increasing ΔE and with increasing J′. The efficiency both of the magnetic-field-induced change of dynamics and of the magnetic quenching of fluorescence is related to the level density of the triplet state coupled to S1 at zero field. A field-induced mixing between the triplet rovibrational levels which are coupled and uncoupled to S1, respectively, seems to play a part in magnetic quenching, besides the field-induced mixing among the spin sublevels belonging to the triplet levels coupled to S1 at zero field. On the basis of the rotational state dependence both of the fluorescence decay at zero field and of the magnetic field effects on intensity and decay profile of fluorescence, the relation between level structure and dynamics is discussed.
The Ar+–He interaction potential and distribution function effects on swarm measurements of Ar++N2 reaction-rate coefficients using helium buffer gas
Authors: Viehland Larry A. , Viggiano A. A. , Mason E. A.
Published: July 2002

A b initio potentials for the X 2Σ+1/2, A1 2Π3/2, and A2 2Π1/2 states of ArHe+ are tested as to their ability to describe swarm measurements of gaseous ion transport coefficients. Also tested are potentials based on spectroscopic measurements and model potentials chosen specifically so as to match the transport data. Ar+-ion velocity distributions in a drift tube containing a helium buffer are calculated from the potentials that best match the mobility data, by solution of the Boltzmann kinetic equation. The velocity distributions are used with estimated cross sections for the charge-transfer reaction Ar++N2 to calculate the effects upon the rate coefficients when the distribution differs from a Maxwell–Boltzmann form. The results indicate that the corrections are small at high buffer gas temperatures (293 K and above) and low to moderate electric-field strengths, but become larger at low temperature (82 K) and high fields. The smallness of the corrections confirms that previous rate coefficient measurements in a drift tube show a dependence of the Ar++N2 reaction upon the rotational temperature of N2.
Reactions of N+2 and N+4 with O2 from thermal to 20 eV center of mass
Authors: Schultz Richard H. , Armentrout P. B.
Published: July 2002

Reactions of N+2 and N+4 ions with O2 are studied under single-collision conditions in a guided-ion beam mass spectrometer. Reactant ions are formed in a flow tube source to ensure thermalization. N+4 is found to undergo charge transfer to O2 more efficiently than does N+2. The cross section for the latter reaction has a complex shape which is interpreted in terms of vertical transitions to form ground and excited state O+2. For reaction of N+4, the necessity of ligand exchange as an initial step is discussed. Cross sections for several other products, N2O+, O+, NO+, and N+, are measured for the first time in both systems and found to be formed only in endothermic reactions. Evidence is presented that the first three of these products arise from an N2O+ intermediate for both systems. The present results are compared to and contrasted with previous measurements.
Monte Carlo eigenvalue and variance estimates from several functional optimizations
Authors: Alexander S. A. , Coldwell R. L. , Monkhorst Hendrik J. , Morgan J. D.
Published: July 2002

Using several simple systems as examples, we show that the choice of optimization functional can have a significant influence on the accuracy of variational Monte Carlo calculations. In addition, we demonstrate that the Monte Carlo analog of the Rayleigh–Ritz procedure, which explicitly orthogonalizes ground and excited states, can be used to produce accurate eigenvalue and variance estimates of excited states.
Photoelectron spectroscopy from the Ã 1A
u state of acetylene: The bending vibrations of C2H+2 X 2Πu
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

Photoelectron spectra are reported for one-photon resonant, two-photon ionization of acetylene via a number of vibronic levels of the Ã 1Au state. Because the Ã 1Au state is trans bent and the X 2Πu ground state of the ion is linear, ionization from the Ã 1Au levels produces a long progression in the trans-bending vibration of the ion. The X 2Πu state is split by the Renner–Teller interaction along the trans-bending coordinate; the analysis of the photoelectron spectra allows the determination of the Renner–Teller parameters and leads to reassignments of the cis and trans-bending vibrations in earlier photoelectron studies.
Reduced partial cross sections for UV photodesorption of molecules from finite temperature surfaces
Authors: Messina Michael , Coalson Rob D.
Published: July 2002

A wave packet dynamical formalism was recently introduced [J. Chem. Phys. 92, 5712 (1990)] for extracting photofragmentation distributions of specified ‘‘primary’’ internal modes while summing implicitly over transitions to all other ‘‘secondary’’ internal degrees of freedom. This reduced partial cross section formalism is useful for analyzing photodesorption of a molecule from a solid surface, since it enables direct computation of internal state distributions of the desorbed molecule without having to explicitly resolve all vibrational transitions of the lattice oscillators. The original formulation of the method considered only zero temperature systems. In the present paper the formalism is extended to treat systems prepared at finite temperatures. Two applications, focusing on vibrational and rotational excitation of diatomic desorbates, respectively, are presented in order to illustrate the versatility of the methodology.
Spectroscopic and dynamical studies of the S1 and S2 states of decatetraene in supersonic molecular beams
Authors: Petek Hrvoje , Bell Andrew J. , Yoshihara Keitaro , Christensen Ronald L.
Published: July 2002

Fluorescence and fluorescence excitation spectra of all-trans-2,4,6,8-decatetraene have been obtained in free jets and in inert-gas clusters. In isolated decatetraene, excitation into 1 1Bu (S2) results in emission from both S2 (1 1Bu→1 1Ag) and S1 (2 1Ag→1 1Ag) on time scales that are faster than the 10 ns experimental resolution. In clusters, rapid electronic and vibrational relaxation leads to long-lived (360 ns) emission from thermally relaxed levels of S1. Direct excitation of low-lying, S1 vibronic levels in cold, isolated molecules also results in long-lived S1→S0 fluorescence, as expected for this symmetry-forbidden transition. The detection of S1 emission in free decatetraene has permitted the first detailed study of the vibronic structure and kinetics of the 2 1Ag state of an isolated, all-trans linear polyene. The S1←S0 fluorescence excitation spectrum is rich in low-frequency vibronic progressions. Analysis of this spectrum suggests that the transition not only is made allowed by vibronic coupling involving low-frequency bu skeletal modes (Herzberg–Teller coupling), as for polyenes in condensed phases, but also gains intensity from interactions between the electronic motion and the hindered rotations (torsions) of the terminal methyl groups. Preliminary analysis suggests that the barriers to internal rotation of the methyl groups must be substantially reduced in the 2 1Ag (S1) state. For isolated decatetraene, the 2 1Ag fluorescence lifetimes show a monotonic decrease with increasing vibrational energy, presumably due to increased mixing with the 1 1Bu state.
An investigation of three-body effects in intermolecular forces. II. Far-infrared vibration–rotation–tunneling laser spectroscopy of Ar2HCl
Authors: Elrod M. J. , Steyert D. W. , Saykally R. J.
Published: July 2002

A second Ar2HCl intermolecular vibration–rotation band centered at 37.2 cm−1 has been measured and assigned as a b-type transition originating from the ground state. Nuclear hyperfine splittings were resolved for both chlorine isotopes. The rotational constants determined from the data indicate coupling between an Ar–Ar stretching or bending coordinate and the Ar2 –HCl vibrational coordinates. As a result of this particular vibrational motion, Ar2H 35Cl undergoes an axis-switching transition while the Ar2H 37Cl isotope does not. In addition, the measured hyperfine projections indicate the possibility of coupling between the Ar2 –HCl stretching and bending modes, preventing an absolute vibrational assignment. These results indicate that the ‘‘reversed adiabatic’’ approximation employed by Hutson, Beswick, and Halberstadt in their theoretical study of Ar2HCl [J. Chem. Phys. 90, 1337 (1989)] is not appropriate for the complicated intramolecular dynamics presently observed in this system.
Thermal diffusion factors at low temperatures for gas phase mixtures of isotopic helium
Authors: Hurly J. J. , Taylor W. L. , Meeks F. R.
Published: July 2002

Vapor phase thermal diffusion measurements with considerably less scatter than previous data for equimolar mixtures of 3He and 4He are examined against the most recent interparticulate potential function for helium. Calculations are done for the purely classical case; for the quantal (phase-shift) case using Boltzmann statistics; and for the quantal case with Fermi–Dirac statistics for 3He and Bose–Einstein statistics for 4He. A ‘‘hump’’ appearing in the quantum-statistical curve, which is in disagreement with the experimental results, indicates that at the vapor densities reported here the use of quantum statistics is unnecessary.
Computation of transport coefficients of liquid benzene and cyclohexane using rigid multicenter pair interaction models
Authors: Luo H. , Hoheisel C.
Published: July 2002

With the use of multicenter Lennard-Jones pair potentials, the liquids benzene and cyclohexane were studied by molecular-dynamics computations. The experimental atomic structure of the molecules was essentially kept by the rigid molecule models. However, the H atoms were only indirectly taken into account. Shear viscosity and thermal conductivity coefficients and the corresponding time correlation functions were determined and their dependence on system size and averaging level investigated. It was found that the time correlation functions must be integrated over extremely long times compared with atomic Lennard-Jones liquids to obtain reliable plateau values for the shear viscosity. The transport coefficients finally resulting from our expansive molecular-dynamics runs agree well with the measured data for the chosen eight liquid states. Apparently, the rigid polycenter interaction models work well for the description of the complicated potential interaction in molecular liquids both for static and dynamic properties.
A perturbation theory guide to open-shell complexes: OH–Ar(X 2Π)
Authors: Green William H. , Lester Marsha I.
Published: July 2002

Perturbation theory is used to understand the experimentally observed stimulated emission spectra of OH–Ar(X 2Π). A useful zero-order Hamiltonian for an open-shell van der Waals complex is presented, and the most important perturbation terms are identified: rotational decoupling ( j⋅s), Renner–Teller coupling (V̂2), and a Coriolis interaction (J⋅j). This treatment reveals those parts of the Hamiltonian which are responsible for various unusual features in the spectra of open-shell complexes, such as the large parity splittings in certain vibrational bands and spin–orbit–induced predissociation of the OH–Ar(X 2Π) complex. The magnitude of the parity splitting is shown to be directly proportional to the change in the intermolecular potential when the odd electron in the free radical lies in or out of the O–H–Ar plane, the A′ and A″ surfaces. The measured splitting is used to infer the magnitude of the difference between the A′ and A″ potential-energy surfaces (∼12 cm−1) in the region sampled by the first excited bend.
Computation of transition dipole moments by Monte Carlo
Authors: Barnett R. N. , Reynolds P. J. , Lester W. A.
Published: July 2002

Three Monte Carlo methods for computing transition dipole moments are presented. Two of these approaches are based on the use of multiple Monte Carlo ‘‘random walks’’ to sample different probability distributions. The remaining technique employs a single Monte Carlo walk and averages an analytic approximation to the Green’s function to sample other distributions. The accuracy and efficiency of each method is investigated by computing the transition dipole moment between the 1s and 2px states of the hydrogen atom. Monte Carlo parameters, such as the time step size and the convergence time, are varied in order to study their effect on computed results. It is found that the approach based on a guided Metropolis walk with quantum Monte Carlo ‘‘side walks’’ and also the approach based on Green’s function averages yield accurate transition dipole moments efficiently. These two methods also yield accurate energies and expectation values for the individual eigenstates. The approach based on two equivalent quantum Monte Carlo walks, one for each state, is found to be least satisfactory.
Intermediate-time collective diffusion in colloidal suspensions
Authors: López-Esquivel Juan Antonio , Arauz-Lara José Luis
Published: July 2002

Collective diffusion in monodisperse suspensions of strongly interacting colloidal particles was studied using a single exponential function to model the memory kernel entering in the evolution equation for the dynamic structure factor, F(q,t). The parameters of the memory function are calculated exactly, using short time conditions for F(q,t), which are derived from the generalized Smoluchowski equation without hydrodynamic interactions. Our theoretical results, for particle interacting via a Yukawa type pair potential, show a good agreement at short and intermediate times with Brownian Dynamics data, for a large range of values of the wave vector q.
All-electron molecular Dirac–Hartree–Fock calculations: Properties of the XH4 and XH2 molecules and the reaction energy XH4→XH2+H2, X=Si, Ge, Sn, Pb
Authors: Dyall Kenneth G.
Published: July 2002

Dirac–Hartree–Fock (DHF) calculations have been carried out on the ground states of the Group IV di- and tetrahydrides. Geometries and infrared data are presented for both sets of molecules, dipole moments for the dihydrides, and the SCF (self-consistent field) reaction energies for the reaction XH4→XH2+H2. The effects of relativity can be seen in shorter bond lengths, higher frequencies for XH4 but lower stretching frequencies for XH2, higher infrared intensities for stretching modes and lower intensities for bending modes, a more positive dipole moment, and decreased stability for XH4 relative to XH2+H2. The results are also compared with relativistic effective core potential (RECP) calculations, first-order perturbation theory (PT) calculations, and with the limited experimental data. PT predicts all properties well for the Ge hydrides, but deviations are noted for the Sn hydrides, which become serious for the Pb hydrides. The full-core RECP calculations generally do not give results as good as PT. The addition of the (n−1)d orbital improves many of the properties. Overall, the quality of the RECPs used is somewhat variable, and no consistent pattern in the deviations from the DHF results is found for the set of RECPs used in this study.
The rotational tunneling dynamics of the ammonium ion in ammonium nitrate
Authors: Weier Jane , Strauss Herbert L.
Published: July 2002

The rotational dynamics of the NH3D+ ion in ammonium nitrate (phase V) have been studied at low temperatures. Each of the four N–D stretching bands has been irradiated with an infrared laser, and this irradiation causes rotation of the ions with a quantum efficiency of about 10−3. With the laser off, the rates of rotation in the vibrational ground state are determined. The four sets of data: the relative rates to different orientations in the excited state, the rates in the ground state, the hydrogen bond lengths determined by diffraction methods, and the frequencies of the N–D stretches do not correlate in a simple manner with one another.
Reduced dimensionality quantum calculations of mode specificity in OH+H2↔H2O+H
Authors: Wang Desheng , Bowman Joel M.
Published: July 2002

We report reduced dimensionality quantum calculations for zero total angular momentum of the OH+H2↔H2O+H reaction using the Walch–Dunning–Schatz–Elgersma potential. The three bending degrees of freedom of the tetra-atomic system are treated adiabatically in the harmonic approximation. The full six-mode potential is minimized with respect to the bending degrees of freedom, and the dynamics of the three radial degrees of freedom is governed by an effective potential given by the sum of this minimized potential plus the local adiabatic bending energy. The Schrödinger equation for the radial degrees of freedom is solved using a modification of the hyperspherical approach, which we have described previously. Vibrational state-to-state reaction probabilities are presented for the ground and first excited bending states of H2O. We examine the effect of exciting the vibrations of H2, OH, and H2O on the reaction probability, and the results are compared qualitatively with experiments, and previous calculations.
One-electron density matrices and energy gradients in second-order electron propagator theory
Authors: Cioslowski Jerzy , Ortiz J. V.
Published: July 2002

A formalism for evaluation of the effective first-order density matrices associated with second-order electron propagator theory is described. Computer implementation of this formalism affords first-order density properties, such as dipole moments, and energy gradients. Given an initial state with N electrons, this approach enables geometry optimization of the ground and excited electronic states of species with N−1 and N+1 electrons. The performance of the present method is assessed with test calculations on the formyl radical.
Photodissociation of bromocarbons at 193, 222, and 248 nm: Quantum yields of Br atom at 298 K
Authors: Talukdar Ranajit K. , Vaghjiani Ghanshyam L. , Ravishankara A. R.
Published: July 2002

The primary quantum yields, ΦBr, for the formation of Br atom in the photodissociation of CF2Br2 and CH3Br at 248, 222, and 193 nm, and of CF3Br at 222 and 193 nm were measured at 298 K. The bromine atoms were directly detected via resonance fluorescence following pulsed laser photolysis of the molecules of interest. The Br atom quantum yields in CF2Br2 photolysis increased with decreasing wavelengths: 1.01±0.15, 1.63±0.19, and 1.96±0.27 at 248, 222, and 193 nm, respectively. The ΦBr values in CH3Br and CF3Br were close to unity at all the wavelengths: 0.92±0.15 and 1.12±0.16 at 222 and 193 nm, respectively, for CF3Br; 1.01±0.16, 1.10±0.20, and 1.05±0.11 at 248, 222, and 193 nm, respectively, for CH3Br. Quantum yield of H atom formation in the photolysis of CH3Br at 193 nm was measured to be 0.002±0.001. H atom could not be detected in the photolysis at 248 and 222 nm. In all cases the ΦBr values were found to be independent of buffer gas pressure or the photolysis laser fluence. Our results suggest that the quantum yields for dissociation of all the molecules considered here are unity; therefore, atmospheric lifetime calculations carried out by assuming a unit dissociation quantum yield are correct. The nature of the photodissociation process is discussed in the light of previous and present results.
On the orientation of molecular photofragments produced in highly excited rotational states
Authors: Beswick J. A. , Glass-Maujean M. , Roncero O.
Published: July 2002

The degree of orientation of highly excited rotational states of molecular fragments produced by photodissociation with circularly polarized light is studied quantum mechanically. It is shown that a significant orientation of the fragments’ angular momentum j can be obtained when two or more dissociative continua correlated to the same final state of the products are excited simultaneously. In addition, the coherently excited continua should correspond to different helicity states, that is, to different projections of j on the reaction coordinate R (the vector joining the centers of mass of the fragments). The particular cases of an initial total angular momentum equal to zero as well as the axial recoil limit are discussed. The theory is applied to a simplified model of the photodissociation of ICN in the A continuum. The calculations have been performed by integration of the time independent quantum close-coupling equations for the coupling between the rotation of CN and the reaction coordinate R, using recently proposed potential energy surfaces and couplings. The results reproduce qualitatively the experimental results of Hasselbrink, Waldeck, and Zare [Chem. Phys. 126, 191 (1988)], in particular, the change of sign and the large degree of orientation found for highly excited rotational states of the CN fragments.
Diffusivity of the deuterated hydrogen molecule HD in NaA zeolite by neutron scattering experiment. Comparison with H2 in NaA
Authors: Cohen De Lara Evelyne , Kahn Rémi
Published: July 2002

The diffusion of HD in NaA is studied by incoherent neutron scattering, in order to be compared with H2. The behavior of HD is similar to the one of H2: Translational motion in a nonrestricted volume and liquidlike diffusivity. The diffusion model which fits the width of the quasielastic scattering in the entire (q,T) range gives a mean length for the isotropic jump l̄=3.3 Å, shorter than the one of H2 (3.9 Å), and equivalent values of the time τ0 between jumps. The comparison of the Arrhenius plots of DH2 and DHD leads to the conclusion that quantum effects have to be taken into account.
A low-energy quasiclassical trajectory study of N++H2 and N++D2. Dynamics, cross sections, and rate constants
Authors: Nyman Gunnar , Wilhelmsson Ulla
Published: July 2002

A quasiclassical trajectory study investigating the N++H2(D2)→NH+(ND+)+H(D) reactions has been performed. Two potential-energy surfaces differing only in their thermicities were employed. Both surfaces are based on the recent ab initio calculations for the N+(3P)+H2(1Σ+g)→NH+ (2Π)+H(2S) reaction by Wilhelmsson et al. Dynamics, forward–backward scattering, product energies, and trajectory lifetimes are discussed. Reaction cross sections and thermal rate constants are presented and compared with experimental results. In general, the agreement with experimental data is good. The thermicity of the reaction can, however, not be established by the trajectory calculations.
Properties of a hard-core fluid with a Yukawa tail studied by molecular dynamics and the mean spherical approximation
Authors: Rey C. , Gallego L. J. , González L. E.
Published: July 2002

A molecular dynamics simulation method for computing the static and dynamic properties of a hard-core fluid with a Yukawa tail is developed. The calculated static bulk properties show good agreement with the Monte Carlo results previously reported. We have also calculated the thermodynamic properties within the mean spherical approximation by the energy, compressibility, and virial routes; the ‘‘exact’’ data are most closely approximated by the energy route. The computed values of the self-diffusion constant are compared with those corresponding to a hard-sphere fluid; the results are consistent with previous findings that the cohesive part of the intermolecular potential plays a significant role in reducing diffusion.
Different surface profiles in surface demixing of polymer blends
Authors: Cifra Peter
Published: July 2002

The surface of the polymer mixture is simulated by Monte-Carlo method via condensing the polymer mixture against the solid wall and examining the free surface parallel with the wall. Depending on interactions present in the mixture and on the composition we find three representative situations of surface segregation effect in polymer mixtures described as segregation in, segregation out, and surface double layer. The segregation occurs across a realistic diffusive surface profile in contrast to theoretical approaches to this phenomenon working with a steplike density profile across the surface. Among the advantages of the present simulation, like the presence of compressibility of the studied system, there are also certain limitations, such as the lattice approximation.
Vibrational energy of the monoalkyl zinc product formed in the photodissociation of dimethyl zinc, diethyl zinc, and dipropyl zinc
Authors: Jackson Robert L.
Published: July 2002

The gas-phase photodissociation of (CH3)2Zn, (C2H5)2Zn, and (n-C3H7)2Zn has been examined at 248 nm using laser-induced fluorescence to detect the monoalkyl zinc radical and zinc atom photoproducts. For each compound, the monoalkyl zinc radical is the primary photoproduct and is formed sufficiently hot that it spontaneously dissociates to an alkyl radical and a Zn atom without absorption of a second photon. Photodissociation was examined in the presence of He buffer gas to measure the probability of quenching the secondary spontaneous dissociation of the monoalkyl zinc species. For all three dialkyl zinc compounds, the probability of quenching the secondary dissociation step increases substantially over the He pressure range of 0–400 Torr. The quenching probability vs He pressure was fit using RRKM theory in conjunction with a time-dependent master equation, treating the nascent vibrational energy distribution of the monoalkyl zinc product as an adjustable function. The quenching data for C2H5Zn and n-C3H7Zn can be fit only if it is assumed that these species are formed with a hot, narrow vibrational energy distribution, much narrower than that predicted by phase-space theory. A dissociation mechanism involving crossover from an optically prepared singlet state to a repulsive triplet state is proposed to explain this observation. Spontaneous dissociation of CH3Zn is quenched much more strongly by He than is calculated using any reasonable vibrational energy distribution function for CH3Zn. This is attributed to the inapplicability of RRKM theory to reactions involving very low-state-density molecules like CH3Zn.