bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectroscopic study of vibrational relaxation and cooling of rare-gas excimers formed in a direct current discharge with supersonic expansion
Authors: Dubé P. , Kiik M. J. , Stoicheff B. P.
Published: July 2002

Emission spectra (in the region 100–200 nm) of the excimers Ar2*, Kr2*, and Xe2* produced in a dc jet discharge with supersonic expansion were used to determine vibrational temperatures of the excimers, and to analyze the process of excimer relaxation from high vibrational levels near dissociation of the A3Σ+u(1u) states to the lowest levels. The observed narrow bandwidths of the second continua yielded population distributions with at least 80% in the v=0 levels at thermal equilibrium, for all three gases, corresponding to vibrational temperatures of 280, 95, and 82 K for Ar2*, Kr2*, and Xe2*, respectively. Changes in relative intensities of the first and second continua with pressure were monitored by optical pumping of atoms in the 1s5 metastable level [precursor of the excimer A3Σ+u(1u) state], and the resulting difference spectra were analyzed in terms of vibrational relaxation to the v∼0 levels. A simple model based on collisional relaxation was developed to calculate the VUV band spectra. Comparisons of the Ar2* spectra with the model showed that vibrational relaxation accounted for the observed difference spectra, up to plenum pressures of ∼7 atm. Departures from this model at higher pressures for Ar2*, and particularly the observed behavior of Kr2* and Xe2* difference spectra, indicated that populations in the lowest vibrational levels were affected by additional processes, and not only by vibrational relaxation of excimers formed by association.
Dipolar interactions in deuteron spin systems. I. Spin diffusion
Authors: Diezemann Gregor
Published: July 2002

The influence of dipolar interactions on the longitudianl relaxation of deuteron spin systems is investigated. Spin diffusion rates are evaluated, including approximate rates due to double quantum spin diffusion and three-spin flip–flop transitions. It is shown that slow molecular rotations in supercooled liquids do not affect the spin diffusion rates significantly provided that the motional correlation times are below the average spin lattice relaxation time T1 which becomes on the order of one second close to the glass transition temperature Tg. However, the broad distribution of deuteron T1 values at T<Tg results in a large effect of spin diffusion upon the long time decay of the longitudinal magnetization in T1 experiments. These effects are estimated in terms of a simple model in agreement with recent experiments. It is also shown that the initial decay determining the average rate 〈T−11〉 remains unaffected by spin diffusion. Finally, we show that small amplitude motions on a time scale of 10−6–10−3 s may cause temperature dependent spin diffusion effects.
The reactivity of molecular and atomic oxygen in oxygen-exchange reaction between NO and O2 coadsorbed on a Pt(111) surface
Authors: Sawabe Kyoichi , Matsumoto Yoshiyasu , Yoshinobu Jun , Kawai Maki
Published: July 2002

Surface reactions between N16O and 18O2 coadsorbed on Pt(111) have been studied by temperature-programmed desorption (TPD), low energy electron diffraction and infrared reflection absorption spectroscopy (IRAS). When the surface covered with N16O and 18O2 is annealed, 18O16O desorbs at 155 K and N18O at 145, 310, and 340 K. In addition, a new absorption line at 1658 cm−1 due to N18O appears above 265 K. However, there is no indication of oxidation products of NO in the TPD and IRAS measurements. Thus, only oxygen-exchange reactions take place on the coadsorbed surface. Reaction yields and mechanisms of the oxygen-exchange strongly depend on the adsorption states of oxygen. At ∼145 K, molecularly adsorbed oxygen and N16O (νNO=∼1930 cm−1) directly interacting with the oxygen molecule are involved in the exchange reaction. Above 270 K the exchange reaction takes place between atop N16O (νNO=∼1720 cm−1) and the metastable oxygen adatoms that do not possess a long-range order. On the contrary, the oxygen-exchange reaction is greatly suppressed when NO is coadsorbed on the surface with well-ordered p(2×2) oxygen atoms.
A large scale molecular dynamics study of chain configurations in the n=100 alkane melt
Authors: Brown David , Clarke Julian H. R. , Okuda Motoi , Yamazaki Takao
Published: July 2002

We describe simulations of a polymer melt sample containing 640 chains, each chain being composed of 100 methylene groups. Using these large scale simulations radii of gyration have been calculated with a statistical error of ∼2%. At this level of precision it is shown that even for this modest chain length configurational equilibrium is not achieved in less than 4 ns at 500 K. Values of the square radii of gyration, square end-to-end distances and proportion of conformers at equilibrium show good agreement with predictions of the Flory model of alkane chains in which all long range interactions along the chains are ignored. These simulations were performed using a Fujitsu AP1000 massively parallel processing machine with 1024 processors in conjunction with a recently described domain decomposition molecular dynamics algorithm [D. Brown, J. H. R. Clarke, M. Okuda, and T. Yamazaki, Comput. Phys. Commun. 83, 1 (1994)] capable of handling systems containing rigid bond constraints and threea are made with previous accurate data obtained for smaller samples containing just 10 n=100-site chains using the alternative cloning method of parallel processing [D. Brown, J. H. R. Clarke, M. Okuda, and T. Yamazaki, J. Chem. Phys. 100, 1684 (1994)].
Molecular quadrupole moment functions of HF and N2. I. Ab initio linear-response coupled-cluster results
Authors: Piecuch Piotr , Kondo Anne E. , Špirko Vladimír , Paldus Josef
Published: July 2002

The orthogonally spin-adapted linear-response coupled-cluster (LRCC) theory with singly and doubly excited clusters (CCSD) was employed to calculate quadrupole moment functions of the HF and N2 molecules in their ground electronic states. We also calculated several potential energy curves for both systems using various CC and non-CC methods, ranging from the limited and full configuration interaction (CI) and first-order CI (FOCI) to finite-order many-body perturbation theory. FOCI and related complete active space self-consistent field (CASSCF) methods were used in both energy and quadrupole moment calculations. Most of the calculations were performed using the medium-size basis set of TZ+2P quality devised by Sadlej [A. Sadlej, Coll. Czech. Chem. Commun. 53, 1995 (1988)] for high-level ab initio calculations of electrostatic molecular properties. In addition, a number of model CC calculations using small basis sets were performed, for which the exact full CI results, both for the energy and multipole moments, are available. It was demonstrated that the CCSD approach provides a realistic description of quadrupole moment functions, for all relevant geometries in the case of HF and for internuclear separations up to 1.5 times the equilibrium bond length for N2. The results of this study will be used for the analysis of the rovibrational dependence of quadrupole moments and for the calculation of quadrupole transition moments for both HF and N2.
Effects of different population, orientation, and alignment relaxation rates in resonant four-wave mixing
Authors: Williams Skip , Rahn Larry A. , Zare Richard N.
Published: July 2002

We present a combined theoretical and experimental study on the effects of different population, orientation, and alignment relaxation rates in resonant four-wave mixing (RFWM). Signal generation in RFWM can be viewed as the formation of and scattering from laser-induced population, orientation, and alignment gratings. We show that the relative contributions from the upper-state and lower-state population, orientation, and alignment gratings to the observed output signal can be changed by varying the polarizations of the three input fields. A theory is developed to account for these changes in collisional environments where the three multipole moments of the total angular momentum distribution, i.e., the population, the orientation, and the alignment, relax unequally. This theory is applied to the OH radical in an atmospheric-pressure H2/O2/He flame for which we have measured the line profiles using high-resolution degenerate and nearly degenerate four-wave mixing. We find that orientation and alignment gratings relax more rapidly than population gratings for low rotational levels of OH in the presence of He but at essentially the same rate for high rotational levels. A discussion is presented of the importance of this effect in the interpretation of RFWM experiments.
Growth of diamond films on a diamond {001}(2×1):H surface by time dependent Monte Carlo simulations
Authors: Dawnkaski E. J. , Srivastava D. , Garrison B. J.
Published: July 2002

Time dependent Monte Carlo (TDMC) simulations are performed on a diamond lattice to determine the effect of surface properties/conditions on the growth of diamond thin films on flat and stepped diamond {001}(2×1):H surfaces under chemical vapor deposition conditions. The gas–surface interface consists of reactions of incoming gas-phase species, such as H2 molecules and H and CH3 radicals with surface radical, π-bond and step edge sites on the diamond {001}(2×1):H surface. The rates and probabilities of adsorption, abstraction, desorption, and incorporation reactions, as well as the reverse reactions, are explicitly calculated either via molecular dynamics or transition state theory methods, or taken from experimental measurements. The TDMC method allows all these reactions to occur simultaneously, though probabilistically, at each time step. The microscopic and macroscopic characteristics of the growing film are observed as functions of time. Diamond films of 10∼100 layers are grown in the simulation and the observed growth rate (∼0.5μm/h at 1200 K) is in agreement with experimental results. The contributions to the activation energy of growth by specific processes such as H abstraction, CH3 adsorption and CH2 incorporation into the trough sites have been determined. The contributions to the activation energies by specific processes are not linearly additive, and the CH3 adsorption at step edges leads to enhanced growth at the edges.
Caging effects in the low-temperature rotational spectra of endohedral diatomic molecules at C60 fullerene
Authors: Hernández-Rojas J. , Bretón J. , Gomez Llorente J. M.
Published: July 2002

Caging effects in the low-temperature (T=50 K) rotational spectra of CO@C60, LiF@C60, and LiH@C60 are investigated by first deriving an effective Hamiltonian to describe the coupled rotation of both, cage and confined molecule. Our results prove the close connection between the degree of anisotropy of the molecule–cage interaction and the features in the rotational spectra. If the anisotropy is weak, as in CO@C60, the spectra are dominated by the free rotation of the confined molecule. The stronger anisotropy in LiF@C60 and LiH@C60 produces more complex spectra dominated by librational motions and hindered rotations. The cage rotation plays a fundamental role in the spectral features, enhancing the CO free rotation in CO@C60, and broadening the librational bands in LiF@C60 and LiH@C60.
Interrogation and control of condensed phase chemical dynamics with linearly chirped pulses: I2 in solid Kr
Authors: Sterling M. , Zadoyan R. , Apkarian V. A.
Published: July 2002

The effect of linearly chirped pulses in condensed phase ultrafast pump–probe experiments is investigated by classical simulations for the model system of I2 isolated in a Kr matrix. The central frequency of the probe laser is selected to monitor exclusively the event of first collision and recoil of atoms from the host cage. It is shown that a chirped probe pulse enables characterization of the magnitude and sign of the momentum of the evolving trajectory flux. This can be understood by transforming the frequency–time profile of the probe pulse to coordinate–time space, and noting that the observable signal is a function of the relative group velocities of the traveling wave packet and the traveling window function. The effect of the pump pulse chirp, is a measure of the controllability of the evolving dynamics. In the particular case studied, breaking and remaking of the I2 bond near the dissociation limit of the bare molecule, it is shown that the memory of the system outlasts the collision with the cage. Negatively chirped pulses produce a more tightly focused wave packet during recoil, leading to a stronger population coherence in the subsequent dynamics.
Quantum mechanical angular distributions for the F+H2 reaction
Authors: Castillo Jesus F. , Manolopoulos David E. , Stark Klaus , Werner Hans-Joachim
Published: July 2002

Quantum mechanical integral and differential cross sections have been calculated for the title reaction at the three collision energies studied in the 1985 molecular beam experiment of Lee and co-workers, using the new ab initio potential energy surface of Stark and Werner (preceding paper). Although the overall agreement between the calculated and experimental center-of-mass frame angular distributions is satisfactory, there are still some noticeable differences. In particular, the forward scattering of HF(v′=3) is more pronounced in the present calculations than it is in the experiment and the calculations also predict some forward scattering of HF(v′=2). A comparison with the quasiclassical trajectory results of Aoiz and co-workers on the same potential energy surface shows that the forward scattering is largely a quantum mechanical effect in both cases, being dominated by high orbital angular momenta in the tunneling region where the combined centrifugal and potential energy barrier prevents classical trajectories from reacting. The possible role of a reactive scattering resonance in contributing to the quantum mechanical forward scattering is also discussed in some detail.
Theory of magnetic circular dichroism in molecules oriented by photoselection
Authors: Goldbeck Robert A. , Che Diping , Kliger David S.
Published: July 2002

Expressions describing the absorption of light by molecules partially aligned by photoselection in the presence of a static external magnetic field are presented in complete form as a function of the molecular moments and the polarization properties of excitation and probe beams. The magnetic circular dichroism (MCD) of such samples is shown to involve terms arising from the coupling of electronic transition moments for excitation and probe photons that are additional to terms present in isotropic samples. The theoretical description presented here provides a basis for understanding the nature of MCD signals in samples partially aligned by photoselection and for interpreting the time evolution of MCD measurements when photoselection-induced alignment decays through rotational diffusion.
Algebraic approach to molecular spectra: Two-dimensional problems
Authors: Iachello F. , Oss S.
Published: July 2002

The Lie algebraic approach is extended to two-dimensional problems (rotations and vibrations in a plane). Bending vibrations of linear polyatomic molecules are discussed. The algebraic approach is particularly well suited to treat coupled bending modes. The formalism needed to treat coupled benders is introduced and a sample case, acetylene, is analyzed in terms of two coupled local benders.
Ab initio collision-induced polarizability, polarized and depolarized Raman spectra, and second dielectric virial coefficient of the helium diatom
Authors: Moszynski Robert , Heijmen Tino G. A. , Wormer Paul E. S. , van der Avoird Ad
Published: July 2002

Symmetry-adapted perturbation theory has been applied to compute the interaction-induced polarizability for the helium diatom. The computed polarizability invariants have been analytically fitted, and used in quantum-dynamical calculations of the binary collision-induced Raman spectra. The predicted intensities of the depolarized spectrum are in good agreement with the experimental data [M.H. Proffitt et al., Can. J. Phys. 59, 1459 (1981)]. The computed polarized spectrum shows agreement with the experiment within the large experimental uncertainties. The calculated trace polarizability was also checked by comparison of computed second dielectric virial coefficients with the experimental data. The ab initio dielectric virial coefficients, including first and second quantum corrections, agree well with the experimental data from indirect measurements.
The Baus elastic constants in interfaces of simple liquids
Authors: Padilla P. , Stecki J.
Published: July 2002

From molecular dynamics simulations of liquid–liquid and liquid–vapor interfaces at low temperatures between two simple liquids we determine the Baus elastic constants [M. Baus, J. Chem. Phys. 74, 2003 (1981)] and their variation across the interface. For certain cases the nonlocal stress–stress correlation functions were also calculated. A two-phase system with a planar interface is characterized by five independent elastic moduli. Two moduli show a dramatic decrease inside the liquid–liquid interface, similar to the drop in viscosity (‘‘lubricating effect’’) we found in sheared interfaces [P. Padilla, S. Toxvaerd, and J. Stecki, J. Chem. Phys. 103, 716 (1995)]. The other three, which are zero in the bulk region, show a oscillatory variation across the interface. One of these three is related to the surface tension.
On the role of conical intersections in photodissociation. V. Conical intersections and the geometric phase in the photodissociation of methyl mercaptan
Authors: Yarkony David R.
Published: July 2002

The geometric, or Berry, phase effect is particularly diabolical when it is associated with a conical intersection of two states of the same symmetry. A recently developed algorithm for determining a conical intersection of two states of the same symmetry provides the basis for a general approach for characterizing paths that give rise to the geometric phase effect in this instance. This approach is used, with wave functions in excess of 1.5 million configuration state functions, to determine a conical intersection and associated paths in the Franck–Condon region of the photodissociation process CH3SH(X 1A′)+hν→CH3SH(1,2 1A″)→CH3S+H or CH3+SH. Derivative couplings along these paths are also determined. The implications for photodissociation dynamics are discussed.
Computational study of the structures and thermodynamic properties of ammonium chloride clusters using a parallel jump-walking approach
Authors: Matro Alexander , Freeman David L. , Topper Robert Q.
Published: July 2002

The thermodynamic and structural properties of (NH4Cl)n clusters, n=3–10 are studied. Using the method of simulated annealing, the geometries of several isomers for each cluster size are examined. Jump-walking Monte Carlo simulations are then used to compute the constant-volume heat capacity for each cluster size over a wide temperature range. To carry out these simulations a new parallel algorithm is developed using the parallel virtual machine (PVM) software package. Features of the cluster potential energy surfaces, such as energy differences among isomers and rotational barriers of the ammonium ions, are found to play important roles in determining the shape of the heat capacity curves.
High pressure study on the Raman spectra of fluid nitrogen and nitrogen in helium
Authors: Scheerboom M. I. M. , Michels J. P. J. , Schouten J. A.
Published: July 2002

A study on the Raman shift and width of nitrogen and nitrogen in helium has been performed as a function of pressure and temperature by means of experiments, molecular dynamics (MD) simulations and hard fluid (HF) theory. The experiments have been performed using Raman spectroscopy in a diamond anvil cell at pressures up to 10 GPa and temperatures between 250 and 400 K. Both the experimental shift and width results of pure nitrogen link up very well with accurate measurements at lower pressures and with less accurate measurements at higher pressures. For the first time the Raman shift and width have been determined as a function of temperature at an isobar, such that a sensitive test of theoretical models can be made. The MD calculations on the linewidth along an isobar show very good agreement with experiment. The influence on the linewidth of the bondlength dependence of the site–site interaction parameters (often called the attractive contribution) appears to be small, which indicates that this has a small anisotropy. For pure N2 the MD and the HF calculations of the repulsive contribution to the Raman shift are about the same. This shows that both ways of calculation are consistent. The experimental Raman shift of nitrogen diluted in helium appears to be much larger than that of pure nitrogen. In contrast, the linewidth is much smaller than that of pure nitrogen. HF calculations were also performed for the Raman shift of N2, infinitely diluted in He. The results for the bondlength independent (repulsive) contribution give clearly smaller values than those of the experiment, which means that the effect of the change of the potential parameters at excitation must be positive. This implies that that part of the intermolecular potential, which is due to the overlap of the molecular charge distributions has a dependence on the bondlength, that results in a positive contribution to the Raman shift. It will be shown that for N2 the good agreement with experiment of earlier HF calculations with an attractive contribution, based on a purely dispersive model, is due to a cancellation of errors. For nondiluted mixtures of He–N2 under noncritical conditions the plot of experimental FWHM values as a function of the volume fraction shows a broad maximum, which is indicative for inhomogeneous broadening. This behavior is described with the help of the Knapp–Fischer model.
Hyperchaos and chemical turbulence in enzymatic reaction-diffusion systems
Authors: Strasser Peter , Rössler Otto E. , Baier Gerold
Published: July 2002

We derive two kinetic models based on commonly occurring, simple enzymatic reactions. The first belongs to the class of activator-inhibitor models, whereas the second is a Selkov-type substrate-depletion model. The bifurcation behavior of both models is studied in a spatially homogeneous environment. We consider one-dimensional arrays of N oscillatory reaction cells coupled by diffusion. For small N we find two kinds of hyperchaos depending on a bifurcation parameter and the ratio of the diffusion coefficients of activator and inhibitor (Da/Di). For large N and Da/Di≳1, we observe spatiotemporally chaotic states characterized by phase defects. For Da/Di<1, we find a chemical turbulent state emerging from the interaction of a Hopf and a Turing instability in both models.
Transport mechanism in nonconventional bismuth cuprate glass
Authors: Hazra S. , Mandal S. , Ghosh A.
Published: July 2002

Temperature and compositional dependence of the electrical transport properties of the nonconventional binary bismuth cuprate glasses are reported for the first time. It has been observed that the phonon assisted small polaron hopping model in the nonadiabatic regime is consistent with the data only at high temperatures. The variable range hopping model can fit the low temperature data qualitatively. The polaron hopping models proposed by Schnakenberg [Phys. Status Solidi 28, 623 (1968)] and Emin [Phys. Rev. Lett. 32, 303 (1974)] can predict quantitatively the conductivity data in the entire temperature range of measurement for all glass compositions. The physical parameters obtained from the best fits of these models are found reasonable and consistent with the glass compositions.
The first excited singlet state of s-tetrazine: A theoretical analysis of some outstanding questions
Authors: Stanton John F. , Gauss Jürgen
Published: July 2002

The equation-of-motion coupled cluster method for excited electronic states (EOMEE-CC) is applied to study the structure and selected properties of the first excited singlet state of s-tetrazine. Adiabatic S1←S0 excitation energies obtained with large basis sets containing up to 270 functions are uniformly somewhat above the experimental 0–0 value of 2.238 eV, but nevertheless are the most accurate calculations reported to date for this quantity. The equilibrium geometry of S1 predicted in this study is in excellent agreement with another high-level calculation, and moreover is quantitatively consistent with both the intensity of vibrational progressions observed in absorption and measured rotational constants for S1. The EOMEE-CC harmonic force field of S1 is the first to satisfactorily describe the low frequency in-plane b3g bending mode, notably its marked reduction in frequency upon excitation and characteristic positive anharmonicity. Both of these effects are due to a strong second-order Jahn–Teller interaction between S1 and the nominal S2(1Au) state, which is also investigated superficially in this work. Finally, results presented for the static dipole polarizabilies of the S1 state join others in calling for a reinvestigation of the experimentally determined parameters.
The local ansatz extended
Authors: Stollhoff Gernot
Published: July 2002

The local ansatz allows to perform correlation calculations for all electronic systems that can be computed in Hartree–Fock approximation, including metals. It distinguishes from standard quantum chemistry schemes in a sizable and physically motivated reduction of the excitation space. So far, this reduction caused a loss of greater than 10% of the otherwise available correlation energy. Here, a new kind of correlation operators are introduced that when added, allows to upkeep all advantages of the scheme but strongly reduces this deficiency.
Molecular dynamics for nonequilibrium systems in which there are a small number of very hot particles in a cold bath: Reference system propagator methods
Authors: Zhou Ruhong , Stuart Steven J. , Berne B. J.
Published: July 2002

In molecular dynamics simulations of systems with very hot particles in contact with a bath of cold particles one must choose a very small time step dictated by the fast (hot) particles. This requires the recalculation of all the forces after each of these small time steps. We show how reference system propagator algorithms (RESPA) can be used to speed up these simulations, and also comment on some interesting physical phenomena involved in these nonequilibrium systems.
The effect of adsorbed noble gas atoms on vibrational relaxation of hydroxyl group in zeolite
Authors: Fujino T. , Furuki M. , Kashitani M. , Onda K. , Kubota J. , Kondo J. N. , Wada A. , Domen K. , Hirose C. , Wakabayashi F. , Ishida M. , Goto F. , Kano S. S.
Published: July 2002

By means of tunable infrared pump–probe experiment, vibrational relaxation rates (v=1→0) of OH stretching mode of Bro/nsted acidic hydroxyl groups in a mordenite zeolite, which were interacting with noble gases (He, Ar, Kr, or Xe), were measured. The population lifetime, T1, decreased with the increase of the interaction with noble gases, e.g., 170 ps and 58 ps for isolated hydroxyl groups and ones interacting with Xe, respectively. Using a simple linear chain model, the mechanism for the enhancement of the relaxation rate was studied.
Spatiotemporal patterns in a heterogeneous model of a catalyst particle
Authors: Sheintuch Moshe
Published: July 2002

A novel problem, of diffusion resistance in porous particles that catalyze kinetically unstable reactions, is introduced, analyzed and simulated in order to unveil the possible spatiotemporal patterns in the direction perpendicular to the surface. Pore-diffusion resistance is a core problem in chemical reaction engineering. The present problem is described mathematically by three variables: a very-fast and long-ranged pore-phase concentration, a fast and diffusing autocatalytic surface species (activator) and a slow and localized surface activity. Unlike homogeneous models of pore disfussion resistance, in which instabilities emerge only with strong diffusion resistance, the present model exhibits oscillatory or excitable behavior even in the absence of that resistance. Patterns are generated by self-imposed concentration gradients. A detailed kinetic model of a simple but reasonable reaction mechanism is analyzed, but the qualitative results are expected to hold in other similar kinetics. The catalyst particle is a three-dimensional system and it may exhibit symmetry-breaking in the directions parallel to the surface due to interaction between the fast diffusion of a fluid-phase reactant and the slow solid-phase diffusivity of the activator. A thin catalyst can be described then by a one-dimensional reaction-diffusion system that admits patterned solutions. We point out this possibility, but refer to another work that investigates such patterns in the general framework of patterns due to interaction of surface reaction and diffusion with gas-phase diffusion and convection.
Response theory for static and dynamic polarizabilities of excited states
Authors: Jonsson Dan , Norman Patrick , Luo Yi , Ågren Hans
Published: July 2002

We demonstrate a fully analytical method for calculations of excited state static and dynamic polarizabilities. From the double residues of a single reference state cubic response function all excited state polarizabilities can be determined. The method forms also a basis for calculations of excited state hyperpolarizabilities through the finite-field approach. A demonstration is given for the first excited state polarizabilities and hyperpolarizabilities of long polyene chains.
Plucking a hydrogen bond: A near infrared study of all four intermolecular modes in (DF)2
Authors: Davis Scott , Anderson David T. , Nesbitt David J.
Published: July 2002

The near ir combination band spectra of supersonically cooled (DF)2 in the 2900 to 3300 cm−1 region have been recorded with a high resolution slit jet spectrometer. Twelve vibration–rotation–tunneling (VRT) bands are observed, representing each of the four intermolecular modes (van der Waals stretch ν4, geared bend ν5, out-of-plane torsion ν6, and antigeared bend ν3) built as combination bands on either the ν1 (free) or ν2 (bound) DF stretches. Analysis of the rotationally resolved spectra provide spectroscopic constants, intermolecular frequencies, tunneling splittings, and predissociation rates as a function of both intra- and intermolecular excitation. The intermolecular frequencies demonstrate a small but systematic dependence on intramolecular mode, which is exploited to yield frequency predictions relevant to far-ir studies, as well as facilitate direct comparison with full 6-D quantum calculations on trial potential surfaces. The tunneling splittings demonstrate a much stronger dependence upon intermolecular mode, increasing by as much as an order of magnitude for geared bend excitation. Conversely, high resolution line shape analysis reveals that vibrational predissociation broadening is only modestly affected by intermolecular excitation, and instead exhibits mode specific behavior controlled predominantly by intramolecular excitation. Detailed H/D isotopic vibrational shifts are obtained by comparison with previous combination band studies of all four intermolecular modes in (HF)2. In contrast to the strong state mixing previously observed for (HF)2, the van der Waals stretch and geared bend degrees of freedom are largely decoupled in (DF)2, due to isotopically ‘‘detuning’’ of resonances between bend–stretch intermolecular vibrations. Four-dimensional quantum calculations of the (HF)2 and (DF)2 eigenfunctions indicate that the isotopic dependence of this bend–stretch resonance behavior is incorrectly predicted by current hydrogen bond potential surfaces.
New operators for electronic density calculation. II. Application to hydrogen, first-row atoms, and first-row diatomic hydrides
Authors: Rassolov Vitaly A. , Chipman Daniel M.
Published: July 2002

The first practical calculations using two new operators specifically designed for determination of electronic spin and charge density at nuclei are reported. Applications are given for hydrogen, first-row atoms, and first-row diatomic hydrides. Numerical grid methods that simulate complete basis set results confirm a number of relations previously derived formally for the new operators. They also serve as benchmarks for testing the practical utility of the new operators in calculations with small to large Gaussian basis sets. In this connection, the new operators are generally found to have performance superior to the usual delta function formulation and to an alternative one based on the Hiller–Sucher–Feinberg identity.
Formation of NO(A 2Σ+,C 2Πr,D 2Σ+) by the ion–ion neutralization reactions of NO+ with C6F5Cl−, C6F5Br−, and C6F−5 at thermal energy
Authors: Tsuji Masaharu , Ishimi Hiroaki , Nishimura Yukio , Obase Hiroshi
Published: July 2002

The ion–ion neutralization reactions of NO+(X 1Σ+:v″=0) with C6F5Cl−, C6F5Br−, and C6F−5 have been spectroscopically studied in the flowing helium afterglow. The NO(A 2Σ+– X 2Πr,C 2Πr–X 2Πr,D 2Σ+–X 2Πr ) emission systems are observed in the NO+/C6F5Cl− reaction with the branching ratios of 0.96, 0.017, and 0.028, respectively, while only the NO(A–X) emission system is found in the NO+/C6F5Br− and NO+/C6F−5 reactions. The vibrational and rotational distributions of NO(A,C,D) indicate that only 1%–11% of the excess energy is deposited into vibration and rotation of NO(A,C,D) for all the reactions. In the NO+/C6F5X− (X=Cl,Br) reactions, a major part of the excess energy is expected to be partitioned into the relative translational energy of the neutral products and the vibrational energy of C6F5X. A comparison of the observed vibrational and rotational distributions with the statistical prior ones indicates that the reaction dynamics is not governed by a simple statistical theory because of the large impact parameter. The excitation mechanism of NO(A,C,D) in the ion–ion neutralization reactions of NO+ with C6F5X− (X=F,Cl,Br,CF3) and C6F−5 is discussed.
Temperature effects in the collisional deactivation of highly vibrationally excited pyrazine by unexcited pyrazine
Authors: Miller Laurie A. , Cook Carolyn D. , Barker John R.
Published: July 2002

Time-dependent infrared fluorescence (IRF) from the C–H fundamental and overtone bands was used to monitor the vibrational deactivation (by unexcited pyrazine) of pyrazine excited at 308 nm with a pulsed laser. The 1-color and 2-color IRF results were modeled with collisional master equation calculations in order to determine the temperature dependence of the energy transfer parameters. The experimental data cannot be modeled without invoking a biexponential collision step size distribution, which implies that ‘‘super collisions’’ are significant. The results show that the energy transfer parameters are essentially constant at temperatures greater than the Lennard–Jones well depth, but at lower temperatures, energy transfer is enhanced. It is likely that vibration–vibration energy transfer dominates in this system.
Structure and bonding in mixed silicon–carbon clusters and their anions
Authors: Hunsicker S. , Jones R. O.
Published: July 2002

Density functional calculations with simulated annealing have been performed for singly charged mixed silicon-carbon cluster anions SinCm− and for neutral SinCm clusters with up to eight atoms. The calculations predict the existence of a number of previously unknown isomers and identify two classes of anion structure: carbon-rich (chainlike) and silicon-rich (three-dimensional), with pronounced differences in the vertical detachment energies (VDE, transitions to states of the neutral clusters). The results provide insight into recent photoelectron detachment measurements on the anions.
Surprising cases of divergent behavior in Mo/ller–Plesset perturbation theory
Authors: Olsen Jeppe , Christiansen Ove , Koch Henrik , Jo/rgensen Poul
Published: July 2002

High-order Mo/ller–Plesset perturbation calculations have been carried out for several small molecules and compared to full configuration interaction (FCI) results. The convergence of the Mo/ller–Plesset series is found to depend crucially on the one-electron basis sets. Addition of diffuse basis functions leads in some cases to divergent behavior of the Mo/ller–Plesset series, even for highly single reference dominated systems as Ne and HF. The results thus questions the usefulness of higher-order perturbation calculations as a vehicle for obtaining arbitrary accuracy of quantum chemical calculations and raises the fundamental theoretical question: When does Mo/ller–Plesset perturbation theory converge for many-electron systems in extended basis sets?
Vibration–rotation excitation of CO by hot hydrogen atoms: Comparison of two potential energy surfaces
Authors: Green Sheldon , Keller Hans-Martin , Schinke Reinhard , Werner Hans-Joachim
Published: July 2002

Collision cross sections for rotational and vibrational excitation of CO by fast H atoms are calculated for two potential energy surfaces, the older Bowman–Bitman–Harding potential and the recently constructed surface of Werner, Keller, and Schinke. Both quantum mechanical and classical calculations are performed. The results obtained with the new potential energy surface are very similar to those obtained with the older potential; in particular, they do not rectify the discrepancies between the experimental and theoretical cross sections for vibrationally elastic transitions into small rotational states of CO.
Variational calculations of vibrational states of N2O using hyperspherical normal coordinates
Authors: Zùñiga José , Alacid Mercedes , Bastida Adolfo , Requena Alberto
Published: July 2002

Accurate variational calculations are presented for highly excited vibrational bound states of N2O using different recently proposed ab initio and empirical potential energy surfaces. All these potential surfaces are expressed as series expansions in terms of internal displacement coordinates. Transformations of them into Simons, Parr and Finlan (SPF) and Morse expansion are also considered. The vibrational state calculations are performed using a set of curvilinear hyperspherical normal coordinates derived from Radau coordinates. The vibrational energies are compared with experimental data and the quality of the potential energy surfaces used is discussed.
Time domain analysis of the bound states and resonances of the hydroperoxy radical HO2: Signature of quantum chaos
Authors: Mahapatra Susanta
Published: July 2002

The nature of the bound states and resonances of the hydroperoxy radical HO2 was analysed by computing ensemble averaged survival probabilities, 〈〈P(t)〉〉, from the Fourier transformation of the spectral autocorrelation function followed by averaging over the initial states and the Hamiltonian. The results are fitted with the 〈〈P(t)〉〉 values derived from the random matrix theory for an intermediate situation (partly regular and partly chaotic) to estimate the fractional irregularity of the states in the spectrum. The estimate is found to be in agreement with the result obtained from an analysis of the nearest-neighbor spacing distribution.
State-specific neutral time-of-flight of CO from ketene photodissociation at 351 nm: The internal energy distribution of CH2(X̃ 3B1)
Authors: Morgan Christopher G. , Drabbels Marcel , Wodtke Alec M.
Published: July 2002

Metastable time-of-flight (TOF) spectroscopy was used to measure the translational energy distribution of specific rotational states of CO formed from ketene photodissociation (CH2CO→CH2+CO) at 351 nm. This distribution could be directly related to the internal energy distribution of the other fragment (X̃ 3B1 CH2) formed in the reaction, thereby giving a correlated distribution of the internal states of the fragments. This technique overcomes the spectral complexity associated with detection the X̃ 3B1 state CH2. Previous measurements of the CO rotational distribution were simulated theoretically using the impulsive model and zero-point vibrational energy considerations. These models predicted that the rotational distributions of CO and CH2 should be uncorrelated, that ∼10% of the CH2 should be vibrationally excited with one quantum in the bending mode, and that the rotational energy distribution of CH2 should peak near zero. Measurements presented in this paper show a slight anticorrelation of CO and CH2 rotations, no vibrational excitation of CH2 and Gaussian-like rotational energy distributions of CH2 that peak at ∼1 kcal/mole and have a full width at half-maximum of ∼0.8 kcal/mol. Qualitative explanations for this behavior are presented.
Homogeneous nucleation in a laminar flow diffusion chamber: The operation principles and possibilities for quantitative rate measurements
Authors: Hämeri Kaarle , Kulmala Markku , Krissinel’ Evgenii , Kodenyov Gennadi
Published: July 2002

The laminar flow diffusion chamber technique for experimental homogeneous nucleation rate measurements is presented. In this type of chamber the flow containing vapor is rapidly cooled and critical supersaturations for homogeneous nucleation are obtained. Here the main operation principles as well as data reduction methods are investigated and the results are applied to dibutyl phthalate (DBP) vapor, which works well in this type of chamber. The nucleation rate has to be calculated using the profiles of saturation ratio and temperature, and using a theoretical expression for the nucleation rate. We have used three different nucleation theories when investigating the experimental nucleation rate: classical nucleation theory, self-consistent correction to classical theory, and the Dillmann–Meier theory. According to investigations in this work the determination of nucleation rate is general and rather independent of the theory used, and therefore the results can be considered reliable. The volume inside which the nucleation event is calculated to take place is examined experimentally. The comparison between model calculations of the actual nucleation volume and experiments show reasonable agreement.
Importance of hindered rotations in the thermal dissociation of small unsaturated molecules: Classical formulation and application to HCN and HCCH
Authors: Kiefer John H. , Mudipalli Partha S. , Wagner Albert F. , Harding Lawrence
Published: July 2002

A standard low-pressure limit Rice–Ramsperber–Kassel–Marcus rate constant is shown to significantly underestimate, by factors of three or more, the measured thermal dissociation rates for HCCH and HCN if the correct value of the bond-dissociation energy is used. An explanation for this discrepancy is sought by examining anharmonic effects due to isomerization. Classical expressions for the density of states and partition function are developed which include isomerization anharmonicity and can be substituted in the standard rate constant expression for corresponding harmonic terms. These expressions are then applied to HCN and HCCH. For HCN, the resulting expression can be compared both to experiment and to a previous quantum mechanical study using the same Hamiltonian form and potential for isomerization. The classical and quantum mechanical agreement is excellent. Good agreement with experiment is obtained with the consensus dissociation energy. For HCCH, electronic structure calculations are performed to produce the required potential for isomerization. With this potential, comparison between measured rate constants and those calculated with the consensus dissociation energy is also good. In both of these applications, adiabatic influences from the two stretching frequencies are argued to reduce the effective isomerization barrier and increase the effective mass of the rotation. Based on these detailed applications, an approximate, closed-form multiplicative factor for the rate constant expression is derived. This expression can be regarded as a generalization of one-dimensional hindered rotor formulas for the inherently multidimensional hindered rotors of isomerization. The expression is parametrized by the height of the hindered-rotor barrier. With the correct barrier height, this expression reproduces the more detailed calculations on HCN and HCCH. Its application to other systems indicates that the kinetic importance of isomerization in olefins is a rather general effect, not relegated only to small molecules.
Composition of critical clusters in ternary nucleation of water–n-nonane–n-butanol
Authors: Viisanen Y. , Strey R.
Published: July 2002

In previous papers the determination of the molecular contents of critical clusters has been performed. Both unary and binary vapor mixtures have been examined. This paper describes the first study of a ternary system. Using a nucleation pulse chamber accurate measurements of homogeneous nucleation rates (105<J/cm−3 s−1<109) of mixed droplets in supersaturated, ternary mixtures of water, n-nonane and n-butanol vapor have been performed. The examined gas phase activities, a1, a2, and a3, respectively, covered the whole ternary composition space at T=240 K. The observed variations of the cluster compositions with vapor phase composition differ substantially from that of ideal mixtures. The experiments seem to indicate phase separation within the critical clusters. The arrangement of the molecules in the cluster remains to be clarified.
IR–rf double resonance studies of dipole moments in the ν1+ν4 and ν1+ν5 states of acetylene-d
Authors: Marshall Mark D. , Izgi Kamil Can , Muenter J. S.
Published: July 2002

Infrared laser–radio frequency double resonance spectroscopy has been used to observe transitions across J=1 ℓ-type doublets in vibrational states of monodeuterated acetylene, HCCD, that have both the C-H stretch and one degenerate bending mode excited. Electric dipole moments and ℓ doublings have been measured for the ν1+ν4 and ν1+ν5 states, giving: μ14=−0.046 44(3) D, qℓ(14)=132.289(6) MHz, μ15=+0.031 26(2) D, and qℓ(15)=107.263(4) MHz.
Thermodynamic properties of an asymmetric fluid mixture with Yukawa interaction in the mean spherical approximation
Authors: Herrera J. N. , Blum L. , García-Llanos E.
Published: July 2002

The analytical solution of mean spherical approximation of the Ornstein–Zernike equation for a Yukawa fluid with factorizable coefficients is used to obtain a simple form of the equation of state for the mixture. These results are an extension of Ginoza’s work.
Viscosity of isotropic hard particle fluids
Authors: Allen Michael P. , Camp Philip J. , Mason Carl P. , Evans Glenn T. , Masters Andrew J.
Published: July 2002

A kinetic theory for the frequency-dependent shear viscosity η(ω) of isotropic fluids, composed of non-spherical hard convex bodies, is extended in two ways. First, the theory is reformulated to allow η(ω) to be expressed directly in terms of matrix elements involving the shear stress tensor rather than in terms of the transverse momentum correlation function. Second, relaxation of the antisymmetric component of the stress, due to coupling with spin angular momentum, is explicitly incorporated; this corrects an error in a previous version of the theory. The revised kinetic theory is compared with computer simulations for hard ellipsoids of revolution of axial ratio 2, 3, 5 and 10. Both the symmetric and antisymmetric contributions to η(ω) are well reproduced. Coupling with the collective molecular second-rank orientation tensor remains an important factor in determining the variation of η(ω) from high to low frequencies; the prediction of the magnitude of the associated dip in η(ω) is significantly improved. The new version of the theory is also more successful in predicting values of the zero-frequency shear viscosity η, the shear-orientation coupling parameter R, and the Stokes–Einstein (–Debye) products Dsη and Drη.