bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of gas-phase collisions in rapid desorption of molecules from surfaces in the presence of coadsorbates
Authors: NoorBatcha I. , Lucchese R. R. , Zeiri Yehuda
Published: July 2002

The effects of gas-phase collisions in mixtures of gases rapidly desorbed from surfaces are studied using direct Monte Carlo techniques. The results are compared with the effects observed in the desorption of pure gases under similar conditions. The translational energy distribution of the desorbed particles are found to deviate from the Boltzmann distribution and are found to be well represented by ellipsoidal Boltzmann distributions. In this respect the rapid desorption process is found to have similarities to the expansion of gases in nozzle sources. The influence of mass, internal degrees of freedom, and surface coverage of the adsorbates on the focusing, accelerating, and cooling effects due to gas-phase collisions are analyzed. The presence of molecules with active internal degrees of freedom is found to increase the average number of collisions experienced by the rapidly desorbed molecules. However, the influence of this increased number of collisions on the focusing effects due to gas-phase collisions is less pronounced compared to the focusing effects due to collisions between the desorbed atoms. In a gas mixture containing molecules as the minor constituents (10%) and atoms as the major constituents (90%), atoms are found to be more focused towards the surface normal than the molecules and the mean translational energies of the molecules are found to be less than those calculated in the desorption of pure molecules under similar conditions. The presence of atoms in the desorbed gas mixture is found to increase the most probable speed of the desorbing molecules and this accelerating effect increases with decrease in the mass of the coadsorbed atoms. The light atoms are found to be more efficient than heavy atoms in cooling the internal degrees of freedom.
Green–Kubo formula for collisional relaxation
Authors: Visscher P. B.
Published: July 2002

In this paper we generalize the Green–Kubo method (usually used for obtaining formulas for transport coefficients involving conserved densities) to relaxation processes occurring during collisions, such as the transfer of energy from vibrational to translational modes in a molecular fluid. We show that the relaxation rate can be calculated without evaluating time correlation functions over long times, and can in fact be written as a sum over collisions which makes the relation between the Green–Kubo method and approximate independent-collision models much clearer.
Equations of change with dimer formation and decay
Authors: Sze P. K. I. , Ali J. , Snider R. F.
Published: July 2002

A kinetic theory of recombination and decay due to Lowry and Snider [J. Chem. Phys. 61, 2320 (1975)] is used as the basis for obtaining the equations of change for a molecular system undergoing recombination and decay. Because of the differing localizations of the hydrodynamic variables for the monomers and the dimers (the dimer center of mass is located halfway between the two monomers making up the dimer), the recombination and decay processes contribute explicitly to how the hydrodynamic fluxes are defined. This is particularly the case for the stream velocity appearing in the equation of continuity. The superoperators describing the recombination and decay collision processes naturally involve both reactive and collisional transfer contributions to the hydrodynamic fluxes. A sum rule for the collision superoperators is introduced which allows a logical separation of these effects to be made. Colliding molecules are allowed to be at different positions during a collision and this implies that kinetic energy is not conserved. An equation for the rate of change of potential energy density is obtained treating the collision processes in a manner analogous to the way the rates of change for the single molecule hydrodynamic variables are calculated. This is shown to give a consistent description for energy conservation.
A crossed-beam experiment on intramultiplet mixing collisions with short-lived Ne** {(2p)5(3p)} atoms
Authors: Manders M. P. I. , Ruyten W. M. , Beucken F. V. D. , Driessen J. P. J. , Veugelers W. J. T. , Kramer P. H. , Vredenbregt E. J. D. , van Hoek W. B. M. , Sandker G. J. , Beijerinck H. C. W. , Verhaar B. J.
Published: July 2002

We describe the design, operation, and calibration of a crossed-beam experiment for the study of intramultiplet mixing collisions of short-lived electronically excited Ne{(2p)5(3p)}≡{α} atoms with ground-state atoms/molecules. The excellent performance of almost 1 kHz/Å2 (number of counts per unit of inelastic cross section) enables us to measure, with good accuracy, absolute total Ne**–X cross sections Q‖Mk‖l←k, for the {α}k→{α}l transition. Here Mk is the magnetic quantum number of the electronic angular momentum J of the initial {α}k state with respect to the asymptotic relative velocity. The polarized {α}k state is produced with a polarized laser. Narrow-band interference filters are used to detect the fluorescence radiation from the short-lived {α}k and {α}l states. An extensive series of measurements has been undertaken to calibrate the experiment. These are related to, e.g., beam properties, the optical-pumping process, and the optical detection system. The basic principles of the collision experiment itself have been thoroughly examined as well. We discuss the kinds of experiments it is possible to perform. These have yielded absolute (within 30%) cross sections between 0.05 and 50 Å2. Very strong polarization effects have been observed, with 0.1≲Q‖0‖l←k/Q‖1‖l←k ≲10. The average collision energy has been varied between 50 and 250 meV (depending to some extent on the collision partner), by using a seeded primary beam and by manipulating the Newton diagram of primary- and secondary-beam velocity vectors. Time-of-flight measurements with a laser chopper have been performed as well. The wide range of Ne**-collision partners offers the option of studying intramultiplet mixing pure (He, Ne), and in conjunction with Penning ionization (noble gas atoms Ar, Kr, Xe) or even angular-momentum coupling and anisotropy effects (molecules, from H2 to CO2, N2O).
Vibrational relaxation and electronic quenching rate coefficients for BiF(A0+,v′) by SF6
Authors: Helvajian H. , Holloway J. S. , Koffend J. B.
Published: July 2002

We have extended our kinetic data base for the A state of BiF to include SF6 as a collision partner. By applying a four level finite difference fitting routine to time-resolved fluorescence traces from v′=0–3, we have obtained rate coefficients for electronic quenching, and for vibrational relaxation within the A-state manifold. As was the case for our previous results in He and Ar, electronic quenching is slow in comparison to radiative decay. Vibrational relaxation does not show a simple v′ dependence, but rather, for the levels examined, increases markedly for higher v′. Our results confirm the suitability of SF6 as a bulk diluent in the proposed NF/BiF chemical laser system.
Comparison of physical adsorption isotherms for planar and cylindrical lattices
Authors: Hock J. L. , McQuistan R. B.
Published: July 2002

The shift operator matrix (SOM) (from which the exact, composite, grand canonical partition function can be obtained) is used to study a system consisting of simple, nearest-neighbor-interacting particles distributed on a planar M×N lattice. Such an SOM is transformed into the corresponding SOM for a cylindrical lattice of the same dimensions. Utilizing these results, we calculate and compare the physical adsorption isotherms for a M×∞ planar lattice (which has two free edges) and for a cylindrical lattice of the same size (which has no free edges) for several particle–particle interaction potentials and for several values of M. By further modification of the SOM for planar lattices, we show explicitly that the differences between the physical adsorption on these two lattices are reflected in anomalous particle densities along the free edges of the planar lattice.
Bond ionicity of the halogen–silver interaction
Authors: Bagus Paul S. , Pacchioni Gianfranco , Philpott Michael R.
Published: July 2002

The nature of the bonding between halogen atoms (F, Cl, and Br) and the Ag (111) surface has been investigated by analyzing ab initio Hartree–Fock wave functions for cluster models of the Ag surface and a halogen atom. Using a variety of criteria, we conclude that the bonding is ionic and that the halogen ionicity is essentially −1. The measures of ionicity reported are (a) the expectation value of a projection operator which provides an indication of the total charge associated with the halogen atom, (b) the analysis of the dipole moment curve as function of distance, (c) the effect on the equilibrium bond distances of a uniform external electric field, and (d) the decomposition of the interaction energy into the sum of different contributions. This latter analysis shows that the bonding arises, almost entirely, from two effects: (1) the Coulomb attraction between the charged halogen and the metal and (2) the intraunit polarization of the metal and halogen subunits.
Model preparation of H2O hyperspherical modes by visible versus infrared multiphoton excitation
Authors: Jakubetz W. , Manz J. , Mohan V.
Published: July 2002

Multiphoton excitation of hyperspherical modes of H2O is simulated using the quasiresonant approximation (QRA) applied to a model of coupled Morse oscillators representing the two O–H stretches. Highly excited hyperspherical modes are more efficiently and more selectively populated by two-photon (vis) than by multiphoton (IR) transitions. The IR excitation mechanism is not dominated by a single ladder of exclusively hyperspherical levels, as in the case of a weakly bound ABA model system, but includes interfering pathways via nonhyperspherical modes, which often provide the dominant contributions. Vis two-photon excitation proceeds via one single (‘‘bond–bond’’) ladder, which to a first approximation is characterized by sequential excitation of the two individual O–H bonds by one photon each. Predicted experimental conditions for selective preparation of highly excited hyperspherical modes call for long (≫ps), intense (≥TW cm−2) laser pulses. These requirements are slightly less demanding than corresponding conditions for the preparation of comparably highly excited local modes.
Dissociative electron attachment to some chlorine-containing molecules
Authors: Petrović Z. Lj. , Wang W. C. , Lee L. C.
Published: July 2002

The electron-attachment rate constants of CH3Cl, C2H5Cl, and C2H3Cl in N2 and Ar were measured as a function of reduced electric field (E/N). These data and the previous data of SOCl2 and CCl2F2 were converted to the electron-attachment cross sections as a function of electron energy. The present results are compared with existing fragmentary data. The dissociative electron-attachment processes of the studied molecules are discussed.
A recursive solution to the time dependent Schrödinger equation using a generalized quasidegenerate Bloch formalism
Authors: Jolicard Georges
Published: July 2002

A unitary time evolution method for quantum systems under the influence of time dependent Hamiltonians is developed. The evolution scheme is iterated for additional time intervals by defining a new nonorthogonal basis at each step. The evolution over an elementary time interval is integrated using a time dependent Bloch effective Hamiltonian formalism. The method is tested on a triatomic molecule model under an intense monochromatic field. It is shown to be strictly unitary and appears stable and economical of storage.
The properties of CO and K coadsorbed on Pd(100) surface
Authors: Berkó András , Solymosi Frigyes
Published: July 2002

Low energy electron diffraction (LEED) measurements showed a few weak and two sharp patterns (2×2)R45° and (×)R45° at θK=0.25 and θK=0.50 (monolayer), respectively, for the K+CO coadsorbed layer. Preadsorbed potassium markedly increased the activation energy for CO desorption from 128 kJ/mol at θK=0 to 163 kJ/mol at θK=0.44 (monolayer). The sticking probability remained constant in the range θK=0.0–0.30, but radically decreased above this potassium coverage. The work function of the potassium dosed surface (θK=0.1–0.25) increased sharply following CO adsorption suggesting a high negative charge on CO molecule. A strong electronic interaction between CO and the K/Pd system was also revealed by electron energy loss spectroscopic (EELS) studies. This consisted of a significant decrease in the intensity of the CO loss observed on a clean Pd surface at 12.5–13.0 eV and the simultaneous development of a new loss feature at 24.5–25 eV.
Charge transfer states in phenothiazine crystal
Authors: Takase Tsugiko , Kotani Masahiro
Published: July 2002

Observation of charge transfer states in phenothiazine crystal which have lower energies than Frenkel excitons is reported. Two weak absorption peaks are observed at 15 K around 420 nm (2.9 eV) in the absorption edge, the absorption coefficient of which can be modulated up to a few percent by an external electric field. The modulation is so large, because the transitions are not overlapped with strong exciton absorption bands and susceptible to direct observation. The final states of the transitions have large dipole moments (∼50 D). The conductivity band gap is estimated to be 3.35 eV.
Variational calculations for metastable systems: Thermodynamic properties of glass transitions
Authors: Nakano H. , Qi D. W. , Wang S.
Published: July 2002

It is demonstrated, both theoretically and numerically, that the Gibbs–Bogoliubov (GB) thermodynamic variational calculation, developed originally for equilibrium systems, can be applied to a thermodynamic metastable system provided the structure of the reference system, used in this calculation, has the same main features as the metastable system under consideration. Following this, the GB thermodynamic variational calculation, employing the hard-sphere system as the reference system, is used to quantitatively determine the thermodynamic features of the metallic glass transition. Good agreement with observation is obtained.
Anharmonic effect of interaction between energetic particles and lattice atoms
Authors: Yi Li , Jianhua Zhang
Published: July 2002

Under consideration of anharmonic effect among lattice atoms, the interaction between incident particles and lattice atoms in ion implantation is discussed. And excited phonon spectrum is obtained by performing Fourier transformation on the interaction.
The rotational spectrum of the weakly bound dimer N2⋅⋅⋅HBr and the dynamics of the N2 and HBr subunits
Authors: Howard N. W. , Legon A. C.
Published: July 2002

The ground-state rotational spectra of the six isotopomers 14N15N⋅⋅⋅H79Br, 15N14N⋅⋅⋅H79Br, 15N2⋅⋅⋅H79Br, 15N2⋅⋅⋅H81Br, 15N2⋅⋅⋅D79Br, and 15N2⋅⋅⋅D81Br of a weakly bound dimer formed between nitrogen and hydrogen bromide have been detected by pulsed-nozzle, Fourier-transform microwave spectroscopy. The spectroscopic constants B0, DJ, χ(14N), χ(Br), and Mbb(Br) have been determined from analyses of the spectra and for the species 15N14N⋅⋅⋅H79Br take the values 1163.2477(5) MHz, 4.517(17) kHz, −4.302(7) MHz, 373.338(13) MHz, and −1.9(5) kHz, respectively. The χ(14N) and χ(Br) values have been corrected for the electrical contribution from the neighboring HBr and N2 subunits, respectively, with the aid of a simple electrostatic model and the corrected values used to determine the oscillation angles αav=cos−1〈cos2 α〉1/2=20.05° and βav=cos−1〈cos2 β〉 =25.45° for the N2 and HBr subunits. The value r(N⋅⋅⋅Br) =3.9474(2) Å has been established for the isotopomers containing H while the corresponding value is 3.9458 Å for D-containing species. The ratio βHav/βDav is discussed in terms of a simple model of the zero-point oscillations of the HBr and DBr subunits.
Molecular dynamics by numerical simulation in zeolites: Methane in NaA
Authors: Cohen De Lara E. , Kahn R. , Goulay A. M.
Published: July 2002

A molecular dynamical study of one methane molecule in a cavity of NaA zeolite is performed in order to compare calculated to experimental data obtained by infrared spectroscopy and neutron scattering experiments in the temperature range 300–30 K. The calculation shows the trajectory of the molecule in the cavity and then the occupied volume as a function of energy. It allows the calculation of average quantities and correlation functions: (i) the mean field felt by the molecule comparable to the field responsible for the induced infrared band ν1, (ii) the average of the potential energy (to be compared to the heat of adsorption) and of the velocity squared, (iii) the external frequency distribution, and (iv) the position autocorrelation function which is related to the dynamical structure factor seen by neutron scattering.
Spectra, radiative lifetimes, and band oscillator strengths of the A 1Π–X 1Σ+ transition of BH
Authors: Douglass C. H. , Nelson H. H. , Rice Jane K.
Published: July 2002

We report new spectroscopic information on the A 1Π–X 1Σ+ transition of BH including the observation of the 0–1 and 1–2 bands using laser induced fluorescence (LIF) techniques. Ratios of Einstein coefficients, band oscillator strengths and transition probabilities have been obtained for the 0–1 compared to the 0–0 band and the 1–0 and 1–2 compared to the 1–1 band. These ratios indicate that the emission observed occurs predominantly within the diagonal elements. Additionally, the radiative lifetimes of the v′=0, 1, and 2 levels have been measured to be 127±10, 146±12, and 172±14 ns, respectively. Using the ratios above and the experimental lifetimes, we have obtained Einstein emission coefficients and band absorption oscillator strengths. These values are compared to several calculations from the literature and in some instances large differences are seen.
Vacuum ultraviolet–visible double resonance spectroscopy of NO. Observation of the high excited n
s and n
d Rydberg series
Authors: Fujii Asuka , Ebata Takayuki , Ito Mitsuo
Published: July 2002

The two-color double resonance multiphoton ionization and fluorescence dip spectra due to the transitions from various rotational levels of the D 2Σ+ (v=1) state of NO to its high Rydberg states have been measured. Coherent vacuum ultraviolet (VUV) light generated by four wave mixing in Hg was used in the first excitation step. The ns(v=1) and nd(v=1) Rydberg series with 7≤n≤32 were observed. The rotational analysis for the d Rydberg series indicates that only the Π− component appears in the MPI spectra. A large dependence of the rotational constant upon n for the nd Π− Rydberg states was found and interpreted in terms of dΠ−–dΔ− mixing. An anomalous intensity distribution was also found for the rotational branches of the ns Rydberg states in the transition from the D 2Σ+ state. The anomaly is explained by the ns–(n−1)d mixing.
Brownian dynamics of freely jointed chains: Chain conformation with excluded volume effects
Authors: Parnas Richard S. , Cohen Yoram
Published: July 2002

A freely jointed bead-rod chain model was developed to describe the configurations of polymer chains in solution with and without excluded volume effects. The dynamic behavior of model chains was investigated with Brownian dynamics simulations and Fourier analysis. Numerical simulation results are in good agreement with the analytical solutions for the conformations of freely jointed chains, which are available only for chains without excluded volume effects. Stationary statistical results obtained by Brownian dynamics simulations show that excluded volume effects significantly expand the model chains in solution. Fourier analysis of computed numerical data demonstrate that configurational transitions occur over a broad range of time scales.
A study of the influence of an amphiphile monolayer on the structure of the supporting liquid
Authors: Cai Zhong-hou , Rice Stuart A.
Published: July 2002

We report the results of self-consistent Monte Carlo simulations of the influence of a close packed monolayer of C20H41OH on the structure of the surface of liquid Cs. The monolayer enhances the stratification of the longitudinal density distribution of Cs in the interface by an amount which can be detected in an x-ray reflectivity experiment. The transverse correlation in the Cs surface is also influenced by the monolayer, but not to a great extent; the transverse ordering in the outermost layer of atoms is found to be somewhat stronger than in bulk liquid Cs. All of these effects of the monolayer on the structure of the supporting liquid are naturally explained in terms of the interatomic forces and electronic structure of the system.
Intermolecular potential for vibrationally excited CH3F measured by light-induced drift
Authors: Hoogeveen R. W. M. , van der Meer G. J. , Hermans L. J. F. , Chapovsky P. L.
Published: July 2002

From changes in the diffusion coefficient upon vibrational excitation of CH3F in He, Ne, Ar, Kr, and Xe, as measured by light-induced drift, the intermolecular potential between two vibrationally excited CH3F molecules is determined on the basis of simple combination rules. The main effect of vibrational excitation is found to be an increase in potential well depth by 3%.
Dissociation of CD3I at 248 nm studied by diode laser absorption spectroscopy
Authors: Hall G. E. , Sears Trevor J. , Frye Joan M.
Published: July 2002

Transient diode laser absorption spectroscopy has been used to monitor the rotational and vibrational populations of CD3 radicals following photodissociation of CD3I by 248 nm light. Rotational lines in the ν2=1–0, 2–1, 3–2, and 4–3 bands have been analyzed to give time-dependent CD3 populations following photodissociation of 100 mTorr samples of neat CD3I. Prompt absorption signals are small, but positive for all observed rotational lines of all observed bands, providing clear evidence against population inversion in the low levels of the ν2 vibration of methyl photoproducts. The vibrational population distribution in ν2 a few μs after photodissociation is 4:3:2 for v=0:1:2 with about 20% uncertainties. A strong variation in time dependence with rotational level is consistent with an initially hot rotational distribution (≥2000 K) that cools on the μs time scale of methyl translational moderation in the bath of undissociated methyl iodide. We believe the origin of the rotational excitation to be translation–rotation energy transfer, rather than the photodissociation itself. Time-resolved Doppler line shapes provide an independent view of the translational energy loss.
A systematic study on the basis set superposition error in the calculation of interaction energies of systems of biological interest
Authors: Sordo J. A. , Sordo T. L. , Fernández G. M. , Gomperts R. , Chin S. , Clementi E.
Published: July 2002

A systematic study on the basis set superposition error in the calculation of interaction energies of strongly bonded molecular associations is presented. Twenty-four different basis sets (ranging from minimal to triple-zeta plus polarization) have been used to compute the interaction energies of four conformations of two alanine molecules interacting through their respective carboxylic groups giving rise to a double hydrogen-bonded association. The basis set superposition error has been calculated in all the cases by using the functional counterpoise method of Boys and Bernardi. It is shown that the basis set superposition error is still important at the double-zeta and valence triple-zeta levels of accuracy. In most cases the correction by the counterpoise method seems to be adequate providing interaction energies which, when compared with the uncorrected ones, are in better agreement with those coming from the largest basis set used in this work ( full triple-zeta plus polarization). The use of basis sets which have a relatively small basis set superposition error (GEOLARGE, GEOTRIPLEZ, 6-31G**, 6-31G**++(ss) , 13,8/5,3*], [13,8/5,3**], and GEOTRIPLEZ**) in projects involving systems of biological interest, require both, computational time and storage capacity, which by far exceed what is generally affordable. The use of appropriately designed minimal basis sets together with the counterpoise correction constitutes a reasonably good approach to such systems.
Scaling relations in the rotational excitation of NH(X 3∑−) N=0 by argon
Authors: Dagdigian Paul J.
Published: July 2002

Integral cross sections for rotational transitions out of the lowest rotational level of NH(3∑−)v=0, namely N=0, J=1 F1, induced by collisions with argon have been measured in a crossed beam experiment. A pulsed beam of rotationally cold NH(3∑−) was produced by 193 nm photolysis of a dilute mixture of ammonia in nitrogen seed gas at the tip of a nozzle, the argon beam was also prepared in a pulsed source. The final rotational and spin–state populations were interrogated in the collision zone by laser fluorescence excitation in the A 3Π←X 3∑−(1,0) band of NH. Within a given N′ manifold, the F1 spin level had the largest values for the cross sections out of N=0. The spin–state distributions are, in fact, well described by the general formulas for inelastic collisions of a case (b) 3∑ molecule. The derived spin–free tensor opacities show a rapid decrease as a function of increasing N′.
A concerted triple dissociation—the photochemistry of s-tetrazine
Authors: Zhao Xinsheng , Miller Walter B. , Hintsa Eric J. , Lee Yuan T.
Published: July 2002

The method of photofragment-translational spectroscopy was used in a molecular beam study of the photochemistry of s-tetrazine. Following 1B3u ←1Ag (S1←S0 excitation) or 1B2u ←1Ag (248 nm excitation), s-tetrazine reverts to the highly vibrationally excited ground electronic state through internal conversion, then decomposes into 2HCN+N2 via concerted triple dissociation. By analyzing the experimental data with a simple model, it was found that the potential energy surface along the reaction coordinate after the transition state is very repulsive, leading to most of the available energy, on average 73.9% in the case of S1 ←S0 excitation, going into translation. The asymptotic angles between N2 and HCN with respect to the center of mass of s-tetrazine were 117.2±0.5° and 114.4±0.5° for S1 ←S0 and 248 nm excitation, respectively.
Rotational–vibrational matrix elements for Kratzer oscillators
Authors: Francés J. M. , Zúñiga J. , Alacid M. , Requena A.
Published: July 2002

Several recursion relations connecting Kratzer oscillator matrix elements of different operators are derived using the hypervirial theorem. These relations can be easily used to calculate such matrix elements, either numerically or by developing analytical expressions. In particular, analytical expressions for the matrix elements of the xα and xαd/dx operators, where x is an internuclear separation coordinate, are obtained.
General transform technique including mode mixing and non-Condon coupling in resonance Raman scattering
Authors: Lu H. M. , Page J. B.
Published: July 2002

Using our exact finite temperature time-correlator model expressions for the optical absorption and resonance Raman (RR) excitation profiles including full linear plus quadratic electron–phonon coupling and linear non-Condon coupling, we have generalized the absorption→RR profile transform technique to normal modes that have both mode mixing (Dushinsky rotation) and linear non-Condon coupling. The new approximate transform provides a useful and convenient method for extracting information on mode mixing from measured RR excitation profiles. Compared with full model calculations, which are often prohibitively difficult for complex multimode systems owing to the fact that values of all of the model parameters are needed, the new transform is advantageous. This is because it allows one to compute a mode’s finite temperature profile using just the measured finite temperature absorption and model parameters of only the Raman mode plus those modes directly mixed with it. The information about the remaining modes is carried through the calculation via the use of the measured absorption. Numerical illustrations will be presented on the use of the new transform as a probe of mode mixing. Comparison with a previous transform relation which includes only mode mixing and is restricted to T=0 K will also be discussed.
Random electron paramagnetic resonance sampling of the modulated structure in incommensurate phases of betaine calcium chloride dihydrate
Authors: Fujimoto M. , Kotake Y.
Published: July 2002

The successive phase transitions in betaine calcium chloride dihydrate crystals were investigated in the range between 300 and 100 K with VO2+ and Mn2+ electron paramagnetic resonance (EPR) probes substituted for Ca2+ ions in the lattice. Preceded by B2u soft phonons in the normal phase, these transitions are continuous, and found to consist of stepwise changes of a collective librational fluctuation of the Ca–betaine complexes. Below the normal-to-incommensurate transition at 164 K, the fluctuation became slower than the EPR frequencies so that the spectra exhibited anomalous line shapes resulting from random sampling of the modulated structure. In addition, such anomalies as observed at 9.2 and 36 GHz indicated clear evidence for time-dependent lattice modulation in these phases. Sampling results by VO2+ and Mn2+ probes yielded complementary information about the modulated structure, which were interpreted by the plane-wave approximation of the anharmonic librational mode.
Infrared fluorescence from NO2 excited at 400–500 nm
Authors: McAndrew James J. F. , Preses Jack. M. , Weston Ralph E. , Flynn George W.
Published: July 2002

NO2 has been electronically excited to the 2B2/2B1 states, using pulsed dye laser radiation at 400–500 nm. Strong mixing of the electronically excited state with the ground electronic state (2A1) leads to highly vibrationally excited NO2(2A1), from which infrared emission has been observed. The time dependence of the IR fluorescence at several wavelengths has been observed, and quenching rate constants for NO2 and other gases have been measured. In addition to IR fluorescence at wavelengths identifiable as vibrational transitions (3.0–4.0, 6.1–6.8, 7.4–8.5, and 10.0–14.0 μm), emission at wavelengths <3.0 μm has been observed and attributed to a transition with electronic character. The emission observed in these experiments has been compared with that of chemiluminescent NO2 produced in the O+NO and O3+NO reactions.
A time-dependent variational principle and the time-dependent Hartree approximation in hydrodynamical form
Authors: Lill J. V. , Haftel M. I. , Herling G. H.
Published: July 2002

A quantum mechanical time-dependent variational principle is generalized using the classical theory of fluids to obtain a variational principle suitable for the fluid dynamical description of mixed state quantum mechanics. A newly derived set of moment equations, in both standard and renormalized form, can be derived with the aid of this principle through minimization of the error in expressing the total derivative of the Wigner function. Coupled systems are studied in the time-dependent Hartree (TDH) approximation using a novel variational principle, and the renormalization procedure used earlier in the examination of single particle dynamics is extended to the TDH analysis. Use of a local Maxwellian ansatz for each particle results in a particularly simple ‘‘two-fluid’’ theory, the TDH/LM approximation, which does not violate the standard and renormalized energy conservation theorems derived earlier for the single particle equations. The fluid dynamical TDH/LM approximation is shown to possess a simple semiclassical interpretation.
N(2D) production by two-photon NO photodissociation
Authors: Jusinski L. E. , Gadd G. E. , Black G. , Slanger T. G.
Published: July 2002

Photodissociation of NO has been investigated in the energy region between 8.89 eV, the energy required to produce N(2D)+O(3 P), and 9.26 eV, the ionization limit. Using the technique of resonance-enhanced multiphoton ionization (REMPI), we have demonstrated that there are areas in this energy region in which there are large yields of N(2D), and others where the yields are small. Localized concentrations, from 2-hν dissociation of NO at 270 nm, are an order of magnitude greater than can be obtained from a discharge in N2.
Dynamics of cluster scattering from surfaces
Authors: Xu Guo-Qin , Holland R. J. , Bernasek Steven L. , Tully John C.
Published: July 2002

We have carried out stochastic trajectory simulations of the scattering of argon clusters, Arn, n=5 to 26, from a Pt(111) surface. At incident energies of 0.1 and 0.5 eV per atom, the clusters fragment almost completely into individual atoms. Some atoms remain trapped on the surface, but the majority scatter. For the larger clusters, the angular distributions of the scattered atoms peak near the surface tangent, and are almost independent of the incident angle. The mean kinetic energy of atoms is largest for those scattered near the surface tangent, and decreases as the scattering angle approaches the surface normal. These trends are in qualitative accord with the results of experimental studies of cluster scattering from surfaces. A multiple binary collision mechanism involving atom–atom collisions within the cluster as well as atom–surface collisions is responsible for this behavior.
Sub-Doppler rotationally resolved overtone spectroscopy of the HCN dimer
Authors: Meyer H. , Kerstel E. R. Th. , Zhuang D. , Scoles G.
Published: July 2002

A new molecular beam color center laser spectrometer, which can be tuned single mode under computer control around 1.5 μm, has been used to obtain the sub-Doppler, rotationally resolved, first overtone spectrum of the nonbonded C–H stretch vibration of the HCN dimer. As for the corresponding fundamental vibration, the linewidth is instrument limited, posing a minimum limit of 11 ns to the lifetime of the excited complex.
A new scheme for determination of the carrier mobility and main-chain conformation of an electroactive polymer chain by transient electric birefringence
Authors: Teraoka Iwao , Hayakawa Reinosuke
Published: July 2002

A new scheme of experimental criterion is proposed for estimating the carrier mobility and main-chain conformation of an electroactive polymer chain by the technique of transient electric birefringence (TEB). The rise response Δnr (t) and the decay response Δnd (t) in the TEB satisfy in the Kerr regime (i) Δnr (t)=Δnst -Δnd (t) for a polymer molecule of arbitrary conformation with an induced dipole moment alone arising from carriers highly mobile along the main chain, (ii) Δnr (t)=Δnst -(3/2)Δnd (t/3)+(1/2)Δnd (t) for a rodlike polymer molecule with a permanent dipole moment alone originating from carriers with low mobility, and (iii) Δnr(t)=Δnst-2Δnd (t/2)+Δnd (t) for a random-coiled polymer with a permanent dipole moment alone due to low-mobility carriers, where Δnst =limt→∞ Δnr(t) . Then, comparison of the TEB rise and decay responses gives us information on the carrier mobility and main-chain conformation. This criterion is valid also for polydispersed polymer samples. By applying the criterion to the exemplifying data of the TEB responses for soluble urethane-substituted polydiacetylenes, it is found that the polydiacetylene molecules take a random-coil conformation with a permanent dipole moment in a good solvent, while in a poor solvent the carrier mobility depends crucially on the solvent condition.
Quantum state-resolved study of pure rotational excitation of CO2 by hot atoms
Authors: Hershberger John F. , Hewitt Scott A. , Sarkar Sisir K. , Flynn George W. , Weston Ralph E.
Published: July 2002

Rotationally inelastic scattering of carbon dioxide by translationally hot H, D, and Cl atoms was studied by time-resolved diode laser absorption. The high J rotational distribution falls off quite rapidly between J=60 and J=80. D atom collisions have roughly twice the excitation cross section versus H atom collisions, with the H*/D* ratio decreasing with increasing J. These results are consistent with a constraint on the total reagent orbital angular momentum available for rotational excitation. Transient Doppler profiles measured immediately after hot atom/CO2 collisions indicate that CO2 molecules excited to high J levels have a larger recoil velocity than molecules excited to lower J levels. This result is consistent with predictions based on a simple model which treats the CO2 potential as a hard shell ellipsoid.
Multiple transition states in chemical reactions: Variational transition state theory studies of the HO2 and HeH+2 systems
Authors: Song Kihyung , Chesnavich Walter J.
Published: July 2002

The results of a microcanonical variational transition state theory study of the HO2 and HeH+2 systems are reported. The calculations were carried out using a modification of the Wardlaw–Marcus flexible transition state theory method in which the sum of states N(E,R) is calculated directly. For the HO2 system, the results obtained using the Melius–Blint

potential surface are in excellent agreement with previously reported variational transition state theory calculations of Rai and Truhlar on this system. In particular, two transition states were found for the HO2→OH+O half-reaction. However, calculations using the Lemon–Hase surface produced only one transition state. From calculations carried out on surfaces constructed by combining the two original surfaces and calculations carried out using an orbiting transition state variational model (i.e., ignoring the angular part of the potential), it is shown conclusively that the two transition states found for the Melius–Blint surface arise from the peculiar shape of the minimum reaction path for this surface. Also, the orbiting transition state calculations

suggest that phase space theory can describe this half-reaction on the Lemon–Hase surface over a large energy range provided that the potential along the minimum reaction path is used in the phase space theory calculations. Other calculations, in which the parameters of the Lemon–Hase surface were adjusted, were also carried out in order to determine the conditions necessary to support multiple transition states. The results of these calculations suggest that multiple transition states are more likely to occur when a strong bottleneck is present in the angular part of the potential, especially when the intermediate complex exists in a shallow well. These predictions were confirmed by the

results obtained for the HeH+2 system using the McLaughlin–Thompson–Joseph–Sathyamurthy surface. Two transition states were found for the HeH+2→HeH++H half-reaction and calculations using the orbiting model showed that the origin of the multiple transition

states is not the minimum reaction path as in the case of the Melius–Blint surface. Furthermore, the energy at which ‘‘transition state switching’’ occurs is in reasonable accord with the energy at which phase space theory calculations begin to diverge from the experimental data on the cross section for this system. Lifetime calculations for the pseudocomplex lying between the two transition states suggest that the observation of multiple transition states may be possible using femtosecond transition state spectroscopy. Finally, calculations on the HeH+2→He+H+2 half-reaction showed that one transition state exists for this half-reaction at low energy, while at higher energy there are no transition states.
Comment on ‘‘A theoretical study of the dissociation energy of BH using quadratic configuration interaction’’
Authors: Martin J. M. L. , François J. P. , Gijbels R.
Published: July 2002

The dissociation energy of boron hydride has been calculated ab initio using a very accurate theoretical model, with an expected accuracy of ±0.2 kcal mol−1. The values of 85.7 and 82.4 kcal mol−1 for De and D0, respectively, confirm the conclusion of Curtiss and Pople [J. Chem. Phys. 90, 2522 (1989)], that the predissociation limit of 82.6±0.4 kcal mol−1 might be close to the actual dissociation energy, and that an earlier proposed De of 78.9 kcal mol−1 is in error.
Two-dimensional spectroscopy: Theory of nonstationary, time-dependent absorption and its application to femtosecond processes
Authors: Fain B. , Lin S. H. , Hamer N.
Published: July 2002

In this paper, we present a formalism for a generalized linear susceptibility applicable to femtosecond (fs) processes. As an application, we analyze the recent experimental results of the real-time fs transition spectra of the photodissociation of NaI and NaBr reported by Zewail et al. In this connection we also investigate in general the behaviors of decaying and trapping of the molecule in an excited electronic state coupled to a continuum.
Azeotropic states in the CO2–C2H6 mixture from the hard sphere Lennard-Jones quadrupolar molecular model
Authors: Scalise O. H. , Gianotti R. D. , Zarragoicoechea G. J. , Rodríguez A. E.
Published: July 2002

Azeotropic states and phase equilibria for the CO2–C2H6 mixture are studied by the application of thermodynamic perturbation theory to the hard sphere Lennard-Jones quadrupolar molecular model. It is shown that theory can predict both phase equilibria and the azeotropic qualitative behavior observed in the real mixture.
Computer simulations of monolayer and bilayer nitrogen films at low temperature
Authors: Bhethanabotla Venkat R. , Steele W. A.
Published: July 2002

Computer simulations are reported for monolayer and bilayer films of nitrogen adsorbed on the basal plane of graphite at temperatures ranging from 15 to 40 K. At these temperatures, the simulations primarily yield reliable information about orientational properties. (The large translational displacements needed to achieve equilibrium layer densities in these solid films do not occur on the time scale of the simulations.) Layer densities were chosen which give commensurate and uniaxial bilayers but most of the studies were concerned with the orientational structure of the high density TI layers. To this end, both monolayer and bilayer films were simulated by choosing lattice spacings and symmetries corresponding to those deduced from recent neutron diffraction experiments. The results of the simulations give clear evidence for the occurrence of pinwheel orientational structure in these films at 15 and 25 K. It is also shown that the pinwheel centers form disordered arrays which have considerable mobility on the picosecond time scale. At 40 K, thermal agitation washes out much of this structure. Results for the commensurate and uniaxial bilayer films are in good agreement with earlier work, although a few pinwheels can be seen in the second layers which were not observed in the previous study.
Bimodal kinetic energy release in the unimolecular dissociation of energy-selected formic acid ion
Authors: Nishimura Toshihide , Meisels G. G. , Niwa Yoshio
Published: July 2002

Dissociation of HCOOH+ and DCOOH+ have been studied using threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The detailed breakdown diagrams were obtained over internal energies of the molecular ion less than 8 eV. Kinetic energy release distributions (KERDs) for HCO+ and DCO+ were extracted from their flight time distributions by using a multiple linear regression method. The breakdown curve of HCO+ not only exhibited several maxima but also the surprisal analysis of its KERD demonstrated a considerable deviation from a single statistical distribution. Bimodal KERDs were observed for the formation of HCO+ and DCO+. The large KER component results from dissociation to the products in the ground state, HCO+ (1Σ+)+⋅OH (2Πi). Average kinetic energy releases observed for this process are, however, about twice higher than those calculated by QET/RRKM. The small KER component shows average KERs remarkably smaller than the statistical expectations and this value remains constant over the energy range of 4.5 eV. The origin of this small KER component was ascribed to dissociation after fluorescent transition of initial vibronic states to the first excited state (1 2A″). In contrast with these results average KERs measured for H2O+ and HDO+ are in good agreement with the RRKM/QET calculations.
Direct determination of scattering time delays using the R-matrix propagation method
Authors: Walker Robert B. , Hayes Edward F.
Published: July 2002

A direct method for determining time delays for scattering processes is developed using the R-matrix propagation method. The procedure involves the simultaneous generation of the global R matrix and its energy derivative. The necessary expressions to obtain the energy derivative of the S matrix are relatively simple and involve many of the same matrix elements required for the R-matrix propagation method. This method is applied to a simple model for a chemical reaction that displays sharp resonance features. The test results of the direct method are shown to be in excellent agreement with the traditional numerical differentiation method for scattering energies near the resonance energy. However, for sharp resonances the numerical differentiation method requires calculation of the S-matrix elements at many closely spaced energies. Since the direct method presented here involves calculations at only a single energy, one is able to generate accurate energy derivatives and time delays much more efficiently and reliably.
A Green–Kubo formula for the sedimentation coefficients
Authors: Raineri Fernando O. , Timmermann Ernesto O.
Published: July 2002

By applying linear response theory to mechanical perturbations, a new Green–Kubo-type formula for the sedimentation coefficients is derived. All species are treated on the same footing, and the dependence of the sedimentation coefficient on the reference frame is carefully taken into account. The connection between the sedimentation and the interdiffusion (or mutual diffusion) coefficients is examined from a microscopic point of view for a two component system, and it is shown that the Svedberg relation follows from the present results by a simple microscopic reference frame transformation.
On the laser-wavelength dependence of plasmon surface polariton field-enhanced Raman spectroscopy
Authors: Knobloch Harald , Duschl Claus , Knoll Wolfgang
Published: July 2002

We analyze plasmon surface polariton field-enhanced Raman spectroscopic data for the CH stretching vibrational range (2800–3000 cm−1) of cadmium arachidate multilayer assemblies, taken as a function of the exciting laser wavelength λL. In addition to the well-known λ−4 dependence of ordinary (photon) Raman scattering we find a variation of the intensity ratio of the methylene antisymmetric to symmetric stretching band (νa=2885 cm−1 and νs=2850 cm−1, respectively) which can be attributed quantitatively to a specific surface plasmon field property: a longitudinal field component parallel to the propagation direction adds to the dominant perpendicular field with increasing relative strength for decreasing wavelength.
The branching of nonradiative processes in isoquinoline
Authors: Sneh Ofer , Amirav Aviv , Cheshnovsky Ori
Published: July 2002

We have measured the absolute rates of internal conversion and of intersystem crossing in isoquinoline excited to its S2 vibronic states (3131–3062 Å) in supersonic beams. The combination of direct detection of the triplets, absorption and excitation spectra allowed us to extract these rates. The intersystem crossing rate barely depends on the excess vibrational energy in the S2 manifold, being about 2×109 s−1. The internal conversion rates grow by more than two orders of magnitude over the range of 0–800 cm−1 of excess vibrational energy in the S2 state, up to 5×1010 s−1. At the S2 electronic origin intersystem crossing is the dominant nonradiative channel, while at vibrational energy exceeding 400 cm−1 internal conversion becomes dominant.