bluuebunny/crossref_metadata_2025 huggingface dataset:
Complex angular momentum theory of molecular collisions: New phase rules for rotationally inelastic diffraction scattering in atom–homonuclear diatomic molecule collisions
Authors: McCabe P. , Connor J. N. L. , Thylwe K.-E.
Published: July 2002

New phase rules have been derived for the rotationally inelastic diffraction scattering of an atom with a homonuclear diatomic molecule. The phase rules are obtained by applying semiclassical complex angular momentum techniques to the infinite order sudden approximation. The new phase rules are of three types, which are denoted weak coupling, intermediate coupling, and strong coupling. Higher-order terms have been derived for each type of phase rule. A systematic computational study of the accuracy of the phase rules has been carried out for a deformed inverse power potential energy of the form ε[r(γ)/r]n, n≥3, where r(γ)=r0+r2P2(cos γ). Additional calculations are reported for the Ne–D2 and He–N2 collision systems, using more realistic interaction potential energies.
Absorption spectroscopy of nonlinear excitations in polyaniline
Authors: Sariciftci N. S. , Smilowitz L. , Cao Y. , Heeger A. J.
Published: July 2002

We present near steady state photoinduced absorption spectra of two forms of polyaniline. Both the emeraldine base (nondegenerate ground state) and pernigraniline (degenerate ground state) exhibit long lived photoinduced nonlinear excitations with transient absorptions within the π→π* gap. In the case of the emeraldine base, two well-resolved photoinduced absorption bands are observed with maxima at 0.9 and 1.47 eV, accompanied by a bleaching of the 2 eV absorption edge. The different relaxation rates of the two bands indicate that they arise from two independent photoinduced nonlinear excitations. The results are discussed in terms of intrachain and interchain excitations. The possibility of deep traps on defect sites in pernigraniline is suggested.
High resolution laser spectroscopy of asymmetrically deuterated cyclopentadienyl radicals: A study of vibronic degeneracy resolution and Jahn–Teller distortion
Authors: Yu Lian , Cullin David W. , Williamson James M. , Miller Terry A.
Published: July 2002

The rotationally resolved, laser induced fluorescence, excitation spectra of the partially deuterated cyclopentadienyl radicals, C5H4D and C5HD4, have been observed at low temperature in a supersonic free jet expansion. The observed electronic transition in the uv region corresponds to the Ã 2A″2■X̃ 2E″1 transition in the symmetric cyclopentadienyl isotopomers with D5h symmetry. In the reduced C2v symmetry of the C5HD4 and C5H4D isotopomers, this electronic transition splits into two distinct vibronic bands, separated by about 9 cm−1, which arise from the two vibronic components X̃1 and X̃2 into which the X̃ state is resolved when the symmetry is lowered. In C5H4D the ground X̃1 state has 2A2 symmetry and a permanently distorted, elongated allyl-like structure while the low-lying X̃2 state has 2B2 symmetry and a compressed dienelike structure. The symmetries of the energy levels and the distortions are reversed for the C5HD4 species. A detailed theoretical model is developed to describe the splitting and the rotational structure of the X̃1 and X̃2 states. Application of this model yields a precise value for the alternation of the C–C bond lengths in the distorted structures. This value is shown to be the same as that describing the dynamically pseudorotating, Jahn–Teller distorted structures in the D5h isotopomers. Additionally, the theory explains anomalous values of the observed inertial defects and relates them to physically meaningful quantities for all the isotopomers in both their X̃1 and X̃2 states. Our results are compared to previous experimental work and ab initio calculations on the cyclopentadienyl radical.
Persistent infrared holeburning spectroscopy of (NH4)2SO4–K2SO4 mixed crystals
Authors: Cho Han-Gook , Strauss H. L.
Published: July 2002

Infrared holeburning has been observed from mixed crystals. NH3D+ ions were imbedded in (NH4)2SO4–K2SO4 mixed crystals, the sample irradiated at the N–D stretching frequency, and resulting spectral hole then continuously monitored by a Fourier-transform infrared (FTIR) spectrometer. The holewidth decreases proportionally to the center frequency of the hole; within the range of 2240–2300 cm−1, the width changes from 10 to 5 cm−1 at 7.5 K. A longer irradiation time (≳10 min), however, leads to a wider spectral hole and also comparatively larger antiholes. The holewidth also shows a T2 type increase with temperature over the range of 7.5–45 K. Decay of the hole (hole filling) is not exponential, indicating that there is a distribution of decay rates. The holeburning quantum efficiency remains almost unchanged with temperature. Similarly, it remains almost constant with the variation of the mole-fraction of (NH4)2SO4 in the solid solution [in the range of 5%–100% mole-fraction of (NH4)2SO4], while the hole decay rate varies more than 20-fold. The constant holeburning quantum efficiency is surprising, considering that the phonon density increases by several orders of magnitude in the experimental temperature range and that the variation of the (NH4)2SO4 concentration changes the height of the rotational barrier of the ammonium ion. Thus, the holeburning must occur by conformational change in the excited vibrational state.
The phenol dimer: Zero-kinetic-energy photoelectron and two-color resonance-enhanced multiphoton ionization spectroscopy
Authors: Dopfer Otto , Lembach Gerhard , Wright Timothy G. , Müller-Dethlefs Klaus
Published: July 2002

The two-color, two-photon (1+1′) resonance-enhanced multiphoton ionization spectrum of the hydrogen-bonded phenol dimer has been recorded in reasonable agreement with previously reported spectra. However, more features are obtained in this work and this has allowed a more detailed analysis of the spectrum. Five intermolecular vibrations (out of a possible six) are observed for the S1donor state, while only two modes are obtained for the S1acceptor state. Zero-kinetic-energy (ZEKE) photoelectron spectra were recorded via different intermediate vibronic states. The spectrum recorded via the vibrationless level of the S1donor state is rich in structure and indicates a large change in the geometry on ionization. Progressions in the intermolecular stretch mode and at least one other mode are obtained. ZEKE spectra were also recorded via a number of S1donor vibronic levels, and the S1acceptor vibrationless level. The lowest value measured for the ionization energy of the donor is 63 649±4 cm−1 (7.8915±0.0005 eV); this is over 2000 cm−1 lower than the previously reported value. No structure is resolved in the ZEKE spectrum of the acceptor, and it is suggested that this could be due to rapid internal conversion between the S1acceptor and S1donor states.
Floquet theory response of two- and three-level systems interacting with pulsed electric fields
Authors: Kavanaugh Theresa C. , Silbey Robert J.
Published: July 2002

Using Floquet theory together with a density matrix formalism, analytic expressions are derived that describe a typical pump–probe experiment on (a) a two-level, one-photon resonant model that mimics a system with two electronic states, and (b) a two-photon resonant three-level model. The time dependence of the amplitude of the pulsed electric fields is approximated by a square envelope. These expressions, which include all orders of the electric field, can be used to describe systems interacting with very intense fields, where perturbation theory fails. This description allows variation of both the order of the pump and probe pulse, as well as the pulse durations. These expressions, once expanded, are equivalent to the usual perturbative series expansion, within the square-envelope approximation. Using a δ function approximation for the pulsed fields, a two-level system with an excited state vibrational manifold is also considered in a nonperturbative manner.
Temperature dependent kinetics of the formation and self-reactions of FC(O)O2 and FC(O)O radicals
Authors: Maricq M. Matti , Szente Joseph J. , Khitrov Gregory A. , Francisco Joseph S.
Published: July 2002

This paper presents the ultraviolet (UV) absorption spectrum of FC(O)O2 and the temperature dependent rate constants for its formation via the addition of O2 to FCO, its self-reaction to form FC(O)O, and for the subsequent dimerization of FC(O)O. The UV spectrum of FC(O)O2 shows two absorption bands in the 190–300 nm range. The lower energy band has a peak cross section of (3.0±0.3)×10−18 cm2 at 232 nm. A higher energy band begins at ∼210 nm and reaches an absorption cross section of 4.3×10−18 cm2 at 190 nm. The FC(O)O2 self-reaction exhibits a negative temperature dependence with k2(T)=(2.5±0.4)×10−12 e(286±40)/T cm3 s−1 over the 213–358 K temperature range. The oxygen addition to FCO has a rate constant of k1=(8.0±0.8)×10−13 cm3 s−1 and the FC(O)O dimerization rate constant is in the range k3a=(2.3–6.5)×10−12 cm3 s−1, at 293 K and 300 Torr total pressure. Both of these rate constants show little variation over the 213–358 K temperature range.
Memory and temperature induced suppression of activated rate processes
Authors: Frishman Anatoli M. , Pollak Eli
Published: July 2002

A detailed study of memory and temperature induced suppression of activated rate processes is presented. Numerical computations demonstrate that long memory in the presence of moderate barriers can induce noticeable deviation of the reaction rate constant from the predictions of the Kramers–Grote–Hynes theory. A canonical variational transition state theory, based on finding the optimized planar dividing surface, is shown to account quantitatively for the observed suppression of the rate. The suppression is associated with an almost perpendicular rotation of the optimal dividing surface away from the usual one. A further generalization of the Pollak–Grabert–Hänggi theory for the Kramers turnover is presented and shown to account for the computed rate constants for the whole range of damping at a fixed bath memory time.
High energy transition state resonances in the H+H2 reaction
Authors: Sadeghi R. , Skodje Rex T.
Published: July 2002

We have identified a progression of 13 high energy transition state resonances in the collinear H+H2 system. Using a new method, we have obtained the position, width, and wave function for each resonance state. Unexpectedly, the resonances are more stable at high energies.
A sum-over-states formulation of the diamagnetic contribution to the indirect nuclear spin–spin coupling constant
Authors: Sauer Stephan P. A.
Published: July 2002

Using an operator identity, a sum-over-states expression for the diamagnetic contribution to the indirect nuclear spin–spin coupling constant is given. As the three other contributions also involve similar sum-over-states expressions, it thus becomes possible to apply the same method to the calculation of all contributions.
Three-dimensional quantum-mechanical computations of the control of the H+OD←DOH→D+OH reaction
Authors: Shapiro Moshe , Brumer Paul
Published: July 2002

A three-dimensional quantum mechanical study of the control of a branching photochemical reaction H+OD←DOH→D+OH is presented. It is shown that with two laser pulses, one pulse used to generate a superposition of the (0,2,0) and the (1,0,0) states, and the other, a VUV pulse, used to dissociate the molecule by exciting it to the B continuum, it is possible to control which of two chemical channels is preferred. The control parameters used are the center frequency of the excitation laser and the time delay between the two pulses. For the above superposition state, a combination of a 200 fs excitation pulse and a 50 fs dissociation pulse is found to yield the widest (10% to 90% yield in the H+OD channel) range of control, essentially irrespective of the photolysis wavelength.
Is a triplet HC4N molecule linear?
Authors: Aoki Kozo , Ikuta Shigeru
Published: July 2002

The purpose of this note is to present an ab initio calculation on the energy difference between the triplet linear and bent HC4N molecules.
Estimation of liquid/liquid interfacial tensions of metallic mixtures using two-scale-factor universality
Authors: Kreuser H. , Woermann D.
Published: July 2002

The two-scale-factor universality hypothesis of critical phenomena is used to estimate the liquid/liquid interfacial tension near the critical point of binary alloys with miscibility gap. Experimental results of measurements of the correlation length of local concentration fluctuations (data for the systems cadmium/gallium and sodium/lithium taken from the literature), and experimental results of measurements of the temperature dependence of the molar heat capacity at constant pressure (data for the system bismuth/gallium taken from the literature) are used for this illustration.
A theoretical model of scanning tunneling microscopy: Application to the graphite (0001) and Au(111) surfaces
Authors: Ou-Yang Hui , Källebring Bruno , Marcus R. A.
Published: July 2002

An expression for the scanning tunneling microscopy (STM) current between the tip and sample is presented using first-order perturbation theory for a two-Hamiltonian formalism (‘‘reactants’’ and ‘‘products’’). The calculated STM current depends on the square of the sample-tip matrix elements, averaged over a selection of random points in wave vector space. In the limit of low voltage and temperature, this averaging is over the Fermi surface of the sample. The model is applied to the graphite (0001) and Au(111) surfaces using a simple model (chain) of a tungsten tip and the tight-binding approximation. Comparisons with experiments and with the result for graphite obtained by Tersoff and Lang using a molybdenum tip are given. The theory is applied elsewhere to STM of adsorbates.
A theoretical study of the initial stages of Si(111)–7×7 oxidation. II. The dissociated state and formation of SiO4
Authors: Schubert Boris , Avouris Phaedon , Hoffmann Roald
Published: July 2002

In a preceding paper (referred to as part I) we presented a theoretical study of the initial stages of the oxidation of the Si(111). Specifically, we discussed the structure of the molecular precursors and the stable products, using approximate molecular orbital calculations of the extended Hückel type. In this contribution (part II) we propose pathways which lead from one oxygen chemisorption configuration to another. A scheme is constructed to account for the reaction sequence from the clean Si(111) surface to molecular precursors, through dissociated states and finally to SiO4 units which are precursors for bulklike SiO2.
Dynamic Monte Carlo simulation of spin-lattice relaxation of quadrupolar nuclei in solids. Oxygen-17 in yttria-doped ceria
Authors: Adler Stuart B. , Smith Jacob W. , Reimer Jeffrey A.
Published: July 2002

Although measurement of spin-lattice relation time (T1) can provide valuable information about atomic motion in solids, T1 data for quadrupolar nuclei are often difficult to interpret because the relevant physical interactions cannot be expressed analytically. In order to address this problem, we have developed an extension to the dynamic Monte Carlo method for simulating spin-lattice relaxation of quadrupolar nuclei in solids. In this paper we develop the simulation method generally, and then apply the method to understanding published T1 measurements of oxygen-17 in yttria-doped ceria. We show that even for simple geometries of motion, multiple time scales for electric field gradient fluctuations are possible, resulting in complex relaxation behavior including multiple T1 minima. The method can be used to explain and deconvolute data in which these effects are present. In the case of yttria-doped ceria, we show that two experimentally observed T1 minima result from simultaneous movement of oxygen vacancies and oxygen ions. We discuss the meaning of the kinetic parameters extracted from the data, and show evidence for the relevance of these parameters to bulk vacancy diffusion kinetics.
Photodecomposition dynamics of Mo(CO)6/Si(111) 7×7: CO internal state and translational energy distributions
Authors: Buntin Steven A. , Cavanagh Richard R. , Richter Lee J.
Published: July 2002

The rotational state and translational energy distributions of CO photodesorption products resulting from the 266 nm photolysis of Mo(CO)6 adsorbed on Si(111) 7×7 with coverages in the multilayer regime are reported. State-resolved measurements show two desorption components with highly disparate energy dispositions. Results for different surface temperatures indicate that the energy content in one component reaches quasi-equilibration with the surface temperature, which is attributed to collisional relaxation of nascent photodecomposition products within the adlayer. The other component exhibits disparate rotational and translational ‘‘temperatures’’ that are significantly greater than, and independent of, the surface temperature. These nascent photodecomposition products are influenced by both energy quenching effects and dynamical constraints imposed by the existence of the adlayer.
Self-diffusion in isotropic and nematic phases of highly elongated hard particles
Authors: Tang Shuang , Evans Glenn T.
Published: July 2002

Velocity time correlation functions (tcfs) and self-diffusion coefficients have been derived by means of an Enskog kinetic theory for hard ellipsoids in isotropic and in nematic environments. The tcfs, calculated on the basis of kinetic theory, track those calculated by Allen by molecular dynamics simulations and show evidence of two exponential decays which we attribute to the differing rates of diffusion parallel and perpendicular to the ellipsoid principle axes. D/DE the ratio of the molecular dynamics (MD) self-diffusion coefficient to our Enskog estimate has a density dependence characteristic of hard sphere systems when diffusion is perpendicular to the macroscopic director; however, diffusion along the director axis is enhanced and this behavior has no hard sphere analog. In the isotropic phase, D/DE follows the same trend as documented previously by Alder in hard sphere systems.
Energy dependencies of fast reactions of positive ions X+ with HCl from near thermal to ≂2 eV center-of-mass collision energy (X+=H+,H2+,H3+,N+,N2+,Ar+,C+,CH+, CH2+,CH3+,CH4+,CH5+)
Authors: Glosik J. , Freysinger W. , Hansel A. , Spanel P. , Lindinger W.
Published: July 2002

The rate coefficients for the reactions of the ions H+, H2+, H3+, N+, N2+, Ar+, C+, CH+, CH2+, CH3+, CH4+, and CH5+ with HCl have been measured as a function of reactant ion/reactant neutral average center-of-mass kinetic energy (KEc.m.). The measurements were performed using a selected ion flow drift tube operated at 300 K with helium buffer gas. Most of these reactions have rate coefficients close to their collisional values at ambient temperature and all of them exhibit a pronounced KEc.m. dependence. A simple model is proposed, in which the measured dependence of the rate coefficients on KEc.m. is expressed in terms of the kinetic-energy dependence of the rate unimolecular decomposition of the intermediate complex formed in the ion-neutral collision.
On the use of time domain methods to study the excitation of a molecule by a strong, long laser pulse
Authors: Bavli Raanan , Metiu Horia
Published: July 2002

Calculations which solve the time dependent Schrödinger equation on a grid are generally useful for studying the cw excitation of a molecule by a weak laser or the excitation by a short, strong laser pulse. Here we show how time dependent methods can be applied to study excitation by strong, long pulses. Moreover, even though the molecule is driven by a time dependent field, one can identify energy eigenstates that provide the time scales on which various observables evolve. The calculations which illustrate the methodology are performed on a model of the bound-to-bound electronic excitation of the I2 molecule. We study the dependence of the excitation probability on time, power, and laser frequency. We find that the I2 molecule driven by a strong laser emits at many frequencies below and above the laser frequency.
Quantum steam tables. Free energy calculations for H2O, D2O, H2S, and H2Se by adaptively optimized Monte Carlo Fourier path integrals
Authors: Topper Robert Q. , Zhang Qi , Liu Yi-Ping , Truhlar Donald G.
Published: July 2002

Converged quantum mechanical vibrational–rotational partition functions and free energies are calculated using realistic potential energy surfaces for several chalcogen dihydrides (H2O, D2O, H2S, H2Se) over a wide range of temperatures (600–4000 K). We employ an adaptively optimized Monte Carlo integration scheme for computing vibrational–rotational partition functions by the Fourier path-integral method. The partition functions and free energies calculated in this way are compared to approximate calculations that assume the separation of vibrational motions from rotational motions. In the approximate calculations, rotations are treated as those of a classical rigid rotator, and vibrations are treated by perturbation theory methods or by the harmonic oscillator model. We find that the perturbation theory treatments yield molecular partition functions which agree closely overall (within ∼7%) with the fully coupled accurate calculations, and these treatments reduce the errors by about a factor of 2 compared to the independent-mode harmonic oscillator model (with errors of ∼16%). These calculations indicate that vibrational anharmonicity and mode–mode coupling effects are significant, but that they may be treated with useful accuracy by perturbation theory for these molecules. The quantal free energies for gaseous water agree well with previously available approximate values for this well studied molecule, and similarly accurate values are also presented for the less well studied D2O, H2S, and H2Se.
Theoretical studies of energy transfer and reaction in H+H2O and H+D2O collisions
Authors: Kudla Kathleen , Schatz George C.
Published: July 2002

We present the results of a quasiclassical trajectory study of vibration–rotation excitation and reaction in H+H2O(000) and H+D2O(000) collisions, including detailed comparisons with experiment. All calculations have used a semiempirical potential surface due to Schatz and Elgersma, and the H2O initial and final states were numerically determined by solving for the good action variables associated with vibrational motions. Our studies of collisional excitation emphasize comparisons with recent experiments by Lovejoy, Goldfarb, and Leone [J. Chem. Phys. 96, 7180 (1992)] in which fast hydrogen atoms produce vibrationally and rotationally excited water. As in the experiments, we find a propensity for the production of rotational states in which the rotational angular momentum vector is predominantly aligned perpendicular to the water molecule plane (c-axis excitation). This propensity is found for all excited vibrational states of H2O, but it is significantly stronger in the experiments [where only the (001) state was studied] than in the calculations. An analysis of trajectory motions indicates that the primary excitation mechanism for states which show the c-axis propensity involves a nearly planar collision in which the incoming H impulsively strikes one of the water hydrogens. Failed reactive collisions associated with either abstraction or exchange as well as reactive exchange collisions give the same propensity but they are not the dominant mechanism for producing aligned water. In studies of the reaction H+D2O→OD+HD, we analyze product vibrational and rotational state distributions in detail, making comparison with recent studies of Adelman, Filseth, and Zare [preceding paper, J. Chem. Phys. 98, 4636 (1993)] as well as earlier work. The product HD energy partitioning is found to be in excellent average agreement with experiment, with the HD receiving much more of the available energy than does OD. There are, however, differences in some of the HD rotational distributions, with the experiment showing a much stronger inverse correlation between HD rotational and vibrational excitation than is found in the calculations.
Hybrid WDA: A weighted-density approximation for inhomogeneous fluids
Authors: Leidl Reinhard , Wagner Herbert
Published: July 2002

We introduce a density functional approach to inhomogeneous fluids which is an interpolation between the original weighted-density approximation (WDA) and its modified version (MWDA). Our ‘‘hybridized’’ ansatz (HWDA) employs both a locally weighted density like the WDA and an additional globally averaged density like the MWDA. To test the HWDA we calculate triplet direct correlation functions of a homogeneous hard-sphere fluid and the density profiles of a hard-sphere liquid near a hard wall. The HWDA is also applied to the problem of surface segregation in a binary hard-sphere mixture. Comparison with simulation data demonstrates that the HWDA improves the popular MWDA qualitatively as well as quantitatively without spoiling the computational convenience of the latter.
Structural study of water confined in porous glass by neutron scattering
Authors: Bellissent-Funel M.-C. , Lal J. , Bosio L.
Published: July 2002

In order to study the structural properties of water at interfaces, we performed neutron scattering experiments of water confined inside interconnected pores of Vycor glass. The structure factors and the corresponding radial distribution functions of the confined water have been determined as a function of temperature from 35 °C down to −100 °C and for two levels of hydration of the glass (full and partial hydration). At 27 °C, the structure factor of confined water is very similar to that of bulk water. When decreasing the temperature, we show that it is possible to supercool water inside pores of Vycor but not to the extent expected for such a small confining space. In fact the presence of large hydrophilic silica interface enhances the nucleation of ice. The observed phase of ice is the cubic ice which appears at about −18 °C in the full hydrated Vycor. The other interesting observation is a greater degree of supercooling of water in partially hydrated samples of Vycor compared to fully hydrated ones. Moreover, the presence of a small amount of liquid water down to −40 °C is identified.
Pure rotational spectrum of Kr...HCl in the excited state (100) observed with a glow-discharge source in a pulsed-nozzle Fourier-transform microwave spectrometer
Authors: Bevan J. W. , Legon A. C. , Rego C. A.
Published: July 2002

The pure rotational spectra of the four isotopomers 84Kr...H 35Cl, 84Kr...H 37Cl, 86Kr...H 35Cl, and 84Kr...D 35Cl of the van der Waals complex of krypton and hydrogen chloride have been observed in the vibrationally excited state (1000) by incorporating a pulsed glow-discharge source of excited-state molecules into a conventional pulsed-nozzle Fourier-transform microwave spectrometer. The rotational constant B1, the centrifugal distortion constant D1, and the Cl nuclear quadrupole coupling constant χ1(Cl) are reported for each isotopomer. The time of flight of the (1000) Kr...HCl molecules between the region of vibrational excitation and the detection region of the spectrometer indicates that the vibrational predissociative lifetime of the state (1000) is τ≥400 μs. Molecular properties of (1000) Kr...HCl determined from the observed spectroscopic constants are compared with those previously available for the ground state.
Thermal diffusion factors and intermolecular potentials for noble gas–SF6 systems
Authors: Taylor William L. , Hurly John J.
Published: July 2002

Experimental thermal diffusion factors for equimolar mixtures of He–, Ne–, Ar–, Kr–, and Xe–SF6 have been measured in the temperature range from 225 to 500 K. The data were obtained in a 20-tube trennschaukel, or ‘‘swing separator.’’ The systems containing the four lighter noble gases all exhibited a ‘‘normal’’ thermal diffusion factor, αT, i.e., concentration of the heavy species, SF6, in the cold region of the apparatus and increase of αT with temperature. Xe–SF6, the system with the smallest mass difference, exhibited ‘‘abnormal’’ behavior. The spherically symmetric Pack potentials were used to calculate the thermal diffusion factor with reasonable success. Recently published dipole–dipole dispersion coefficients were used to construct intermolecular potentials of the Hartree–Fock-dispersion functional form with individually damped attractive terms. The potentials, when tested against the available transport and thermodynamic data, improved the fit to experiment in almost all cases.
Stationary space analysis of the electronic structure of a polymer system: From cluster model to cluster-series model
Authors: Maekawa Koji , Imamura Akira
Published: July 2002

It is pointed out that, in order to use cluster molecules as a model of a polymer system, only the electronic structure within the stationary space, which consists of molecular orbitals unaltered by the extension of a cluster, should be dealt with. As a first example of stationary space analysis, simple procedures to extract both extreme states of the polymer are presented, that is, the periodic extended states and the localized states or stationary canonical orbitals. This approach is essentially based on direct comparison among the electronic structures of a series of clusters not only in the extraction of stationary space but also in its analysis. So it should be called the cluster-series model rather than the usual cluster model.
The electric dipole moment of (NH3)2 for G: ‖K‖=1
Authors: Linnartz Harold , Kips Annemieke , Meerts W. Leo , Havenith Martina
Published: July 2002

From the results of Stark measurements on the (NH3)2 van der Waals complex formed in a molecular jet expansion, it was possible to determine the electric dipole moment for the G: ‖K‖=1 state. The partially quenched inversion in the complex gives rise to quadratic Stark effect. We find an electric dipole moment of ‖μ‖=(0.10±0.01) D in the ground state and an upper limit of 0.09 D for the dipole moment in the excited state. These small values give evidence that for the G: ‖K‖=1–1 states, the ‘‘antiparallel’’ (cyclic) structure is more likely than the hydrogen bonded one.
Solvent effect on translational diffusivity and orientational mobility of polymers in solution: A molecular dynamics study
Authors: Bahar Ivet , Badur Bertan , Doruker Pemra
Published: July 2002

Molecular dynamics simulations have been performed for a bead–spring model chain of 30 beads immersed in 738 solvent molecules. The solvent–solute interaction energy εbs has been varied in the range 0.1≤εbs≤0.8 kcal/mol to assess the role and importance of solvent type on the dynamic and equilibrium properties of the chain. Radial distribution functions for polymer bead–solvent and bead–bead pairs indicate the enhancement of more expanded chain configurations with increasing quality of the solvent. The translational diffusivity D of the chain exhibits an inverse linear dependence on εbs, thus decreasing in the presence of more favorable polymer–solvent interactions. Molecular dimensions of the chain such as the mean-square end-to-end distance 〈r2〉 and the radius of gyration Rg are examined in various solvent environment. The ratio 〈r2〉/Rg2 approximates the limiting value of 6 corresponding to infinitely long freely jointed chains. A linear dependence of D on 1/Rg is observed, in conformity with the Zimm theory of dilute polymer solutions subject to hydrodynamic interactions. The orientational motion of internal chain vectors is also found to slow down with increasing strength of intermolecular interactions, in parallel with the translational diffusivity. Characteristic orientational relaxation times τ are calculated for chain segments of various sizes n, using the initial decay rates of the corresponding orientational autocorrelations functions. These are found to obey a scaling law of the form τ∼na for a given εbs. The exponent a therein decreases with the quality of the solvent, assuming values in the interval 1.0≤a≤1.5 throughout the investigated range of polymer–solvent interactions.
Time-dependent quantum mechanical study of intramolecular vibrational energy redistribution in benzene
Authors: Iung Christophe , Wyatt Robert E.
Published: July 2002

Large scale quantum calculations on relaxation from the CH(v=3) overtone in benzene are reported. Plots of time-dependent local mode stretch and bend energies were used to elucidate the relaxation mechanism. Sequential local mode excitation was observed at early times (t<0.2 ps). Persistent oscillations were found superimposed upon the steady state bond energies at later times.
Excited electronic states of protonated acetylene. II. Anisotropy of the potential energy surface
Authors: Schneider F. , Palma A. , Semprini E. , Gianturco F. A.
Published: July 2002

Extensive ab initio configuration interaction calculations have been carried out for different directions of approach of a proton to a rigid acetylene molecule. The calculated potential energy curves of several singlet states are presented. The anisotropy we found allows us to explain features of the charge transfer mechanism during proton–acetylene collisions as observed in recent molecular beam experiments. An important theoretical conclusion is the preference of the linear approach of protons in collisions with acetylene for the formation of acetylene ions.
Dynamics of liquid acetonitrile at high frequencies
Authors: Kunz W. , Calmettes P. , Bellissent-Funel M.-C.
Published: July 2002

Quasielastic neutron scattering spectra were measured for pure acetonitrile at 25 °C. In the framework of a simple model of translation and rotation, it was found that the short-time self-diffusion coefficient of liquid acetonitrile is similar to the long-time one measured by NMR. As far as the rotational motion is concerned, the characteristic time was found to be close to the typical value of the molecule spinning motion. The complete inelastic and quasielastic spectra may be further used to check the results of molecular dynamics simulations of acetonitrile.
Statistical behavior of elementary collinear exchange reactions A+BC→AB+C
Authors: Bonnet L. , Rayez J. C. , Halvick Ph.
Published: July 2002

In this paper, we revisit the analysis of the classical statistical behavior of three-atom collinear exchange reactions A+BC→AB+C. We begin with the intuitive reason why the statistical assumption (all the states of the available phase space are equiprobable) can, a priori, be applied without any restriction to a collisional process. To check the validity of this hypothesis, we show that an extention of the method of Wagner and Parks allows one to compute the statistical distributions of recoil energy for reactions involving a long lifetime intermediate complex. A comparison between these numerical distributions and the theoretical ones leads to some discrepancies. In order to understand the origin of these unexpected results, we implement a numerical experiment showing that trajectories ‘‘lose the memory’’ of their initial conditions in a reduced area of the region where the three atoms interact. As a consequence, the statistical assumption is only applied in this area which we call the statistical region. The agreement between the resulting theoretical distributions and the numerical ones is now very satisfactory. Thus, the statistical assumption defined above fails. This surprising result shows the originality of the statistical behavior of unbounded systems with a few degrees of freedom, as compared with the larger systems usually treated by statistical mechanics.
Theory of growth rate of nucleating droplets
Authors: Mouti M. , ten Bosch A.
Published: July 2002

The growth law for spherical droplets during a first order phase transition is derived and shown to fulfill a variational procedure. This permits the use of simple trial functions for the interfacial profiles in calculating the critical radius, the growth rate, and the surface tension. Results agree with previous theories for conserved and nonconserved order parameters.
Theoretical study of the electronic spectrum of the CoH molecule
Authors: Freindorf Marek , Marian Christel M. , Hess Bernd A.
Published: July 2002

Calculated potential energy curves and spectroscopic parameters of the ground and various low-lying excited states of the cobalt hydride molecule are presented. Over 30 electronic states of singlet, triplet, and quintet multiplicity have been obtained with adiabatic excitation energies below 4 eV. In addition, the electronic structure of several negative ion states has been determined. CoH− possesses a 4Φ electronic ground state and at least three other electronic states that are stable with respect to electron autodetachment. The calculations include relativistic effects variationally by employing a one-component no-pair operator with external-field projectors. Electron correlation is accounted for by using multireference single and double excitation configuration interaction methods. Several experimentally observed bands in the optical spectrum of CoH are ascribed to transitions between the X 3Φ ground state and excited states of 3Φ, 3Δ, and 1Γ symmetry.
Invariant imbedding method for multichannel Schrödinger equations with first derivative coupling
Authors: Band Y. B. , Tuvi I.
Published: July 2002

We derive and demonstrate an invariant imbedding method for calculating the S matrix for scattering problems involving a first derivative term in the Schrödinger equation for the relative motion. This method can be used directly on scattering problems derived using molecular structure calculations that determine adiabatic potential energies and their derivative coupling matrix elements. A critical discussion of the procedures that transform from an adiabatic to a diabatic representation is presented.
Breakdown of global coupling in oscillatory chemical reactions
Authors: Mertens Florian , Imbihl Ronald , Mikhailov Alexander
Published: July 2002

The effects of global coupling through the gas phase in oscillatory surface chemical reactions are investigated using a model which represents the complex Ginzburg–Landau equation with an additional integral term. Depending on the parameters of the model, global coupling is found to have either a synchronizing or desynchronizing effect. Respectively, the breakdown of global coupling requires the presence of strong supercritical inhomogeneities or spontaneously occurs in a uniform system.
Quantum stochastic approach for molecule/surface scattering. I. Atom–phonon interactions
Authors: Bittner Eric R. , Light John C.
Published: July 2002

We present a general, fully quantum mechanical theory for molecule surface scattering at finite temperature within the time dependent Hartree (TDH) factorization. We show the formal manipulations which reduce the total molecule–surface–bath Schrödinger equation into a form which is computationally convenient to use. Under the TDH factorization, the molecular portion of the wavefunction evolves according to a mean-field Hamiltonian which is dependent upon both time and temperature. The temporal and thermal dependence is due to stochastic and dissipative terms that appear in the Heisenberg equations of motion for the phonon operators upon averaging over the bath states. The resulting equations of motion are solved in one dimension self consistently using quantum wavepackets and the discrete variable representation. We compute energy transfer to the phonons as a function of surface temperature and initial energy and compare our results to results obtained using other mean-field models, namely an averaged mean-field model and a fully quantum model based upon a dissipative form of the quantum Liouville equation. It appears that the model presented here provides a better estimation of energy transfer between the molecule and the surface.
Rydberg states of the Kr2 molecule
Authors: Kane D. J. , Abele C. C. , Fraser R. D. , Shannon D. C. , Eden J. G. , Ginter M. L.
Published: July 2002

Spectra attributed to transitions from the 5sσ a 3Σu+ metastable state of Kr2 to excited Rydberg states have been observed by intracavity and multipass absorption and by laser excitation spectroscopy. Four distinct Rydberg series converging on the X 2Σu+ core of Kr2+ have been identified. Rotational structure is not resolved in any of the observed spectra. Most of the spectra can be assigned to vibronic transitions in the nfλ(3Πg,3Σg+) and npλ(3Πg,3Σg+)←a 3Σu+ series by analogy with similar spectra in the lighter rare gas dimers. The 6pπ 3Πg←a 3Σu+ and 8pσ 3Σg+←a 3Σu+ transitions observed in the ∼19 200–18 200 cm−1 region exhibit many bands with v′≠0 in their (v′–v″) vibrational band system developments. The higher n value members of these and other observed series are dominated by (0–0) transitions. All series are perturbed in a few energy regions by states built on the excited ion core states A 2Π3/2g and B 2Π3/2u. Laser excitation spectra for the 8pσ 3Σg+←a transition obtained by monitoring various 5p→5s emission lines of atomic krypton suggest that the 8pσ 3Σg+ Rydberg state of Kr2 is predissociated by a repulsive potential correlated with Kr 5p[3/2]1+Kr(1S0). The lowest ionization limit of Kr2 (relative to a 3Σu+,v=0) was determined to be 28 484±5 cm−1.
State-specific unimolecular reaction of NO2 just above the dissociation threshold
Authors: Miyawaki Jun , Yamanouchi Kaoru , Tsuchiya Soji
Published: July 2002

Photofragment excitation (PHOFEX) spectra of NO2 are observed by monitoring the specific quantum states of a fragment NO (2Π1/2;v=0, J=0.5–6.5) in the energy region 0–160 cm−1 above the dissociation limit to NO (2Π1/2) and O (3P2). Preparation of NO2 in a quasibound eigenstate above the dissociation limit is attained by the combination of extremely cooled (∼1 K) parent NO2 in a supersonic jet and a high resolution (∼0.05 cm−1) photolysis laser. The dissociation rate constants are obtained from the peak width of PHOFEX spectra and the smallest rate constant is k=8.5×109 s−1, in the energy region where only J=0.5 of NO (2Π1/2; v=0) is produced. The observation that the rate constant increases stepwise when a new product channel J=1.5 opens implies that the transition state is a loose complex. This behavior of the rate constant is direct experimental proof of the statistical theory of the unimolecular reaction process. The product state distribution of NO fluctuates depending on the quasibound state of NO2, though the average value is consistent with the calculation by phase space theory. This state specificity of the rate constant is interpreted in terms of quantum fluctuations associated with individual quasibound eigenstates.
Vibrational modulation effects on the hyperfine coupling constants of fluoromethyl radicals
Authors: Barone Vincenzo , Grand André , Minichino Camilla , Subra Robert
Published: July 2002

A general quantum-mechanical protocol for the study of nonrigid free radicals has been applied to the series CH3, CH2F, CHF2, and CF3. Electronic structures have been computed by highly correlated ab initio methods and vibrational modulation effects have been studied by a nonrigid invertor Hamiltonian. The effect of small amplitude vibrations perpendicular to the inversion motion has been taken into account by an adiabatic model. The results are in close agreement with experiment, and can be interpreted quite straightforwardly in terms of the interplay between the potential energy and the property hypersurfaces. This allows a more dynamically based analysis of hyperfine coupling constants extensible to large, nonrigid radicals.
On the electronic structure aspects of spin-forbidden processes in N2O
Authors: Chang Agnes H. H. , Yarkony David R.
Published: July 2002

The crossing surfaces corresponding to the intersection of the lowest singlet surface of N2O and the three triplet surfaces correlating with N2+O(3P) are studied using multireference configuration interaction wave functions comprised principally of 300 000–500 000 terms, but with some calculations using expansions as large as 1 700 000 terms. These (1 1A′,1 3A′), (1 1A′,1 3A″), and (1 1A′,2 3A″) crossing surfaces are characterized in the vicinity of their minimum energy points. The minimum energy crossing structures are each linear and correspond to (X 1Σ+,3Π) and (X 1Σ+,3Σ−) intersections. The minimum energy point on (X 1Σ+,3Π) crossing surface, was found to be ∼58 kcal/mol above the N2O(X 1Σ+) minimum. The minimum energy point on (X 1Σ+,3Σ−) crossing surface was found to be 71 kcal/mol above the N2O(X 1Σ+) minimum. The N–N bond distance is similar at the (X 1Σ+,3Π) and (X 1Σ+,3Σ−) minimum energy crossing structures, being 1.116 and 1.113 Å, respectively, and approximately equal to that in isolated N2(X 1Σg+). The N–O bond is 1.72 and 1.96 Å for the (X 1Σ+,3Π) and (X 1Σ+,3Σ−) minimum energy crossing structures, respectively, and significantly stretched when compared with its value, 1.18 Å, at the equilibrium geometry of N2O(X 1Σ+). The spin–orbit couplings were also evaluated on each of the (1 1A′,1 3A′), (1 1A′,1 3A″), and (1 1A′,2 3A″) crossing surfaces. The (X 1Σ+,3Π) and (X 1Σ+,3Σ−) spin–orbit interactions were found to be ∼90 and ∼9 cm−1 at their respective minimum energy crossing structures. The crossing surfaces persist in the vicinity of the minimum energy crossing points. However for increased deviations from collinearity avoided crossings are found.
Renner–Teller coupling in the 3dδ 1Φu Rydberg state of acetylene
Authors: Zhu Yi Fei , Shehadeh Rana , Grant Edward R.
Published: July 2002

The ungerade Rydberg states of acetylene in the energy range from 79 400 to 85 000 cm−1 have been investigated by (3+1) ionization-detected absorption spectroscopy. Bending vibrational transitions induced by Renner–Teller coupling have been discovered and analyzed. The structure observed corresponds closely with analogous bands in C2H2+ recently obtained and assigned to trans-bending (ν4) by Pratt et al., using electron spectroscopy following resonant two-photon photoionization via the A 1Au state [J. Chem. Phys. 95, 6238 (1991)]. Based on intensities observed in our higher-resolution Rydberg spectrum, we offer a reassignment of this bending level structure that suggests an acetylene cation, both isolated and at the core of nonpenetrating Rydberg states, in which trans-bend is of relatively high ffrequency and strongly coupled by Renner interaction to the cation 2Π electronic degeneracy.
Persistent infrared hole burning spectroscopy of NH3D+ doped in [(NH4)x,Rb1−x]2SO4 mixed crystals
Authors: Cho Han-Gook , Strauss H. L.
Published: July 2002

[(NH4)x,Rb1−x]2SO4 mixed crystals (0.16≤x≤1) were doped with NH3D+. Four of the eight N–D stretching bands of the NH3D+ ion gradually disappear with increasing Rb+ ion concentration while the widths of the N–D stretching bands increase, indicating that Rb+ ions first substitute NH4+ ions only in one type of crystal site, and that addition of Rb+ ions introduces glasslike disorder into the (NH4)2SO4-type crystalline structure. Infrared hole burning has been demonstrated in the broadened N–D stretching band of NH3D+ ion using a combination of a diode laser and a Fourier-transform infrared spectrometer. The initial hole width decreases proportionally with the center frequency of the hole at all Rb+ ion concentrations and agrees with the measurements of the [(NH4)x,K1−x]2SO4 mixed crystals. The similar proportionality, long known for many hydrogen-bonded systems in solution, suggests that the widths observed in solution are homogeneous. A longer irradiation time (≳10 min), however, leads to a wider spectral hole. Measured hole decay rates decrease with decrease of the center frequency of the hole, showing that the rotational tunneling barrier increases with the strength of the hydrogen bond. The change of the rotational tunneling barrier with Rb+ ion concentration is also observed as a change of the hole decay rate (more than tenfold in the experimental range). On the other hand, the hole burning quantum efficiency shows little change with the Rb+ ion concentration, or temperature. The observed steady holeburning quantum efficiency supports the infrared hole burning mechanism proposed in our previous study: The configurational change of the hole burning must occur in the excited vibrational state.