bluuebunny/crossref_metadata_2025 huggingface dataset:
The liquid–glass transition in LiCl/H2O: Impulsive stimulated light scattering experiments and mode-coupling analysis
Authors: Halalay I. C. , Nelson Keith A.
Published: July 2002

Impulsive stimulated light scattering (ISS) experiments have been conducted on aqueous LiCl solutions to characterize the structural relaxation dynamics at temperatures above, near and below the glass transition temperature Tg. The data provide acoustic velocities, attenuation and thermal diffusion rates over wide ranges of temperature and wave vector. At temperatures in the glass transition region, the dynamics of thermal expansion and subsequent contraction (not due to thermal diffusion) are observed as well. The acoustic information from ISS as well as from earlier Brillouin scattering results is used to determine the frequency-dependent elastic modulus in the MHz–GHz range. Analysis in terms of an empirical (stretched exponential) relaxation function is carried out and shown to be inadequate. The main analysis is inspired by predictions of mode-coupling theory concerning susceptibility spectra. Simple scaling for the α-peak and identical power law dependencies on frequency for both electrical and mechanical moduli, as predicted by theory, were confirmed.
OH A 2∑+→X 2Π chemiluminescence measurements of N+2, Ar++H2O hydrogen-atom-transfer reactions at suprathermal energies
Authors: Gardner James A. , Dressler Rainer A. , Salter Richard H. , Murad Edmond
Published: July 2002

Chemiluminescence attributable to OH A 2∑+→X 2Π emission is observed in hydrogen-atom transfer reactions of N+2 and Ar+ with H2O. High-resolution OH A→X spectra [0.5 nm full width at half maximum (FWHM)] are presented as a function of collision energy in the range Ec.m.=5–20 eV. The spectra clearly show an increase in rotational broadening with collision energy. To reasonably fit the spectral data, simulations require contributions from two Boltzmann distributions with different rotational temperatures. It is proposed that the OH A state is formed via two channels. In the dominant channel, A state OH is formed in both v′=0 and v′=1, with a collision energy-dependent rotational temperature ranging from 1000 K at Ec.m.=6 eV to 3400 K at Ec.m.=19 eV. The rotationally excited products are proposed to be formed in a two-step reaction involving charge transfer, followed by the transfer of a proton to the ‘‘primary’’ species. The second channel is characterized by formation of A state OH primarily in the v′=0 vibrational level with a rotational temperature of 300 K. Comparison of low-resolution (14 nm FWHM) OH A→X and H2O+ Ã 2A1→X̃ 2B1 emissions provides an estimate for the OH A state cross sections of σ<0.3×10−16 cm2 for N+2+H2O at Ec.m. between 6 and 20 eV, representing less than 10% of the hydrogen atom pickup channel at these energies. For Ar++H2O at Ec.m. between 11 and 32 eV, the estimated OH A state cross section is σ<0.5 Å2, which may represent all of the hydrogen atom pickup channel at the higher energies.
Hemiquantal time dependent calculation of the absorption spectrum of a photodissociating triatomic
Authors: Jacon M. , Guérin P. , Atabek O. , Le Rouzo H.
Published: July 2002

A half-collision process is described within a hemiquantal time dependent model to calculate the absorption line shape. The photodissociation of ICN in a collinear geometry serves as an illustrative example with the ‘‘light’’ CN motion treated quantum mechanically, whereas the ‘‘heavy’’ I–CN motion is treated classically. The emphasis is put on (i) the way to introduce the Wigner distribution which appears to be the equivalent of the classical density matrix, and (ii) the choice of the initial positions and momenta in the sampling of trajectories. The so-obtained converged results compare, within good accuracy, with the full quantal ones. Both conceptually and computationally, the hemiquantal approach appears as an interesting alternative to the full quantum wave packet calculation.
Theoretical study on the excited and ionized states of titanium tetrachloride
Authors: Nakatsuji Hiroshi , Ehara Masahiro , Palmer Michael H. , Guest Martyn F.
Published: July 2002

The excitation and ionization spectra of TiCl4 have been studied theoretically by the symmetry adapted cluster (SAC)/SAC-CI and multireference CI (MRD-CI) methods. The calculated spectra show good agreement with the observed spectra. The present results indicate several new assignments for the excitation spectrum. The peaks below 8.0 eV are assigned to valence excitations and those at 9.35 and 10.04 eV are assigned to Rydberg-type excitations within chlorine ligands. The ordering of the ionized states in the outer valence region is (1t1)−1 < (3t2)−1 < (1e)−1 < (2t2)−1 < (2a1)−1, which supports the result of earlier Green’s function calculations.
Structure and dynamics of n-alkanes confined by solid surfaces. I. Stationary crystalline boundaries
Authors: Ribarsky M. W. , Landman Uzi
Published: July 2002

Structural and dynamical properties of n-hexadecane films confined between two crystalline solid substrates are investigated using molecular-dynamics simulations for a range of molecule-to-solid interaction strengths, external loads, and film thicknesses. The confined films are infinite in extent in one of the directions parallel to the solid surfaces and finite in the other direction exposing a liquid-to-vacuum interface (i.e., semidroplet configuration). At sufficiently high loads, whose magnitude depends upon the molecule-to-surface interaction strength, layering, i.e., oscillations in the film density in the direction normal to the boundary, occurs accompanied by an enhancement of surface-parallel orientational ordering of the molecules in the interfacial region. In addition, preferential interfacial adsorption of chain ends, as well as molecular conformations enhancing bridging of the two solid surfaces, are observed. The dynamics of thinning, from a three- to a two-layer film, exhibits a marked dependence on the molecule-to-surface interaction strength. At a critical load the relatively strong adsorption system exhibits a sudden collapse to two layers, resulting in lateral stress accumulation, and subsequent spreading via a cooperative mechanism. On the other hand, thinning of the weakly adsorbed film is gradual, with no stress accumulation, accompanied by spreading occurring via diffusion and adsorption of molecules at the liquid-to-vacuum interface.
Electron degradation and thermalization in CH4 gas
Authors: Kowari Ken-ichi , Demeio Lucio , Shizgal Bernie
Published: July 2002

The relaxation to equilibrium of an ensemble of electrons dilutely dispersed in a large excess of CH4 is studied with solutions of the Boltzmann equation. Elastic and vibrationally inelastic collision processes are included in the analysis. The relaxation time for the approach to equilibrium defined for the relaxation of the average electron energy is determined for two different cross section sets. The kinetic theory formalism, based on the Boltzmann equation, is compared with the formalism used in radiation chemistry and physics and based on the Spencer–Fano equation.
A theoretical study of the surface diffusion of large molecules. I. n-alkane-type chains on W(100)
Authors: Cohen D. , Zeiri Y.
Published: July 2002

In the present study the surface diffusion of model n-alkane-type chains adsorbed on a W(100) surface were simulated. The simulations were performed using molecular dynamics calculations where the thermal motion of the surface atoms was introduced via the generalized Langevin method. The potential function among the chain atoms used in these calculations described the nearest-neighbor interaction by a Morse potential while next-nearest-neighbors and next-next-nearest-neighbors interaction was described by a Lennard-Jones 12-6 and a repulsive exponential function, respectively. The length of the chains, N, considered were N=3, 6, 10, and 20. For each value of N the chain diffusion at three or four surface temperatures was examined. For all values of N it was found that the diffusion coefficient could be described by an Arrhenius expression. It was found, in good agreement with the experimental results, that the activation energy for the diffusional motion scales with the chain length while the preexponential factors were practically independent of N. In addition, various static (e.g., average mean square end-to-end distance and average mean square radius of gyration) and dynamic (e.g., autocorrelation functions) properties of the simulated systems were computed. The results of these simulations were used to obtain a qualitative understanding of the mechanism by which such chains diffuse on a solid surface.
Electronic states and nature of bonding in the molecule YC by all electron ab initio multiconfiguration self-consistent-field calculations and mass spectrometric equilibrium experiments
Authors: Shim Irene , Pelino Mario , Gingerich Karl A.
Published: July 2002

In the present work we present results of all electron ab initio multiconfiguration self-consistent-field calculations of eight electronic states of the molecule YC. Also reported are the calculated spectroscopic constants. The predicted electronic ground state is 4Π, but this state is found to be separated from a 2Π state by only 225 cm−1, and by 1393 cm−1 from a 2Σ+ state. The chemical bond in the 4Π ground state is mainly due to the formation of a bonding molecular orbital composed of the 4dπ of Y and the 2pπ on C. The 5s electrons of Y are partly transferred to the 2pσ orbital on C, and they hardly contribute to the bonding. The chemical bond in the YC molecule is polar with charge transfer from Y to C giving rise to a dipole moment of 3.90 D at 3.9 a.u. in the 4Π ground state. Mass spectrometric equilibrium investigations in the temperature range 2365–2792 K have resulted in the dissociation energy D0°=414.2±14 kJ mol−1 for YC(g), and a standard heat of formation ΔHf,298.15°=708.1±16 kJ mol−1.
Control of HOD photodissociation dynamics via bond-selective infrared multiphoton excitation and a femtosecond ultraviolet laser pulse
Authors: Amstrup Bjarne , Henriksen Niels E.
Published: July 2002

A scheme for controlling the outcome of a photodissociation process is studied. It involves two lasers—one intense laser in the infrared region which is supposed to excite a particular bond in the electronic ground state, and a second short laser pulse in the ultraviolet region which, at the right moment, excites the molecule to a dissociative electronic state. We consider the HOD molecule which is ideal due to the local mode structure of the vibrational states. It is shown that selective and localized bond stretching can be created in simple laser fields. When such a nonstationary vibrating HOD molecule is photodissociated with a short laser pulse (∼5 fs) complete selectivity between the channels H+OD and D+OH is observed over the entire absorption band covering these channels.
Electron-spin resonance of the Co, Rh, and Ir dihydride molecules
Authors: Van Zee R. J. , Li S. , Hamrick Y. M. , Weltner W.
Published: July 2002

Electron-spin-resonance (ESR) spectra of 59CoH2, 103RhH2, 191,193IrH2 molecules and their deuterated counterparts have been observed in argon and krypton matrices at 4 K. Analysis indicates that each is a nonlinear molecule with S=1/2. In all cases, the spin density at the two attached hydrogen atoms is small and the s character of the unpaired electron on the metal varies from about 4% to 20% to 15%, respectively. Relatively large g shifts Δgi=gi−ge (i=x,y,z) are observed, increasing with increasing mass of the metal atom. These findings for RhH2 and IrH2 are in complete agreement with the theoretical calculations of Balasubramanian, Liao, and Dai, indicating that both molecules have 2A1 ground states and low-lying 2A2 and 2B1 excited states. However, the experimental results on CoH2 cannot be accounted for by the 4Φ ground state predicted by other theoretical studies.
Electron propagator calculations on the adiabatic electron binding energies of C3
Authors: Ortiz J. V.
Published: July 2002

New techniques of electron propagator theory (EPT) are applied to C3, C3+, and C3−. Gradients of second-order EPT ionization energies and electron affinities are combined with gradients of second-order many-body perturbation theory for the neutral to produce gradients of the ion total energies. Optimized geometries of the ions, vibrational frequencies, and adiabatic electron binding energies are calculated with these methods. A renormalized self-energy is used to produce improved vertical and adiabatic ionization energies and electron affinities. For the cation, the 2B2 state with C2v symmetry and the 2Σ state with C∞v symmetry are very close in energy. The optimized 2Σu structure is a transition state with an imaginary frequency of σu symmetry that lies 2.8 kcal/mol above the 2B2 state. The adiabatic ionization energy is calculated to be 11.9 eV. The anion in the 2Πg state lies 1.8 eV below the neutral in these calculations.
Convergence properties of coupled Hartree–Fock theory of intermolecular interactions
Authors: Cybulski Sl/awomir M.
Published: July 2002

The interaction between a helium atom and a proton is analyzed using coupled Hartree–Fock perturbation theory in the polarization approximation. It is shown that even for relatively short interatomic separations the perturbation expansion converges and its sum is identical with the supermolecule interaction energy. Similar behavior is observed for molecule–proton complexes although the convergence may be worse and, as an illustration, calculations for H2O–H+ are presented. A detailed analysis of the electrostatic component of the interaction energy between He and H+ shows the failure of a long range multipole expansion in correcting the secondary basis set superposition error. Polarization approximation perturbation theory is also used to analyze interactions for He–Li+ and He–Na+. The results for interatomic distances of 8.0 and 10.0 a0 for both the basis sets of the monomers, and the basis set of the entire complex are in excellent agreement with the counterpoise-corrected interaction energies, and thus show the need to remove the basis set superposition error.
The stereochemistry of the O(1D)+N2O→NO+NO reaction via velocity-aligned photofragment dynamics
Authors: Brouard M. , Duxon S. P. , Enriquez P. A. , Simons J. P.
Published: July 2002

Velocity-aligned, superthermal O(1D) atoms generated via the photodissociation of N2O have been employed to investigate the stereodynamics of the title reaction. The power of this experimental technique, when coupled with Doppler-resolved, polarized laser-induced fluorescence probing of the reaction products, is demonstrated by reference to the specific reaction channel leading to NO(υ′=0)+NO(υ′=16,17), which is shown to proceed via direct stripping dynamics. Furthermore, the observed product-state selective linear and angular momenta disposals imply that the reaction is stereodynamically constrained to occur via collinear collision geometries.
Determination of the D2O ice VII–VIII transition line by Raman scattering up to 51 GPa
Authors: Pruzan Ph. , Chervin J. C. , Canny B.
Published: July 2002

The phase boundary between ice VII and ice VIII of deuterated ice (D2O) was determined by Raman scattering up to 51 GPa. From 2 to 51 GPa, the temperature of the stability range of ice VIII was found to decrease from ∼278 to 158 K.
Absolute state-selected and state-to-state total cross sections for the Ar+(2P3/2,1/2)+CO2 reactions
Authors: Flesch G. D. , Ng C. Y.
Published: July 2002

Absolute spin–orbit state-selected total cross sections for the reactions, Ar+(2P3/2,1/2)+CO2→CO+2+Ar [reaction (1)], CO++O+Ar [reaction (2)], O++CO+Ar [reaction (3)], C++2O+Ar [reaction (4)], ArC++2O [reaction (5)], ArO++CO [reaction (6)], and ArCO++O [reaction (7)] have been measured in the center-of-mass collision energy (Ec.m.) range of 0.26–131 eV. The ratio of the charge-transfer cross section due to Ar+(2P1/2) to that associated with Ar+(2P3/2) varies in the range of 0.5–0.8. The appearance energies observed for CO+ (Ec.m.=4.2±0.5 eV), O+ (Ec.m.=3.7±0.5 eV), and C+ (Ec.m.=12.6±0.5 eV) are in agreement with the thermochemical thresholds for reactions (2), (3), and (4), respectively. The comparison of the absolute cross sections for CO+, O+, and C+ from CO2 by photoionization and by dissociative charge transfer [reactions (2)–(4)] is made. The kinetic-energy analysis of product CO+2, CO+, O+, C+, ArO+, and ArC+ suggests that reactions (2)–(7) proceed via a charge-transfer predissociation mechanism. This experiment, together with the previous studies of Ar+(2P3/2,1/2)+N2(O2,CO), supports the conclusion that product ions formed by dissociative photoionization are also produced by dissociative charge transfer. We find that the absolute cross sections for product ions formed in the dissociative charge-transfer processes [reactions (2)–(4)] are substantially greater than those formed in the dissociative photoionization of CO2, a finding consistent with the general observation that photoionization cross sections are significantly smaller than charge-transfer cross sections.

The relative cross sections for CO+, O+, and C+ formed by reactions (2)–(4) are also found to be different from those for photoionization of CO2. This difference is attributed to the anisotropic interaction potential surface responsible for the Ar+(2P3/2,1/2)+CO2 reactions.
Two-photon induced exciton mediated dissociation of N2O and photomobility of O atoms in crystalline Xe
Authors: Lawrence W. G. , Apkarian V. A.
Published: July 2002

Two-photon induced access of Xe excitons at 248 nm leads to efficient dissociation of N2O impurities via ionic potentials. The product O atoms are probed via the Xe+O− charge transfer transitions, and the N2O disappearance is probed by infrared (IR) spectroscopy. Charge transfer excitation of O atoms leads to atomic mobility, such that with extensive irradiation a photochemical steady-state is reached between N2O, oxygen atoms trapped in the Xe bulk, and within the same cage as N2. A detailed kinetic analysis of these processes is presented. Among the extracted parameters are the two-photon absorption cross section of Xe at 248 nm, 3.7×10−48 cm4 s, the free exciton migration length, 47 Å, and the excitonic dissociation probability of N2O, 0.85. The mobility of photoexcited O atoms is attributed to the topology of electronically excited surfaces which show minima at the ground state cage barriers.
Vibrational frequencies of ozone: A multiconfigurational approach
Authors: Borowski P. , Andersson K. , Malmqvist P.-Å. , Roos B. O.
Published: July 2002

The electronic ground state of ozone and, in particular, its equilibrium geometry and harmonic vibration frequencies was studied by a variety of multiconfiguration and single-configuration methods. It is well known that the antisymmetric stretch frequency cannot be correctly obtained by single-reference methods unless at least triple excitations are included. Extensive comparison with other work in the literature shows that basis-set effects must be taken into account since the ω3 frequency is very sensitive to computational details. The multiconfiguration methods are shown to give good results provided that an adequate configuration space is used. In particular, the second-order complete active space perturbation method performs very satisfactorily. Traditional multireference configuration interaction (MRCI) methods, using a few reference functions, do not perform so well. A two-reference CI is able to give reasonable results, but only when the orbitals have been prepared by some properly correlated method. Adding several reference functions gives small improvements, and the result is capriciously dependent on the type of reference functions included. The success of the perturbation method, as well as an extreme type of MRCI, indicates that it is far more important to include a large number of diverse configurations in the reference than to treat the remaining dynamical correlation accurately.
Pair correlation functions of uncharged and weakly charged Brownian particles
Authors: Kunz W. , Calmettes P. , Turq P. , Cartailler T. , Morel-Desrosiers N. , Morel J.-P.
Published: July 2002

Small-angle neutron scattering measurements are reported for solutions of cryptand 222 and cryptates K+222, ClO4− and Ba2+222, 2ClO4− in deuterated acetonitrile at 25 °C. Solute–solute correlation functions are inferred from the experimental scattering spectra by means of hypernetted chain (HNC) calculations with semiempirical solvent-averaged potentials. Due to their specific structure, cryptands and their complexes are convenient models for uncharged and weakly charged Brownian particles.
Diffusive reaction rates from Brownian dynamics simulations: Replacing the outer cutoff surface by an analytical treatment
Authors: Luty Brock A. , McCammon J. Andrew , Zhou Huan-Xiang
Published: July 2002

The algorithm of Northrup, Allison, and McCammon [J. Chem. Phys. 80, 1517 (1984)] for calculating diffusive reaction rates using Brownian dynamics simulations is reexamined. A new method is described in which a time-consuming portion of the algorithm is replaced by an analytical solution. When applied to two illustrative model systems, the new method is found to reduce the computational work by a factor of 2 or more.
Effect of ion vibration for the proton-transfer reaction of the ammonia cation. II. Classical trajectory calculation of an orientational effect
Authors: Tachibana Akitomo , Suzuki Tetsuo , Yamabe Tokio
Published: July 2002

The microscopic capture cross section σ for the proton-transfer reaction of the ammonia cation has been studied by using the classical trajectory method. The orientation of the pulsing dipole moment of the ammonia cation affects σ considerably. This orientational effect is inherent in the vibrating ion-polar molecule reaction. The reactivity is further depressed as the rotational energy of the polar molecule increases, where σ is a monotone decreasing function of rotational energy.
Calculation of planewise dipole sums for anthracene crystal
Authors: Mothersdale S. E. , Munn R. W.
Published: July 2002

Planewise dipole sums are calculated for various lattice planes in anthracene, treated both as a single point and as three point submolecules. Conventional methods, generalized to treat submolecules, are used as well as a new method using partial reverse Fourier transformation of modulated bulk lattice dipole sums. The new method gives an independent check but is not otherwise preferable. The submolecule treatment makes the interaction between adjacent planes not much less than within a plane, essentially because of the molecular packing. Beyond adjacent planes, the sums decrease exponentially by typically a factor of 100 per 10 Å separation. Calculations for arbitrary planes are conveniently performed by transforming the conventional crystal unit cell to contain the desired plane as a face. Wave vector-modulated planewise sums have also been calculated for point molecules and the (001) plane: They decrease less rapidly with separation than unmodulated sums.
Photogeneration of ions via delocalized charge transfer states. II. HX2− (X=Cl,Br,I) in rare gas matrices
Authors: Räsänen Markku , Seetula Jorma , Kunttu Henrik
Published: July 2002

Photolysis of HX (X=Cl,Br,I) doped rare gas matrices at excitation energies well below direct ionization of the solids is shown to yield efficient formation of hydrogen bihalide anions, HX2−. The formation dynamics of these ions is ascribed to a charge separation process initiated by optical access of the delocalized charge transfer states. The vibrational spectra of HX2− are presented and compared with previous work on M+⋅⋅⋅(HX2)− ion pairs, and the isoelectronic triatomic rare gas hydride cations.
A density functional investigation of the ground- and excited-state properties of ruthenocene
Authors: Daul Claude , Güdel Hans-Ulrich , Weber Jacques
Published: July 2002

Quantum chemical calculations based on density functional theory have been performed on ruthenocene. Excellent agreement is obtained with ground- and excited-state properties derived from optical spectroscopy. In particular, the energies of the first d–d excitations, the unusually large Stokes shift, the structural expansion of Ru(cp)2 and the substantial reduction of the Ru-cp force constant in the first triplet excited state are almost quantitatively reproduced. The lowest-energy excitation is found to have substantial charge transfer character.
Electron paramagnetic resonance spectroscopy of tetrahedral Cr4+ in chromium-doped forsterite and åkermanite
Authors: Whitmore Michael H. , Sacra Ann , Singel David J.
Published: July 2002

The observation of near IR laser action in chromium activated forsterite by Petriĉević et al. [Appl. Phys. Lett. 52, 1040 (1988)] has spawned considerable interest in identifying and characterizing the quadrivalent chromium centers purportedly responsible for laser action in this material. The EPR (electron paramagnetic resonance) spectroscopy reported here of chromium-doped forsterite and structurally related åkermanite provides compelling evidence for the presence of tetrahedral Cr4+ centers in both materials. The EPR spectra reveal substitutional incorporation of chromium ions with triplet spin multiplicity, assignable to Cr4+. The fine structure and electron Zeeman interaction parameters of the centers in both materials are found to be very similar. In åkermanite, the Cr4+ center is unequivocally accommodated at tetrahedral lattice sites. The similarity of the spectroscopic properties of the Cr4+ ions in the two materials thus implicates incorporation at the analogous tetrahedral sites in forsterite. In conjunction with the findings of recent optical Zeeman studies [K. R. Hoffman et al., Phys. Rev. B 44, 12589 (1991); T. S. Rose et al., OSA Proceedings on Advanced Solid State Lasers 13, 17 (1992)], these EPR results demonstrate that tetrahedral Cr4+ is responsible for the near IR laser activity in chromium-doped forsterite.
Multiple charge-transfer-induced local exciton levels in doped naphthalene crystals
Authors: Brovchenko Ivan V. , Eilmes Andrzej , Petelenz Piotr
Published: July 2002

Measurements of the absorption spectra of naphthalene crystals doped with benzene derivatives are reported. For a typical dopant, a set of bands (some genuinely local, some due to resonances) is observed. The patterns of local levels are interpreted in terms of the recently proposed two-dimensional cluster model. It gives a good qualitative description of the general features of the observed spectra. Probable reasons of quantitative discrepancies are discussed.
(KCl)32 and the possibilities for glassy clusters
Authors: Rose John P. , Berry R. Stephen
Published: July 2002

The alkali halide cluster (KCl)32 is demonstrated to be large enough to exhibit ordered and highly disordered structures. The disordered high-energy stable packings of (KCl)32 are identified as amorphous structures. We term the large collection of amorphous (KCl)32 structures the ‘‘microamorphous’’ state, on the basis that they are as disordered as the bulk glass phase. Liquid (KCl)32 was quenched to investigate how fast the cooling rate must be to trap (KCl)32 in one of the high-energy amorphous structures. Even at unrealistically fast cooling rates, (KCl)32 was able to locate its microcrystal structure. A shielded Coulomb interaction potential was used to test whether reducing the range of the pairwise potential would make it possible to prepare amorphous binary clusters. Several different values of the shielding range were tested. These results are discussed in terms of the structure of the underlying potential energy surface. A short study was conducted of the temperature dependence of the time it takes, on average, for supercooled (KCl)32 to relax into the crystalline regions of its potential energy surface. Lastly, we rationalize, in terms of the potential energy surface, the qualitative temperature dependence of the heat capacity for both the first-order and glass transitions.
Improved propagators for the path integral study of quantum systems
Authors: Ramalho J. P. Prates , Cabral B. J. Costa , Fernandes F. M. S. Silva
Published: July 2002

A local harmonic (LH) propagator has been derived through an adequate splitting of the Hamiltonian operator. This propagator has been applied to study several quantum systems including a quartic oscillator, a double well potential, and the Morse potential. We have calculated equilibrium and dynamical properties using the transfer matrix path integral method. It has been verified that the LH propagator is significantly more accurate than other propagators proposed by several studies. For the LH propagator, higher order corrections to the Trotter formula lead to an excellent short time approximation. It is suggested that the LH propagator can be of wide interest for path integral studies of quantum systems.
Comment on the rate of isomerization of 3-phospholene
Authors: Zhao Meishan , Rice Stuart A.
Published: July 2002

We report calculations of the rate of isomerization of 3-phospholene based on the theory of Gray and Rice as extended by Zhao and Rice. This system is of interest for two reasons. First, the potential energy surface for motion in the isomerization coordinates is well known because of the spectroscopic studies of Harthcock and Laane. Second, the potential energy surface generates a Hamiltonian which has important off-diagonal kinetic energy coupling terms, which provides a different test of the accuracy of the approximations used in the Gray–Zhao–Rice theory than posed by previous applications. We find that the predictions of the Gary–Zhao–Rice theory are in good agreement with the results of simulations of the rate of isomerization of 3-phospholene. We also briefly discuss the relationship between the theory presented in this paper and the Reactive Island theory of isomerization.
Statistical modeling of capture, association, and exit-channel dynamics in the CH3+/CH3CN system
Authors: Smith S. C. , Wilson P. F. , Sudkeaw P. , Maclagan R. G. A. R. , McEwan M. J. , Anicich V. G. , Huntress W. T.
Published: July 2002

The ion–molecule reaction CH3++CH3CN is known to have an association channel leading to CH3CNCH3+ in competition with the exothermic binary channels H2CN++C2H4 and C2H5++HCN. This reaction has been modeled using a master equation treatment incorporating weak collisions. The parameters required for the Rice–Ramsberger–Kassel–Marcus (RRKM) treatment have been found from an ab initio investigation of the CH3+/CH3CN energy surface. A means of including capture rate coefficients in the RRKM approach is developed, in which only the hindered dipole rotation is coupled with the reaction coordinate at large separations. Existing experimental data from ion cyclotron resonance (ICR) spectroscopy and a selected ion flow tube are fitted by the model in the pressure range 10−7–0.3 Torr. The low pressure experimental results are accounted for by weak collisions of the complex with the bath gas (when M=He, <ΔEdown≳ and <ΔRdown≳∼100 cm−1) corresponding to a collision efficiency β=0.05 for M=He and 0.14 for M=CH3CN. Unimolecular rate coefficients for the (CH3CNCH3+)* complex are calculated for all product channels at a range of temperatures from 300 to 600 K. The rate coefficient for radiative stabilization was found to be 225 s−1 at the conditions of the ICR experiment. The average lifetime of the complex was calculated to vary between 29 μs at 600 K to 0.47 ms at 300 K and the termolecular association rates from 3.4×10−24–9.8×10−23 cm6 s−1 (M=He) and from 6.7×10−23–2.2×10−21 cm6 s−1 (M=CH3CN) over the temperature range 600–300 K.
Influence of retardation on the vibrational wave function and binding energy of the helium dimer
Authors: Luo Fei , Kim Geunsik , McBane George C. , Giese Clayton F. , Gentry W. Ronald
Published: July 2002

Because of the extremely small binding energy of the helium dimer, the nuclear wave function is delocalized over an extremely large range of separations. One might therefore expect the properties of this extraordinary species to be sensitive to the potential at very large internuclear distances, r, where relativistic corrections to the usual van der Waals interaction may be important. We have estimated the effect of retardation, which changes the r−6 dependence of the potential to r−7 in the limit of large r, and have found that the binding energy and expectation value 〈r〉 are indeed significantly affected by its inclusion.
Surface diffusion of xenon on Pt(111)
Authors: Meixner D. Laurence , George Steven M.
Published: July 2002

The surface diffusion of xenon on the Pt(111) surface was investigated using laser induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The surface diffusion coefficient at 80 K decreased dramatically from D=8×10−7 cm2/s at θ=0.05θs to approximately D=2×10−8 cm2/s at θ=θs, where θs denotes the saturation coverage at 85 K, corresponding to a commensurate monolayer coverage of 5.0×1014 xenon atoms/cm2. This coverage dependence was consistent with attractive interactions between the adsorbed xenon atoms and the existence of two-dimensional condensed phases of xenon on Pt(111). The kinetic parameters for surface diffusion at θ=θs were Edif=1.3±0.1 kcal/mol and D0=1.1×10−4±0.2 cm2/s. The magnitude of Edif at θ=θs represented the combined effect of the intrinsic corrugation of the adsorbate–surface potential and attractive interactions between the adsorbed xenon atoms. LITD experiments at θ=0.25 θs revealed diffusion kinetic parameters of Edif=1.2±0.2 kcal/mol and D0=3.4×10−4±0.5 cm2/s. The constant Edif at low and high coverage was attributed to the ‘‘breakaway’’ of xenon atoms from the edges of condensed phase xenon islands. The coverage dependence of the surface diffusion coefficient for Xe/Pt(111) was explained by a multiple site diffusion mechanism, where collisions with xenon islands limit diffusional motion. Thermal desorption kinetics for xenon on Pt(111) were determined using TPD experiments. Using the variation of heating rates method, the desorption parameters were Edes=6.6±0.2 kcal/mol and νdes=1.3×1013±0.4 s−1, in good agreement with previous studies. The xenon TPD peak shifted to higher temperature versus initial coverage at a fixed heating rate, providing further evidence for attractive interactions between the adsorbed xenon atoms.
Vibrational-state-to-state collision-induced intramolecular energy transfer N2(A 3Σu+, v″→B 3Πg, v′)
Authors: Bachmann R. , Li X. , Ottinger Ch. , Vilesov A. F. , Wulfmeyer V.
Published: July 2002

Absolute cross sections for collision-induced intramolecular energy transfer from the metastable A 3Σu+ state into the radiating B 3Πg state of N2 have been measured for the first time under single-collision conditions, using a thermal energy molecular beam of N2(A). The collision partners studied were the five rare gases, H2, N2, NO, and O2. The product vibrational levels (B, v′=4–10) were separated using spectrally resolved detection by means of filters as in our earlier related work [R. Bachmann, X. Li, Ch. Ottinger, and A. F. Vilesov, J. Chem. Phys. 96, 5151 (1992)]. In addition, in the present study the contributing reactant state vibrational levels (A,v″) were labeled, using optical pumping by a specially developed broad-band (∼1 nm) pulsed tunable dye laser. A depletion of up to 30% of a given v″ level could be achieved, about one-half of the theoretical maximum, at a pump pulse energy of 4 mJ. This quantity was also measured directly using a second synchronized probe laser. Pumping on a particular A,v″ level reduces the emission from the collisionally coupled B,v′ level by an amount which is a measure of the state-to-state cross section. Quasiresonant energy transfer was found to be strongly preferred, the cross section decreasing exponentially with an increasing energy gap. Absolute cross sections were obtained from a simultaneous measurement of the intensity of the fluorescence induced by the laser pumping of the selected A,v″ level, with corrections for predissociation of the excited upper state. Cross sections on the order of 0.1 Å2 for He to 15 Å2 for Xe were found for closely resonant (ΔE≊100 cm−1) processes. These results as well as our earlier, absolute measurements of the analogous intramolecular N2(W→B) transfer, are discussed in terms of interaction potential models.
Theoretical investigation of the insertion of nickel in the CH bond of CH4. Electronic structure calculations and dynamics
Authors: Burghgraef H. , Jansen A. P. J. , van Santen R. A.
Published: July 2002

The insertion of a nickel atom in the CH bond of CH4 is calculated using density functional theory by determining the transition state and the dissociated state of HNiCH3. A barrier for nickel insertion of 40.7 kJ/mol is found and its origin is discussed. The insertion is exothermic by 34.0 kJ/mol. From the potential energy surface at the transition state and the dissociated state vibrational and rotational frequencies are obtained. Unimolecular and bimolecular transition state theory is used for the calculation of rate constants, sticking coefficients, and activation energies for the nickel insertion reaction as well as the nickel elimination reaction. Activation energies for nickel insertion in both CH4 and CD4 are small compared with other theoretical work. A moderate kinetic isotope effect for the insertion reaction is found when all hydrogens are substituted by deuterium, whereas no significant kinetic isotope effect is found for nickel elimination. Hydrogen tunneling corrections on rate coefficients are also evaluated, but their effect is negligible. Sticking coefficients are small, which is consistent with experimental sticking coefficients of CH4 on nickel surfaces.
Nonlinear resonance and correlated binary collisions in the vibrational predissociation dynamics of I2(B,v)–Ar13 clusters
Authors: Borrmann Angela , Li Zhiming , Martens Craig C.
Published: July 2002

The vibrational predissociation dynamics of the van der Waals cluster I2(B,v)–Ar13 are studied theoretically from a classical mechanical perspective. The focus is on the ejection of the first Ar atom from the excited cluster. This process is found to occur by two competing mechanisms (1) evaporation of a cluster atom induced by the slow vibrational relaxation of the I2 impurity, and (2) direct ejection of a geometrically favored Ar atom by two or more impulsive collisions with the vibrating I2. In contrast with the picture of independent binary collisions, the multiple argon–iodine interactions leading to successful direct dissociation are correlated in time. The relative propensity of the direct channel is a highly structured function of v, the initial vibrational level of the I2. This behavior results from the energy-dependent frequencies of motion of the diatomic and the localized Ar cluster mode, and is a novel example of a nonlinear resonance between ‘‘system’’ and ‘‘bath’’ degrees of freedom.
Predictions of adiabatic ionization energies and electron affinities for CH3S and CH2SH by density functional method
Authors: Chong Delano P. , Ng C. Y.
Published: July 2002

Using density functional method, we have obtained predictions of ionization energies and electron affinities for CH3S and CH2SH. These predictions are in good accord with experimental values and theoretical values based on the Gaussian-2 theory.
Photodissociation dynamics of (C6H6)2+
Authors: Ohashi Kazuhiko , Nishi Nobuyuki
Published: July 2002

Photodissociation of (C6H6)2+ is studied with photon energies between 1.29 and 2.81 eV. Mass-selected ion beam of (C6H6)2+ is photodissociated by a pulsed laser beam in the field-free region of a reflectron-type time-of-flight mass spectrometer. The average relative translational energy, 〈εt〉, and the angular distributions of the photofragments (C6H6+ and C6H6) are measured as a function of photon energy. With a photon energy of 2.81 eV, the (C6H6)2+ ions are promoted to a bound upper state correlated to C6H6+(ππ)+C6H6(X). Only a small fraction (≊3%) of the available energy is partitioned into the translational energy of the fragments and the product angular distribution is isotropic. Absorption of a photon in the range of 1.29–2.14 eV induces a charge resonance transition to a repulsive upper state which correlates to C6H6+(X)+C6H6(X). The observed values for 〈εt〉 are at most 10% of the available energy, although the statistical phase space calculation shows that the complete randomization of the available energy is not achieved. As the photoexcited (C6H6)2+ moves apart on the dissociative potential surface, ≊90% of the available energy flows into the intramolecular modes of the fragments. The energy partitioning becomes almost statistical regardless of the photoexcitation to a dissociative state.
Quantum simulation studies of singlet and triplet bipolarons in liquid ammonia
Authors: Martyna Glenn J. , Deng Zhihong , Klein Michael L.
Published: July 2002

The properties of bipolarons in liquid ammonia have been investigated using path integrals and local density functional theory. In both schemes, the singlet state consists of a large ‘‘peanut’’-shaped bubble with the peaks in the electron density separated by about 7 Å. However, the triplet state electrons exist in well-separated isolated cavities. This difference in behavior between the singlet and triplet states is consistent with the large drop in the susceptibility observed in dilute alklai metal–ammonia solutions at a concentration of about 1 mol % alkali metal. The adiabatic dynamics of the two states were studied using the Car–Parrinello local density function theory method. In the singlet state, novel hopping events are observed in which the electrons play leap frog over solvent molecules. The dynamics in the triplet state resembled that of two isolated excess electrons. In general, the results are in accord with a wide range of experimental data on metal–ammonia solutions.
Path integral calculation of hydrogen diffusion rates on metal surfaces
Authors: Sun Ying-Chieh , Voth Gregory A.
Published: July 2002

Path integral quantum transition state theory is implemented to calculate the diffusion constant for atomic hydrogen on metal surfaces at low coverage. The path integral theory provides a unified computational methodology to study the influence on the diffusion constant from multidimensional tunneling, vibrational mode quantization, surface distortion, and phonon thermal fluctuations. An approximate technique has also been employed to incorporate the dissipative effect from the electron–hole pair excitations of the metal. The hydrogen diffusion rates on two model metal surfaces are calculated. These surface models are (1) a simple rigid model of the Cu(100) surface allowing a comparison with previous theoretical results, and (2) a more realistic moving model of the Cu(100) surface to examine the effects of surface atom motion. The quantum diffusion constant for hydrogen is calculated over a temperature range of 100–300 K. The largest effect from the moving lattice atoms is found to be the surface distortion effect, leading to a 5% modification of the activation free energy for site-to-site hopping. The phonon thermal fluctuations are not found to significantly enhance or dissipate the tunneling at low temperatures. The electron–hole pair dissipation is, however, estimated to have an effect on the tunneling behavior at the lowest temperature studied (100 K).
Calculation of hyperfine coupling constants of the CN and CP ground state radicals
Authors: Fernández Berta , Jo/rgensen Poul , Simons Jack
Published: July 2002

By carrying out a systematic basis set and electron correlation investigation, we have determined accurately the hyperfine coupling constants of the ground states of CN and CP. The basis set studies began with Dunning’s correlation consistent bases, after which systematic uncontractions and extensions with diffuse and tight functions were introduced until saturation was achieved. The basis set self-consistent-field (SCF) wave function results compare favorably with numerical Hartree–Fock (HF) results. The electron correlation study was based on extending systematically the active space of a complete valence orbital spin-restricted multiconfiguration self-consistent-field (MCSCF) wave function and using the MCSCF restricted–unrestricted response function approach to obtain the hyperfine coupling constants.
Quantum mechanical reaction probabilities with a power series Green’s function
Authors: Auerbach Scott M. , Miller William H.
Published: July 2002

We present a new method to compute the energy Green’s function with absorbing boundary conditions for use in the calculation of quantum mechanical reaction probabilities. This is an iterative technique to compute the inverse of a complex matrix which is based on Fourier transforming time-dependent dynamics. The Hamiltonian is evaluated in a sinc-function based discrete variable representation, which we argue may often be superior to the fast Fourier transform method for reactive scattering. We apply the resulting power series Green’s function to the calculation of the cumulative reaction probability for the benchmark collinear H+H2 system over the energy range 0.37–1.27 eV. The convergence of the power series is found to be stable at all energies and accelerated by the use of a stronger absorbing potential.
Quantum calculations of mode specificity in reactions of H with HOD and H2O
Authors: Wang Desheng , Bowman Joel M.
Published: July 2002

We present approximate quantum calculations of cumulative reaction probabilities, rotationally averaged cross sections, and branching ratios for the reactions H+HOD and H+H2O, using the Walch–Dunning–Schatz–Elgersma potential. The calculations are done using the reduced dimensionality, adiabatic-bend approach in which the three internal bending degrees of freedom are treated adiabatically. The H+HOD calculations focus primarily on the effect of vibrationally exciting the OH or OD local modes of HOD on the reactivity, the branching ratios to form the products HD+OH and H2+OD, and their final vibrational state distributions. The calculations of the H+H2O reaction focus on two initial vibrational states, one with three quanta of vibrational excitation in the local mode stretch, and one with two quanta in one local mode and one quantum in the other local mode. The results are generally in good agreement with experiments and previous calculations.
Effects of a fluctuating electronic coupling matrix element on electron transfer rate
Authors: Tang J.
Published: July 2002

The effects of a fluctuating electronic coupling matrix element on an electron transfer process are examined theoretically. An analytical expression for the electron transfer rate constant was derived using the spin-boson model. In the limit of long correlation time (τv) for the fluctuation, our results are identical to the Marcus expression for the static coupling. The electron transfer rate in the intermediate range of τv, however, can be either enhanced or reduced, depending on the free energy gap and the reorganization energy. In the limit of very short τv, the electron transfer rate decreases with τv, and depends only on τv and the coupling matrix element.
A variational model for the thermodynamical and structural properties of impurities in low temperature solids
Authors: Li Daohui , Voth Gregory A.
Published: July 2002

A variational model is formulated to study the effect of substitutional impurities in solids at very low temperature. The aim of this approach is to improve upon the independent harmonic oscillation assumption in the Einstein model. Since the impurities may vibrate partially independent of the media, an Einstein model is used to describe their motion, while the motion of impurities that correlates with the lattice and the collective motion of the host particles are characterized by a Debye model. By variationally determining the parameters of the model, this approach is used to investigate the effects of various impurities on the thermodynamical and structural properties of solid H2 and solid D2. The averaged effect of the anharmonicity in the pair potentials is included self-consistently in the variational model. However, as suggested from the comparison between the model calculations and computer simulations, a detailed consideration of the anharmonicity is necessary for a highly quantized solid such as H2.