bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of silicon-atom diffusion on the dimer–adatom-stacking fault and Binnig e
t a
l. models of the reconstructed Si(111)-(7×7) surface
Authors: Agrawal Paras M. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

The dynamics of silicon-atom diffusion on the dimer–adatom-stacking fault model (DAS) of the reconstructed Si(111)-(7×7) surface suggested by Takayanagi et al. have been investigated using variational phase-space theory methods. The site-to-site jump frequency is obtained from the variationally minimized total flux across a right cylindrical dividing surface whose cross section in the surface plane is formed from straight line and elliptical segments. This minimized flux is corrected for surface recrossings by the computation of trajectories starting from phase-space points in the transition-state region that are obtained in the Markov walk used to evaluate the phase-space integrals in the expression for the total classical flux. The jump frequencies are used as input to the set of differential equations that describes the diffusion rates on the DAS surface. Values of the diffusion coefficient D are computed from the slopes of plots of the time variation of the root-mean-square displacements obtained from the solution of the rate equations. Arrhenius plots of the results at 300, 600, and 1000 K yield D=0.124 exp[−2.18 eV/kT] cm2/s. These rates are orders of magnitude smaller than the corresponding rates we have previously obtained for silicon-adatom diffusion on the Binnig et al. model of the Si(111)-(7×7) surface. In addition, it is found that the diffusion pattern on the DAS surface is uniform with no preferential directions for silicon-atom flow. In contrast, diffusion on the Binnig surface was found to occur via gateways at three of the four corners of the unit cell. This led to preferential directions for adatom flow. These differences lead us to suggest that careful measurements of silicon-adatom diffusion rates on the Si(111)-(7×7) surface may be a very sensitive measure of the extent to which these surface models accurately describe the experimental Si(111)-(7×7) reconstruction.
Asymptotic wave functions for Be, alkaline earths, and their isoelectronics
Authors: Patil S. H. , Setlur G. S.
Published: July 2002

We have developed equations for obtaining asymptotic wave functions and used them to construct simple, model wave functions for Be, alkaline earths, and their isoelectronics. The general approach is also useful for getting perturbed wave functions. It allows us to obtain reliable expressions for many properties of these systems, such as diamagnetic susceptibilities, polarizabilities, van der Waals constants, hyperpolarizabilities, higher order Zeeman effects, etc.
Brownian dynamics simulation of linear and star polymers
Authors: Bishop Marvin , Smith William
Published: July 2002

Brownian dynamics computer simulations are performed for off-lattice polymer systems in order to investigate the shapes of linear and star polymers. In this manner, proposed equations for the overall radius of gyration, the end-to-end distance and radius of gyration of an individual arm, and the asphericity are tested and found to be in reasonable agreement with the simulation data for a small number of arms.
Photodissociation of the hydroxyl radical (OH) at 157 nm
Authors: Cody R. J. , Moralejo C. , Allen J. E.
Published: July 2002

The photodissociation of the OH radical was studied at 157 nm via the detection of the product H atoms with the resonance fluorescence technique. OH radicals were produced in a fast flow cell from the reaction between H and NO2 and subsequently photodissociated by an excimer laser operating on the F2 emission. The quantum yield for photodissociation of OH was measured to be 1.10±0.28. The photodissociation cross section was calculated to be 6.6×10−18 cm2 (±25%) which is in good agreement with the work of Nee and Lee (Ref. 8).
Forces on nuclei in interacting molecules: New analytical results obtained with nonlocal polarizability densities
Authors: Hunt K. L. C. , Liang Y. Q.
Published: July 2002

When the charge overlap between interacting molecules or ions A and B is weak or negligible, the first-order interaction energy depends only upon the molecular positions, orientations, and the unperturbed charge distributions of the molecules. In contrast, the first-order force on a nucleus in molecule A as computed from the Hellmann–Feynman theorem depends not only on the unperturbed charge distribution of molecule B, but also on the electronic polarization induced in A by the field from B. At second order, the interaction energy depends on the first-order, linear response of each molecule to its neighbor, while the Hellmann–Feynman force on a nucleus in A depends on second-order and nonlinear responses to B. One purpose of this work is to unify the physical interpretations of interaction energies and Hellmann–Feynman forces at each order, using nonlocal polarizability densities and connections that we have recently established among permanent moments, linear response, and nonlinear response tensors. Our theory also yields new information on the origin of terms in the long-range forces on molecules, through second order in the interaction.

One set of terms in the force on molecule A is produced by the field due to the unperturbed charge distribution of B and by the static reaction field from B, acting on the nuclear moments of A. This set originates in the direct interactions between the nuclei in A and the charge distribution of B. A second set of terms results from the permanent field and the reaction field of B acting on the permanent electronic moments of A. This set results from the attraction of nuclei in A to the electronic charge in A itself, polarized by linear response to B. Finally, there are terms in the force on A due to the perturbation of B by the static reaction field from A; these terms stem from the attraction of nuclei in A to the electronic charge in A, hyperpolarized by the field from B. For neutral, dipolar molecules A and B at long range, the forces on individual nuclei vary as R−3 in the intermolecular separation R; but when the forces are summed over all of the nuclei, the vector sum varies as R−4. This result, an analogous conversion at second order (from R−6 forces on individual nuclei to an R−7 force when summed over the nuclei), and the long-range limiting forces on ions are all derived from new sum rules obtained in this work.
Millimeter-wave laboratory detection of HNCCN+
Authors: Amano T. , Scappini Flavio
Published: July 2002

The ground-state rotational spectrum of protonated cyanogen, HNCCN+, has been identified following the infrared detection by Warner and Amano [J. Mol. Spectrosc. 145, 66 (1991)], using a hollow cathode discharge as production source. The rotational and centrifugal distortion constants have been determined to be 4438.010 13(71) MHz and 0.529 95(24) kHz, respectively.
Pressure-induced phase transition in solid hydrogen sulfide at 11 GPa
Authors: Shimizu H. , Nakamichi Y. , Sasaki S.
Published: July 2002

The Raman spectra of hydrogen-bonded molecular solid H2S have been measured up to 23 GPa at 300 K in a gasketed diamond-anvil cell. In the orientationally disordered phase I between 0.47 and 11 GPa, the symmetric stretching mode ν1 shows a red-shift in frequency (dν1/dP=−10.1 cm−1/GPa ) and a dramatic broadening with pressure. At about 11 GPa, the antisymmetric stretching band ν3 appears at the higher-frequency side of ν1. Near this same pressure five low-frequency vibrational modes also appear and show pressure-sensitive features. These results indicate a pressure-induced phase transition near 11 GPa. This new solid phase, which persists to at least 23 GPa at 300 K, seems to be the same phase as previously found above 3.3 GPa at 25 K.
Atomic relaxation at ordered electrode surfaces probed by scanning tunneling microscopy: Au(111) in aqueous solution compared with ultrahigh-vacuum environments
Authors: Gao Xiaoping , Hamelin Antoinette , Weaver Michael J.
Published: July 2002

Detailed surface atomic rearrangements at the ordered Au(111)-aqueous interface triggered by negative electrode charges are examined by in situ scanning tunneling microscopy (STM). Not only the (√3 × 22) atomic reconstruction, but also the longer-range superstructures, formed at this electrochemical interface are seen to be remarkably similar to those observed previously by STM on clean Au(111) in ultrahigh vacuum.
Resonant two-photon ionization spectroscopy of coinage metal trimers: Cu2Ag, Cu2Au, and CuAgAu
Authors: Bishea Gregory A. , Arrington Caleb A. , Behm Jane M. , Morse Michael D.
Published: July 2002

The jet-cooled coinage metal triatomic molecules Cu2Ag, Cu2Au, and CuAgAu have been investigated using resonant two-photon ionization spectroscopy. One band system, labeled as the Ã–X̃ system, has been observed for each species, with origin bands at 13 188, 17 217, and 17 470 cm−1, respectively. Vibrational progressions have been assigned and vibrational constants have been extracted using a linear least-squares fitting procedure. For Cu2Ag, 47 vibrational bands have been assigned within the Ã–X̃ system. The upper states of these bands derive from combinations of two symmetric (a1) and one antisymmetric (b2) mode in the C2v point group. For the Ã–X̃ system of Cu2Au, only seven vibrational bands have been observed, all occurring within a 500 cm−1 range. Lifetime measurements for the observed vibrational levels support the possibility that predissociation may be occurring in the Ã excited state of Cu2Au and this may be limiting the number of vibrational levels observed within this state. Finally, in the case of CuAgAu, 92 vibrational bands have been assigned, corresponding to excitations of three totally symmetric (a′) vibrational modes in the Cs point group. For this molecule, a complete set of vibrational frequencies (ωi) and anharmonicities (xij) have been obtained for the excited Ã state. In addition, the observation of weak hot bands in the spectrum permits the three vibrational modes of the X̃ ground state to be characterized by ν1=222.83±0.29, ν2=153.27±0.22, and ν3=103.90±0.28 cm−1 for 63Cu107Ag197Au (1σ error limits).
On the effect of centrifugal stretching on the rotational partition function of an asymmetric top
Authors: Martin J. M. L. , François J. P. , Gijbels R.
Published: July 2002

Correction terms (up to third order in temperature) for the effect of centrifugal distortion on the rotational partition function of linear molecules, spherical, symmetric, and asymmetric tops are evaluated by means of the classical partition function. It is shown that for the linear, spherical, and symmetric cases, the expressions thus obtained differ from the exact quantum expressions only in the absence of a very small constant correction term. It is then proposed that the partition function for a nonrigid asymmetric top, for which no exact expression has as yet been derived, be evaluated as the product of Watson’s asymptotic expansion for the rigid rotor and the centrifugal correction factor derived in the present work. Numerical comparison with direct numerical summation shows that, even for troublesome cases, this approximation holds very well even at 2000 K. Similar performance is observed for the heat content function, except for pathological cases at high temperatures. In the rigid-rotor case, Watson’s asymptotic series holds very well. As an example application, pilot calculations have been performed on the thermodynamic functions of water and formaldehyde. The present method yields functions in excellent agreement with those obtained by direct rovibrational summation, whereas computer-time requirements are reduced by 3 or 4 orders of magnitude.
Heterodyne spectroscopy of carbon monoxide lines perturbed by hydrogen and helium
Authors: Mannucci Anthony J.
Published: July 2002

The pressure-induced broadening and shift of CO spectral lines has been measured at room and liquid-nitrogen temperatures, using He and H2 as perturber gases. The R(2), P(8), and R(14) rovibrational transitions for the fundamental vibrational band were studied, which absorb in the 5 μm region of the infrared. A novel heterodyne spectrometer operating near 5 μm wavelengths has been used for these measurements. The local oscillator (LO) for the spectrometer was a frequency-doubled CO2 laser, with CdGeAs2 as the nonlinear crystal. This LO was step tunable and could be frequency stabilized to better than 0.5 MHz absolute accuracy. A spectral resolution of 20 MHz was achieved. The CO–He line-broadening measurements were accurate enough (∼2%–3% absolute accuracy) to distinguish between two proposed intermolecular potential models for this system. Also, a measured increase in the broadening coefficient as a function of rotational quantum number j was observed for CO–He at temperatures of 80 K. This is not easy to explain in the context of so-called ‘‘sudden approximations’’ which have been used to interpret pressure-broadening data for this system. The shift measurements are the first reported for the fundamental vibrational band of CO with He or H2. For H2, there appears to be a decrease of the shift with the rotational state j of the transition. For both He and H2, there is a pronounced temperature dependence of the shift for the P(8) transition, the shift becoming ‘‘redder’’ (toward lower frequency) as the temperature decreases from 298 to 80 K.
Photoionization mass spectrometric study of CH3OF
Authors: Ruscic B. , Appelman E. H. , Berkowitz J.
Published: July 2002

The vacuum ultraviolet photoionization mass spectrum of CH3OF displays a prominent parent ion peak, whose adiabatic onset is 11.340±0.008 eV, although much lower energy fragmentation processes (CH2O++HF, 8.0 eV; CH2OH++F, 9.3 eV) are possible. These lower energy processes have very low intensity. Two higher energy processes, to CH+3+OF and CH2O++H+F, are observed. Their thresholds are used to determine ΔH0f0 (CH3OF)≥−23.0±0.7 kcal/mol, or D0 (CH3O–F)≤47.4±1.2 kcal/mol. CH2OF+ is a significant fragment, whose appearance energy leads to ΔH0f0 (CH2OF+)≊215.1±0.8 kcal/mol.
Anisotropic intermolecular potentials for HeC6H6 and HeC5H5N from total differential cross section measurements
Authors: Brandt R. , Henkel M. , Pfeil B. , Seidel W.
Published: July 2002

Differential cross section (DCS) measurements are reported for scattering of a He atomic beam by crossed beams of C6H6 and C5H5N. Damping of the DCS diffraction oscillations is used to extract reliable anisotropic intermolecular potentials, applying the infinite order sudden approximation (IOSA). The isotropic average for the HeC6H6 potential has a well depth of 3.06 meV and an interatomic separation of 614 pm, while the HeC5H5N potential has a well depth of 3.19 meV and an interatomic separation of 620 pm.
Resonant third-harmonic generation in all-trans β-carotene: The vibronic origins of the third-order nonlinear susceptibility in the visible region
Authors: van Beek J. B. , Kajzar F. , Albrecht A. C.
Published: July 2002

The dispersion of the modulus of the third-order nonlinear susceptibility, ‖χ(3)zzzz(−3ω;ω,ω,ω)‖, of an all-trans β-carotene doped polystyrene thin film has been measured by third-harmonic generation throughout the 1 1Bu absorption band of the β-carotene. The observed dispersion is interpreted in terms of a three-electronic state model including explicit regard to six Franck–Condon active normal modes in the resonant electronic state. Both the sum-over-states method and, equivalently, the time domain approach are used. A fit of the three-state model to the data shows that the large nonlinear susceptibility of β-carotene in this wavelength range can be explained by strong dipole coupling between the strong one-photon allowed 1 1Bu state and a nearby two-photon allowed n 1Ag state. The electric dipole transition moment between the 1 1Bu excited state and the n 1Ag excited state, located at 22 200 cm−1, was determined to be 31 D.
Photochemistry of SO2 on Ag(111)
Authors: Castro M. E. , White J. M.
Published: July 2002

The thermal and photon-induced chemistry of SO2 on Ag(111) has been studied using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), and work function change (ΔΦ). SO2 adsorbs reversibly on Ag(111) and desorbs in peaks at 180 and 130 K for monolayer and multilayer, respectively. The work function increases to 0.33 eV upon saturation of the chemisorbed monolayer (ML), but drops to 0.20 eV for a physisorbed multilayer. UV photons (250–542 nm) induce desorption, but not decomposition, of chemisorbed SO2. The photodesorption rate is first order in the photon flux, increases linearly with coverage up to 1 ML, but decreases sharply at higher coverages. For coverages up to 1 ML, the photodesorption cross section is (2.8±0.2)×10−20 cm2 at (313±10) nm. The wavelength dependence is consistent with substrate, including bulk plasmon, excitation. Photodesorption mechanisms involving hot electron and hole attachment processes, only in the first monolayer, are discussed.
Methane dissociation on a nonplanar MgO(001) surface. Theoretical modeling of surface defects
Authors: Bo/rve Knut J.
Published: July 2002

Accurate ab initio quantum mechanical calculations are presented for the abstraction of hydrogen from methane over three- and four-coordinated oxygen sites at an imperfect MgO surface. A local portion of the lattice close to the reaction site is described using a cluster model, whereas the surrounding crystal is represented by its Madelung potential. The four-coordinated oxygen has a closed-shell electronic structure and shows no activity towards hydrogen abstraction. The three-coordinated oxygen is a singly charged radical with high hydrogen affinity and the abstraction takes place here with a barrier of 15 kcal/mol. Subsequent adsorption of the methyl radical is found to be feasible and the adsorption geometry is partly optimized. An extension of the point-charge model in terms of projection operators is found useful when describing Pauli repulsion between the adsorbed methyl entity and neighboring Mg2+ cores.
Computer simulation of polymers in thin layers. I. Polymer melt between neutral walls – static properties
Authors: Pakula Tadeusz
Published: July 2002

Systems of linear chains on the face centered cubic lattice with all lattice sites occupied are simulated by the cooperative motion method. Static properties of chains are analyzed and various statistical models of chain shapes are presented, differing in details of individual chain states taken into account. The structure of thin layers of a polymer melt confined between two parallel neutral walls is compared with the melt structure in bulk. It is shown that the effect of neutral walls consists mainly in reorientation of cigar-shaped chains in the vicinity of walls, but not in changes of dimensions and shapes of chains.
Mapping of transition-state wave functions. II. A model for the photodissociation of ClNO(T1)
Authors: Vegiri Aliki , Untch Agathe , Schinke Reinhard
Published: July 2002

Reflection-type structures are investigated in fragment rotational state distributions following the decay of long-lived resonances with k*=0–2 quanta of bending vibration of the intermediate complex. The calculations are performed in relation to the photodissociation of ClNO(T1). In the absence of an ab initio potential energy surface we construct a model potential energy surface which reproduces some of the main features of this system in qualitative agreement with experimental data. In particular, excitation of the lowest bending state of the ClNO(T1) complex yields a Gaussian rotational distribution while excitation of the first two excited bending states gives rise to bi- and trinodal distributions, respectively. Employing a simple classical model demonstrates that the structures of the final distributions can be explained as a mapping of the corresponding wave function along the transition line which separates the complex region from the exit channel. Within this model the final fragmentation step will be treated as direct dissociation which starts at the transition state with the quantum mechanical coordinate distribution serving for the weighting function. In order to illustrate the sensitivity of the rotational NO distributions we carry out calculations for three potentials with different anisotropy in the exit channel.
The sensitivity of absorption experiments with phase locked ultrashort pulses to the excited state potential energy surface
Authors: Bavli Raanan , Engel Volker , Metiu Horia
Published: July 2002

It is often believed, with some justification, that the excitation of a molecule with an ultrashort pulse generates wave packets whose behavior is well approximated by classical mechanics. Therefore, it seems that such pulses are not particularly useful for accurate studies of quantum dynamics of an excited molecule; at least not when compared to cw spectroscopy. In this paper we show that this shortcoming can be overcome if one studies the emission of a molecule which absorbs one photon by interacting with two phase locked ultrashort pulses. The excited state population is affected by quantum interference and the dependence of the emission on the delay time between the pulses is very sensitive to small changes in the excited state potential. This sensitivity is as high at room temperature as it is at zero degrees Kelvin. The use of phase locked pulses can distinguish between two potential energy surfaces for which the traditional pump–probe experiments give practically identical results. These statements are documented by exact numerical calculations for the case of the I2 molecule and by analysis. We also discuss the reasons why the rotational motion affects absorption from two phase locked pulses at very short delay times between the pulses.
A wave-packet study on nonadiabatic transition dynamics in photodissociation: The importance of parent bending motion
Authors: Guo Hua
Published: July 2002

The influence of parent molecule bending motion on nonadiabatic transitions during photodissociation is investigated using a model involving two linear excited-state surfaces. The two dissociative states are strongly coupled near the so-called conical intersection. Due to symmetry constraints, the two adiabatic surfaces are not allowed to interact in linear configurations and the coupling is only switched on when the molecule is bent. Therefore, electronic transitions from one adiabatic surface to another are only possible when the dissociation process involves bending motion. A quantum-mechanical model including both the dissociation and bending degrees of freedom is established to assess the importance of parent bending motion on nonadiabatic transition dynamics. The coupling between the two electronic states is explicitly taken into account and the dissociation dynamics is described by a time-dependent wave packet. Although the approach is applicable to any triatomic molecular systems, we choose to model a pseudotriatomic system, i.e., methyl iodide CX3I (X=H, D, etc.). In our calculations, the three X atoms are treated as a single pseudoatom and the C–X3 umbrella bending coordinate is frozen at its equilibrium geometry. The two dynamically active coordinates are the I–CX3 stretch and the I–C–X3 bend. Both the ground and two excited states are represented by linear potential-energy surfaces and the coupling of the two dissociative surfaces is a conical intersection in nature. Several dissociation processes are modeled with different initial bending wave functions and different isotopic substitutions. It is found that parent bending motion has a significant effect on the final electronic branching of dissociation fragments. The calculation generates a larger I* yield from the CD3I dissociation than that from CH3I, in agreement with experimental observations which could not be reproduced by previous theoretical calculations. Our model also predicts that the dissociation of the first bending overtone of methyl iodide gives a smaller I* yield than that of its ground-state counterpart, which is consistent with a recent experiment on the CF3I photodissociation.
Electron-spin resonance and ground states of the boron and aluminum trimers
Authors: Hamrick Y. M. , Van Zee R. J. , Weltner W.
Published: July 2002

Electron-spin-resonance (ESR) spectra of 11B3(i=3/2) in neon, argon, and krypton, 10B3(i=3) in argon, and 27Al3(i=5/2) in argon, krypton, and methane matrices at 3–30 K indicate that both trimers contain one unpaired electron (S=1/2) and three equivalent nuclei (D3h symmetry). The derived hyperfine (hf) parameters for 11B3 in argon are isotropic ‖A∥‖=‖A⊥‖=130 MHz, yielding about 15% s character in the wave function of the unpaired spin. Rotation of B3 in the matrices, causing spin relaxation and consequent linewidth variation as a function of MI, also causes the anisotropic hf contributions to average to zero. Some hf anisotropy was observed for 27Al3 in solid argon ‖A∥‖=109, ‖A⊥‖=100 MHz and varied with the matrix gas, indicating some hindering of free rotation. The derived s character for Al3 in its ground state is ∼8%. It is concluded that both trimers have 2A′1 ground states, in agreement with ab initio theory and, in the case of Al3, with the Stern–Gerlach experiment of Cox et al.
Reversible electron transfer dynamics in non-Debye solvents
Authors: Zhu Jianjun , Rasaiah Jayendran C.
Published: July 2002

The general solutions obtained earlier [J. Chem. Phys. 95, 3325 (1991)] for the coupled diffusion-reaction equations describing reversible electron transfer reactions in Debye solvents, governed by Sumi–Marcus free energy surfaces, are extended to non-Debye solvents. These solutions, which depend on the time correlation function of the reaction coordinate Δ(t), are exact in the narrow and wide window limits for Debye and non-Debye solvents and also in the slow reaction and non-diffusion limits for Debye solvents. The general solution also predicts the behavior between these limits and can be obtained as the solution to an integral equation. An iterative method of solving this equation using an effective relaxation time is discussed. The relationship between Δ(t) and the time correlation function S(t) of Born solvation energy of the reacting intermediates is elucidated.
The decarboxylation and dehydration reactions of monomeric formic acid
Authors: Goddard John D. , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

Two unimolecular dissociation reactions of formic acid have been investigated theoretically. The decarboxylation reaction yields molecular hydrogen and carbon dioxide, while the dehydration reaction produces water and carbon monoxide. The 1,2-hydrogen shift rearrangement from formic acid to dihydroxymethylene has also been considered. Methods ranged from double zeta plus polarization self-consistent field to triple zeta plus double polarization coupled cluster singles and doubles. For certain key structures, the coupled cluster method including single, double, and linearized triple excitations (CCSDT-1) was applied as well with the double zeta plus polarization basis set. A barrier height of ∼71 kcal mol−1 with zero point vibrational energy correction is predicted for the dissociation to molecular hydrogen and carbon dioxide. A rather comparable value of ∼68 kcal mol−1 is predicted for the barrier to the dehydration reaction. The 1,2-hydrogen shift transition state is somewhat higher in energy at ∼79 kcal mol−1. These predicted energy barriers are discussed with reference to the existing experimental results on the thermal decomposition of formic acid.
A consistent treatment of clustering effects in electrolytes
Authors: Given James A. , Stell George
Published: July 2002

We use the methods of continuum percolation theory to develop a consistent, essentially analytic theory for the properties of the restricted primitive model (RPM) of electrolytes. Our preliminary results are in good agreement with simulation data for the free energy and osmotic coefficient of the RPM.
Monte Carlo study of mixed electrolytes in the primitive model
Authors: Bešter M. , Vlachy V.
Published: July 2002

Canonical Monte Carlo results are presented for mixtures of primitive model electrolytes with a common ion. Both symmetric mixtures, where the ions differ only in size, and charge asymmetric mixtures were studied for ionic strengths ranging from 0.003 to 1.0 M. The hypernetted chain (HNC) equation theory and the simple ‘‘exponential’’ (EXP) approximation are both applied to the same electrolyte models. The electrolyte mixing coefficients w0 and w1 were calculated from osmotic coefficients. Comparison with Monte Carlo data indicates that the HNC equation yields accurate predictions for the zeroth mixing coefficient w0, while the simple EXP approximation yields qualitatively correct results. The Monte Carlo results for the first mixing coefficient, w1, are not precise enough to allow a quantitative comparison with other theories. However, a strong concentration dependence of w1 for dilute solutions, found previously for the nonprimitive models in the HNC approximation, is confirmed by the Monte Carlo results.
Mode selectivity in reactions of H with HOD(100), HOD(001), and HOD(002)
Authors: Bowman Joel M. , Wang Desheng
Published: July 2002

Mode selectivity in reactions of hydrogen with deuterated water is studied. (AIP)
Intermolecular energy transfer: Retardation effects
Authors: Andrews David L. , Juzeliūnas Gediminas
Published: July 2002

An extension of previous theoretical work on the unified theory of radiative and radiationless intermolecular energy transfer is presented. A generalized transfer rate accounting for molecular vibronic structure is derived, enabling the formal connection with the classical Förster formula to be fully established. The solution to an apparent paradox concerning the long-range R−2 dependence of the intermolecular energy transfer rate is demonstrated. It is shown that the inverse square behavior should be modified by inclusion of an exponential factor due to the presence of other acceptors. A corrected Förster decay rate including an R−4 contribution, in addition to the conventional R−6 term, is obtained and the means of characterizing distinctive features of the unified approach are discussed with reference to some model systems. Finally the relation between retardation and quantum uncertainty effects in molecular energy transfer are considered.
Pressure dependence of zero-field splittings in organic triplets. III. Vibronic systems
Authors: Baber J. L. , Chan I. Y.
Published: July 2002

We report optically detected magnetic resonance (ODMR) experiments at pressures up to 40 kbar for p-benzoquinone (BQ) in dibromobenzene (DBB) and for a deep x-trap in a BQ-d4 neat crystal. It is known from previous works that the phosphorescent triplet state (3B1g) of these systems is vibronically coupled to a nearby 3Au state, such that the former exhibits a double-minimum potential well. The zero-field splitting (ZFS) parameter D of both systems changes by several gigahertz over our pressure range. The greatest slope occurs at zero pressure, and the rate of change slows down exponentially at higher pressure. In addition, we observed a phase transition in BQ-d4 crystals at 6 kbar, which causes a huge step-discontinuity in both D and E. In contrast to the enormous change in D, the E-value is not sensitive to pressure except for the step-change during the phase transition. Triplet state kinetics and phosphorescence spectra are used to establish the existence of an in-plane distortion at least in the high-pressure phase. A model is proposed to interpret these results based on the pressure-dependent vibronic interaction, leading to a reduction of the spin–orbit coupling contribution to the ZFS.
Weighted density functional theories of drying at solid–fluid interfaces: Clarification of recent controversies
Authors: Henderson J. R. , Tarazona P. , van Swol F. , Velasco E.
Published: July 2002

The statistical mechanics of contact angle phenomena and wetting/drying transitions at wall–fluid interfaces has benefitted from a series of recent computer simulation studies and density functional treatments. Various groups have contributed to this effort and not all of the results published to date appear to be in agreement. In particular, seemingly conflicting reports of the nature of the drying transition at model wall–liquid interfaces has generated significant confusion. Our purpose here is to clarify the situation by (i) demonstrating full consistency between two weighted density functional studies of wall–fluid interfaces, previously thought to be in conflict, and by (ii) reporting on a new density functional analysis of an earlier controversy surrounding the measurement of contact angles by computer simulation procedures involving ‘‘live’’ walls.
Rotational dynamics and orientational melting of C60: A neutron scattering study
Authors: Neumann D. A. , Copley J. R. D. , Kamitakahara W. A. , Rush J. J. , Cappelletti R. L. , Coustel N. , Fischer J. E. , McCauley J. P. , Smith A. B. , Creegan K. M. , Cox D. M.
Published: July 2002

Well-defined librational excitations have been observed at energies of 2–3 meV in the low temperature ordered phase of solid C60. These relatively high energies imply a stiff orientational potential below the transition. The sharpness of the peaks indicates that this potential does not depend strongly on the axis of the angular displacement. The modes soften and broaden as the temperature approaches that of the orientational melting transition which occurs when the librational amplitude is a considerable fraction of nearest-neighbor interatomic angles.
A perturbation theory for the calculation of the average survival time of diffusion influenced chemical reactions
Authors: Pagitsas Michael
Published: July 2002

A systematic perturbation theory is developed for the calculation of the average survival time, τ̄, and the long-time limit of the effective rate coefficient, k̄, of Brownian particles undergoing diffusion influenced chemical reaction. The chemical reaction is taking place in the bulk and is represented by a sink term, originally introduced by Wilemski and Fixman in the Smoluchowski equation, and depends on the reaction coordinate. The perturbation theory is based on a small dimensionless parameter, ε, which is a measure of the strength of the diffusive process relative to the reactive. Analytical expressions are obtained for the average survival time and the effective rate coefficient for an arbitrary nonequilibrium initial distribution of the reaction coordinate. The effective rate coefficient is reduced to that obtained by Wilemski and Fixman, who used the closure approximation, when the initial distribution of the reaction coordinate is the equilibrium one and keeping only the lowest order term in the perturbation expansion.
State-selectivity of excited-state intramolecular proton transfer in a ‘‘double’’ benzoxazole: Jet spectroscopy and semiempirical calculations
Authors: Ernsting N. P. , Arthen-Engeland Th. , Rodriguez M. A. , Thiel W.
Published: July 2002

The fluorescence excitation spectra of free 2,5-bis(2′-benzoxazolyl)-hydroquinone (BBXHQ) cooled in a supersonic jet are examined. By using double-resonance saturation spectroscopy, we show that adjacent vibronic transitions, leading to either blue or red fluorescence, belong to a single molecular species in the electronic ground state. The molecular geometries of the enol and keto forms which are relevant for intramolecular transfer of a single hydrogen atom in the S1 state are obtained by the MNDO/H method. The translocation distance is calculated to be 0.43 Å. The complexity of the vibronic spectrum, near the electronic origin for the S0(1Ag) → S1(1Bu) transition of the enol form at 4178.1 Å, is attributed to vibronic coupling with the nonplanar excited keto form due to H tunneling.
Dynamics of molecular surface diffusion: Origins and consequences of long jumps
Authors: Dobbs K. D. , Doren D. J.
Published: July 2002

The mechanics of molecular surface diffusion have been studied in a theoretical model of CO/Ni(111). Using molecular dynamics, diffusion rates have been calculated over a wide range of temperatures and interpreted using methods typically applied to experimental measurements. This interpretation is based on transition state theory and a model of uncorrelated hops between near neighbors. An Arrhenius plot of diffusion constants from the simulations is linear from 175 to 1000 K. However, the underlying dynamics do not conform to the model of uncorrelated hops. Instead, molecules that have been excited to a transition state tend to fly past several sites before settling onto a new one. These multiple site flights (‘‘long jumps’’) make the Arrhenius prefactor larger than the transition state theory prediction by more than an order of magnitude. Transition state recrossings have a small effect on the diffusion rate. Long jumps are typical of a ‘‘low friction’’ regime in which energy exchange is slow between lateral translation and other modes. Completely freezing the surface motion has a relatively small effect on flight lengths, and coupling of adsorbate lateral translations to other adsorbate modes is as important as coupling to the surface. The dependence of these results on details of the model is discussed and the frictional forces in this model are compared to other theoretical and experimental estimates of these forces.
Autoionizing-resonance enhanced preferential photodissociation of CO2 in superexcited states
Authors: Ukai Masatoshi , Kameta Kosei , Kouchi Noriyuki , Nagano Kazunori , Hatano Yoshihiko , Tanaka Kenichiro
Published: July 2002

Fluorescence excitation spectra of CO2+(Ã 2Πu→X̃ 2Πg), (B̃ 2∑u+→X̃ 2Πg), and CO(A 1Π→X 1Σ) emissions produced in the photoionization and neutral photodissociation of CO2 have been obtained in the 30–92 nm region. A strong competition between autoionization and neutral dissociation observed in the region near the ionization thresholds to form CO2+(Ã 2Πu) and CO2+(B̃ 2Σu+) states clearly shows the preferential enhancement of the neutral dissociation, which is explained mainly by the intrinsic autoionization rate of the individual superexcited Rydberg states. A drastic step-down decrease in a fluorescence excitation spectrum in the vacuum ultraviolet region at the thresholds has been ascribed to a dramatic density dilution of the superexcited states into the continuum.
High resolution spectroscopy of 1,2-difluoroethane in a molecular beam: A case study of vibrational mode-coupling
Authors: Mork Steven W. , Miller C. Cameron , Philips Laura A.
Published: July 2002

The high resolution infrared spectrum of 1,2-difluoroethane (DFE) in a molecular beam has been obtained over the 2978–2996 cm−1 spectral region. This region corresponds to the symmetric combination of asymmetric C–H stretches in DFE. Observed rotational fine structure indicates that this C–H stretch is undergoing vibrational mode coupling to a single dark mode. The dark mode is split by approximately 19 cm−1 due to tunneling between the two identical gauche conformers. The mechanism of the coupling is largely anharmonic with a minor component of B/C plane Coriolis coupling. Effects of centrifugal distortion along the molecular A-axis are also observed. Analysis of the fine structure identifies the dark state as being composed of C–C torsion, CCF bend, and CH2 rock. Coupling between the C–H stretches and the C–C torsion is of particular interest because DFE has been observed to undergo vibrationally induced isomerization from the gauche to trans conformer upon excitation of the C–H stretch.
Axis-switching transitions and the stimulated emission pumping spectrum of HCN
Authors: Jonas David M. , Yang Xueming , Wodtke Alec M.
Published: July 2002

Six of the 14 unidentified bands in the stimulated emission pumping (SEP) spectrum of HCN are shown to be forbidden transitions to l″=1 e parity levels of the ground state. The band origins agree with predictions within the error of the anharmonic expansion; the rotational constants, when corrected for rotational-l doubling, agree within experimental error. Rotational-l resonance between l″=0 and l″=2 is found in highly excited bending levels, confirming the extrapolation of the rotational-l resonance and doubling constant q2 from microwave and infrared measurements to 17 000 cm−1. The rotational intensity of the l″=1 bands due to the axis-switching mechanism of Hougen and Watson [Can. J. Phys. 43, 298 (1965)] is shown to be greater than some of the observed allowed rotational transitions to l″=2 when laser polarization effects are taken into account. A qualitative Franck–Condon analysis of the SEP spectrum provides unusually strong evidence for the axis-switching mechanism. The eight remaining unassigned bands are evidently perturbed and are assigned based on agreement between sums of observed rotational constants and sums of zero-order (unperturbed) rotational constants predicted by the anharmonic expansion, the magnitude of the rotational-l resonance, and the expected Franck–Condon factors.
Green’s function calculation of the valence photoemission spectra of PF3 and NiPF3
Authors: Ohno M. , von Niessen W. , Zakrzewski V.
Published: July 2002

We have performed ab initio algebraic diagrammatic construction [ADC(3)] Green’s function calculations of the valence photoemission spectra of PF3 and NiPF3. We obtained overall good agreement with experiment for both the free PF3 molecule and the PF3 molecule chemisorbed on a Ni(111) metal surface. A comparison to NiCO shows that there are certain similarities between NiPF3 and NiCO, not only in the σ donor–π acceptor bonding mechanism in the ground state, but also in the metal–ligand CT excitations associated with the creation of a valence hole in the ligand. However, it appears that the many-body effect, such as the configuration interactions in the final ionized state, seems to be weaker for NiPF3 than for NiCO, judging from the main line spectral intensity. The quasiparticle picture of the 4e level breaks down completely as in the case of the 1π level of NiCO.
The vibrational predissociation of Ar–CO2 at the state-to-state level. II. Rotational propensity rules and vector correlations
Authors: Bohac E. J. , Marshall Mark D. , Miller R. E.
Published: July 2002

The vibrational predissociation of Ar–CO2 has been studied at the state-to-state level with the emphasis of the present work being on the determination of the rotational propensity rules governing this process and their dependence on the initial state of the parent complex. We find that for the low J, K=1 initial states associated with the (101)/(0201) Fermi diad of the parent complex, there is a strong propensity for the production of the CO2 fragments in odd j states. This propensity rule breaks down as J′ of the complex increases and preliminary results show that the final state distributions are also strongly dependent on K. The dependence of the photofragment angular distributions on the laser polarization direction is also reported, which provides additional dynamical information (vector correlations) on the nature of the photodissociation process.
Generalized Mo/ller–Plesset and Epstein–Nesbet perturbation theory applied to multiply bonded molecules
Authors: Murphy Robert B. , Messmer Richard P.
Published: July 2002

A generalized Mo/ller–Plesset theory based upon a multiconfigurational reference wave function is applied to the problem of the dissociation of multiple bonds, in particular, to the molecules N2, C2H2, O2++, and C2H4. The second order generalized Mo/ller–Plesset theory is shown to be nearly as accurate as corresponding multireference configuration interaction (CI) calculations with less computational effort. An analysis is given of the size of the multiconfigurational reference necessary for a perturbative treatment of the dissociation of multiple bonds. The relative merits of the computationally simpler multiconfigurational Epstein–Nesbet perturbation theory are presented.
Surface π bonding and the near-first-order desorption kinetics of hydrogen from Ge(100)2×1
Authors: D’Evelyn Mark P. , Cohen Stephen M. , Rouchouze Eric , Yang Yuemei L.
Published: July 2002

We show by temperature-programmed desorption that hydrogen desorbs from Ge(100)2×1 near 570 K with near-first-order kinetics, similar to the behavior of hydrogen on Si(100)2×1. The near-first-order desorption kinetics are attributed to pairing on surface dimers induced by the π bond on unoccupied dimer atoms, and a pairing energy of 5±1 kcal/mol is inferred. However, a comparison between the pairing energies for H atoms on Ge(100) and Si(100) with the electronic structure of the respective clean surfaces indicates that estimates of the π bond strength based on the surface band structure do not correlate with the propensity for pairing.
Molecular expression for dielectric friction on a rotating dipole: Reduction to the continuum theory
Authors: Bagchi B. , Vijayadamodar G. V.
Published: July 2002

Recently we presented a microscopic expression for dielectric friction on a rotating dipole. This expression has a rather curious structure, involving the contributions of the transverse polarization modes of the solvent and also of the molecular length scale processes. It is shown here that under proper limiting conditions, this expression reduces exactly to the classical continuum model expression of Nee and Zwanzig [J. Chem. Phys. 52, 6353 (1970)]. The derivation requires the use of the asymptotic form of the orientation-dependent total pair correlation function, the neglect of the contributions of translational modes of the solvent, and also the use of the limit that the size of the solvent molecules goes to zero. Thus, the derivation can be important in understanding the validity of the continuum model and can also help in explaining the results of a recent computer simulation study of dielectric relaxation in a Brownian dipolar lattice.
Emission from OH(A) produced in the dissociative recombination of H2O+ with electrons
Authors: Sonnenfroh D. M. , Caledonia G. E. , Lurie J.
Published: July 2002

The production of OH(A) from the dissociative recombination of H2O+ with electrons has been studied in a crossed jet experiment. The branching ratio into the OH(A)+H product channel has been estimated to be 7.6%+8%/−4% for an electron temperature of ∼1000 K. The internal energy of the OH(A) has been determined from modeling the emission spectrum. The vibrational distribution is 1.00:0.87 for v=0:v=1. The estimated error is ±15% for both states. The rotational temperatures were found to be 4000 K for both v=0 and 1. The estimated error is +500, −1000 K. The present work represents one of the first measurements of the internal energy distribution of the molecular products of the dissociative recombination of a molecular cation and an electron.
Effect of a single solvent atom on bimolecular reactions: Collisions of O(3P) with hydrocarbon–argon clusters
Authors: Hurwitz Y. , Rudich Y. , Naaman R. , Gerber R. B.
Published: July 2002

The reaction between an O(3P) atom and a hydrocarbon molecule weakly bound to an argon atom was studied by classical trajectory simulations. The results are compared to those obtained for the reaction of a free hydrocarbon. A simplistic model system was constructed in which the hydrocarbon was represented as a pseudodiatomic molecule. Although simple, the model reproduced correctly the internal energy distribution in the OH produced in the reaction of the free species. It was found that the OH, produced from the reaction of the van der Waals complex, emerges with less internal energy and less translational energy than the OH from the monomeric process. In the case of the complexed reagents, the collision complex lifetime is longer and the oxygen explores portions of the potential energy surface that are not available in the monomeric reaction.