bluuebunny/crossref_metadata_2025 huggingface dataset:
Field-dependent C-13 chemical shifts in solids: A second-order dipolar perturbation
Authors: VanderHart D. L.
Published: July 2002

The observation of field-dependent C-13 chemical shifts in the presence of high-power proton decoupling and magic angle sample spinning (MAS) is documented. While the principal data were taken at fields of 1.4 and 4.7 T, the difference in chemical shift, in ppm, between the crystalline resonance of polyethylene and a reference resonance of solid adamantane varied as (a+bB−20) in measurements taken at six different fields in as many laboratories. At a given field there is no dependence of this shift difference on proton resonance offset, proton rf field strength, or sample spinning speed. In rigid solids, the ‘‘b’’ term in the foregoing relationship is twice as large for a methylene as for a methine carbon. Results of chemical shift measurements at two fields are reported for polyethylene, polypropylene, and three molecular solids including the normal alkane, nonadecane, which exhibits fast well-defined molecular rotation in the solid rotator phase. The observed shift differences for several kinds of carbons at 1.4 and 4.7 T agree very well with the explanation that the b term in the above expression results from a second-order energy perturbation involving the nonsecular ‘‘C’’ and ‘‘D’’ terms of the dipolar Hamiltonian. A simple theory is presented for a C-13-proton spin pair, and the role of MAS is examined. A line broadening equal to about one-half of the mean shift is also associated with this phenomenon. The behavior of this second-order shift is also considered for cases where fast molecular motion is present or where multiple protons are bonded to a carbon. Even at 1.4 T the shifts are rather small, i.e., 0.72 and 0.36 ppm, respectively, for a methylene and a methine carbon. Corrections which will account for this second-order shift are easily made in powder samples of rigid molecules; thus, true chemical shifts can be recovered. Above an operating field of, say, 3 T, the degradation of linewidth due to the spread of these second-order shifts is negligible (<3 Hz) even for a methylene group. Finally, a challenge is offered to consider whether similar second-order shifts operate in homonuclear coupled systems in the presence of homonuclear decoupling and MAS.
Electric conductivity measurements on the nematic states of a micellar solution of potassium laurate/1-decanol/D2O
Authors: Photinos P. , Melnik G. , Saupe A.
Published: July 2002

We present electric conductivity measurements on a mixture of 25.15 potassium laurate/6.33 1-decanol/68.52 D2O by weight. The mixture shows two isotropic phases, two uniaxial nematics, and a biaxial nematic phase. The variation of the conductivities with temperature shows the phase transitions, which were also optically verified. The average conductivity does not change at the phase transitions. It shows a temperature dependence that is almost fully due to the effect of the viscosity of the aqueous phase on the ion mobility. The anisotropy of the conductivity in the uniaxial nematic phase is relatively weak. An analysis of the conductivity in terms of micellar structures suggests relatively small micelles of ellipsoidal shape. The reorientation of NC in magnetic fields of 10 kG has relaxation times of 20–40 s. The rotational viscosity is in the order of 10 P.
Gas phase studies of Na diffusion in He and Ar and kinetics of Na+Cl2 and Na+SF6
Authors: Talcott Carol L. , Ager Joel W. , Howard Carleton J.
Published: July 2002

A fast flow reactor, using an oven source and resonant fluorescence detection, was built to study the kinetics of sodium atoms in the gas phase. The rate coefficients for Na+Cl2 and Na+SF6 are (7.80±1.6)×10−10 and (1.17±0.2)×10−12 cm3 molecule−1 s−1, respectively. Since collisions with the wall remove sodium with approximately unit efficiency, gaseous diffusion coefficients of sodium in the carrier gas can be measured. DNa,He=325±33 cm2 Torr s−1 at 290 K and DNa,Ar=140±14 cm2 Torr s−1 at 281 K. The experimental results are compared with previous studies and simple theoretical models.
Resonance rotational Raman scattering of symmetric tops: A probe of molecular photodissociation
Authors: Ziegler L. D.
Published: July 2002

Expressions are derived using irreducible tensor notation describing resonance rotational Raman scattering intensities for symmetric top molecules. Isotropic, antisymmetric, and anisotropic scattering contributions to the resonance O, P, Q, R, and S rotational Raman transitions are considered. Excited state lifetimes in the range of ∼0.02 to 1 ps or ∼1 to 100 vibrational periods can be determined by fitting the pattern of observed relative ΔJ=0 and ΔJ≠0 rotational or rovibrational scattering intensities. UV Raman scattering spectra resonant with photodissociative levels of the X̃→Ã Rydberg transition of NH3 and ND3 are used to illustrate this technique. Rates of ammonia photodissociation are found to have a strong dependence on the excited bending level quantum number.
The ν2 fundamental band of triplet CH2
Authors: Marshall Mark D. , McKellar A. R. W.
Published: July 2002

The ν2 (bending) fundamental band of CH2 in its X̃ 3B1 ground electronic state has been studied using tunable diode laser spectroscopy and also the LMR (laser magnetic resonance) technique in the 800–911 cm−1 and 1030–1173 cm−1 regions. For the diode laser study, a multiple-traversal absorption cell was used with Zeeman modulation, and the CH2 radicals were produced by a mild discharge in a flowing mixture of ketene (∼0.3 Torr) and helium (∼2 Torr). Under these conditions, the apparent lifetime of CH2 following cutoff of the discharge was about 1 ms. A total of 53 new transitions with rotational quantum numbers up to N=10, K″a =3, and Ka =2 were observed to add to the 11 transitions previously measured in this band. A combined analysis of the present data and all the previous pure rotational and ν2 band results on CH2 has yielded an improved set of molecular parameters, including the first determination of centrifugal distortion effects for the dominant spin–spin interaction parameter, D. The present data have also been used in a comprehensive new analysis of triplet methylene rotation–vibration energies using the nonrigid bender Hamiltonian by Bunker et al. in an accompanying paper.
Effective dephasing theory of the optical Anderson transition as probed by four-wave-mixing spectroscopy
Authors: Loring Roger F. , Mukamel Shaul
Published: July 2002

A novel theory of quantum mechanical transport in disordered systems is developed. The theory is based on the effective dephasing approximation (EDA), in which the ensemble averaged Liouville space propagator is mapped into the propagator of an ordered lattice with an effective frequency-dependent dephasing rate. This generalized dephasing rate is determined self-consistently. This approach is applicable to strongly disordered systems and yields results that interpolate between the limits of coherent and incoherent excitation transport and that predict the optical analog of a metal–insulator phase transition (Anderson localization). Our results agree with the predictions of the scaling theory of the Anderson transition. We apply the EDA to the calculation of the transient grating signal from a crystal with an inhomogeneously broadened absorption spectrum (static, site energy disorder). The transient grating experiment is shown to be a sensitive probe of the optical Anderson transition.
Discovery of an emission in NO++
Authors: Besnard M. J. , Hellner L. , Malinovich Y. , Dujardin G.
Published: July 2002

Doubly charged NO++ ions were produced by double photoionization of neutral nitric oxide molecules with the synchrotron radiation from ACO as a photon source of variable energy in the 35–68 eV range. Two stable states of NO++ were observed in the double photoionization spectrum with energies of 38.6 and 40.0 eV. An emission from NO++ was discovered by using a new photoion–photon of fluorescence coincidence (PIFCO) experiment. The fluorescence efficiency was measured as a function of the excitation energy, indicating that the emitting state, whose onset energy was observed at 42.5 eV, is a stable or slowly predissociating state of NO++. On the basis of the comparison with calculated energies, the three stable states of NO++ with energies at 38.6, 40.0, and 42.5 eV were assigned to, respectively, the X 2Σ+, A 2Π, and B 2Σ+ electronic states. The detected emission was ascribed to a B 2Σ+→X 2Σ+ emission around 3170 Å.
Effect of the potential well on low temperature pressure broadening in CO–He
Authors: Palma Amedeo , Green Sheldon
Published: July 2002

Previously reported low temperature pressure broadening calculations for CO–He interacting via an SCF-CI potential are compared with new calculations in which the attractive part of the potential is (1) reduced by half and (2) eliminated entirely. Results demonstrate that the attractive well is responsible for low temperature enhancement of pressure broadening cross sections and suggest that agreement with recent experimental values at 4 K can be obtained by a modest reduction, probably within the expected uncertainty, in the attractive part of the SCF-CI potential.
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o calculations of electronic and vibrational energies of HCO and HOC
Authors: Bowman Joel M. , Bittman Joseph S. , Harding Lawrence B.
Published: July 2002

The ab initio calculation of electronic energies for numerous configurations of HCO and HOC, and a novel method for fitting the energies to a global surface are reported. This surface is used to calculate all the bound vibrational states of nonrotating HCO and DCO using the Watson Hamiltonian. Some quasibound vibrational states are also reported for nonrotating HOC for energies below the HOC saddle point energy. Comparisons of the HCO and DCO vibrational energies are made with recent experimental results.
The assignment of cluster vibrational predissociation spectra by mass spectroscopic detection: The use of isotopes
Authors: Kolenbrander Kirk D. , Lisy James M.
Published: July 2002

A technique for identifying and eliminating larger cluster contamination in vibrational predissociation spectra of van der Waals clusters using mass spectroscopic detection is presented. The application of this technique to the study of benzene dimer is discussed.
CCl2(Ã 1B1) radical formation in VUV photolyses of CCl4 and CBrCl3
Authors: Ibuki Toshio , Takahashi Norio , Hiraya Atsunari , Shobatake Kosuke
Published: July 2002

In vacuum ultraviolet photolyses of CCl4 and CBrCl3 a diffuse emission band was observed in the region of 410–750 nm by Ar i resonance and H Lyman-α line irradiation. The band was attributed to a CCl2(Ã 1B1 → X̃ 1A1) transition from the measurements of the appearance energies of the emitters produced from photodissociative excitation of both CCl4 and CBrCl3 using synchrotron radiation. The fluorescence decay of the CCl2(Ã → X̃) transition showed a superposition of two lifetime components of 2.17±0.26 and 4.0±0.12 μs at pressures from 10 to 140 mTorr. The pressure dependence of the amplitudes for the two lifetimes suggests the occurrence of collision-induced intersystem crossing between 1B1 and 3B1 states of CCl2 radicals. The absorption cross section of CBrCl3 was measured for the first time in the 106–200 nm wavelength region and tentative assignments of Rydberg transitions are presented.
Anisotropy and asymmetry of the polarizability of anthracene, studied by NMR
Authors: Ruessink B. H. , MacLean C.
Published: July 2002

NMR of dielectrically aligned apolar molecules in solution is presented as a new technique for the study of their electric polarizability tensor (α). Spectral changes in the 2H NMR spectrum of anthracene-d10 (in dilute benzene solution), subjected to a very strong external dc electric field, were interpreted in terms of Δα and δα, the anisotropy and asymmetry of the polarizability. The results are: Δα=(15.4±0.4)×10−24 cm3 and δα=(−9.6±1.2)×10−24 cm3. The new NMR method is outlined and compared with existing methods for the determination of α.
Excitation of multiple quantum transitions under magic angle spinning conditions: Adamantane
Authors: Meier B. H. , Earl William L.
Published: July 2002

A pulse sequence designed for the excitation of multiple quantum transitions in magic angle spinning solid state NMR spectroscopy is presented. It is shown that under the action of the standard time-reversal pulse sequence, the change in the sign of the dipole coupling (which is used to generate the multiple quantum coherences) upon rotation causes the multiple quantum intensity to vanish after each rotor period. This effect is demonstrated both in calculations and in experimental 1H spectra of adamantane. A modification of the time-reversal pulse sequence, which involves switching the phase of the rf pulses every half-rotor period causes the spin part of the Hamiltonian to switch sign in synchrony with the modulation of the spacial part. This allows the creation of multiple quantum coherences in solids with magic angle spinning. The effectiveness of this pulse sequence is demonstrated through calculations and experiments.
Mutual neutralization and chemiionization in collisions of alkali metal and halogen atoms
Authors: Cooper D. L. , Bienstock S. , Dalgarno A.
Published: July 2002

A semiempirical valence bond method is used to obtain realistic potential curves for the three lowest 1∑+ curves of NaF, KF, LiCl, NaCl, KCl, Lil, and Nal. They are used together with a model functional form for the diabiatic coupling to compute quantal chemiionization and mutual neutralization cross sections. The accuracy of one-electron estimates of the interaction energies in the crossing regions is explored. The Landau–Zener method is found to be very reliable except for collisions involving iodine, for which modifications are necessary.
A simple kinetic theory model of reactive collisions. III. Convex loaded bodies
Authors: Evans Glenn T.
Published: July 2002

Derived is a classical ‘‘line-of-normals’’ model for the treatment of the orientation dependence of the reaction cross section for nonspherical convex bodies intended to represent loaded diatoms in reactive collisions with (spherical) atoms. For the case of nonspherical molecules with small loading, simple formulas are obtained which display explicitly the dependence of the orientational cross section on the nonsphericity and the loading parameters. Applications to realistic systems are presented.
Theory of carrier motion in dynamically disordered systems
Authors: Loring Roger F. , Sparpaglione Massimo , Mukamel Shaul
Published: July 2002

We present a quantum mechanical theory of the dynamics of a charge carrier or an electronic excitation in a condensed phase system, in which the solvent degrees of freedom that couple to the electronic excitation are characterized by a correlation time of arbitrary magnitude. We consider a charge carrier moving among active sites that are randomly distributed in space. The site energies undergo stochastic modulation with a finite correlation time, through the interactions with the solvent. A mode-coupling self-consistent equation is derived from which transport properties such as the ac conductivity, the mean-squared displacement, and the time-dependent probability that a carrier remains on the initial site are calculated. A metal–insulator transition is predicted in three dimensions, but not in one or two dimensions, in agreement with the scaling theory of Anderson localization. The present treatment allows the investigation of carrier dynamics even when there is no separation of time scales between the dynamics of carrier and solvent.
Path integral Monte Carlo studies of the behavior of excess electrons in simple fluids
Authors: Coker D. F. , Berne B. J. , Thirumalai D.
Published: July 2002

The behavior of an excess electron in helium (at T=309 K) and xenon (at T=309 K and T=248 K) is studied over a range of fluid densities (ρ*=ρσ3=0.1–0.9). A path integral Monte Carlo technique is used to model the ‘‘quantum’’ electron which interacts through pseudopotentials with the ‘‘classical’’ solvent particles. In helium, the electron becomes confined in a cavity in the solvent and behaves like a particle in a spherical box. We observe contrasting behavior in the more polarizable xenon solvent where the electron exists in a ‘‘quasifree’’ state. A variety of equilibrium properties of the electron and the solvent are presented to characterize the structure of the different systems. The anomolous density dependence of the experimental electron mobility along the coexistence curve in xenon can be understood qualitatively in terms of the equilibrium structures we observe at the different solvent densities.
Monte Carlo simulations of gas-phase collisions in rapid desorption of molecules from surfaces
Authors: NoorBatcha I. , Lucchese Robert R. , Zeiri Yehuda
Published: July 2002

We have examined the effects of collisions among the molecules desorbing from solid surfaces by means of Monte Carlo simulations, and have identified the conditions under which and to what extent these collisions would influence the experimentally observed product distributions. By simulating the experiment performed by Cowin et al. [Surf. Sci. 78, 545 (1978)] on the laser induced desorption of D2 from tungsten, we have found that at high coverages each desorbate makes on average 2.9 collisions which decreases to no collisions at very low coverages. These collisions affect the product distribution at high coverages to such an extent that even if the nascent desorbed flux is under thermal equilibrium, these post-desorption collisions could lead to nonequilibrium distributions. The effect of the post-desorption collisions is influenced by the rate of heating the surface and the kinetics of the desorption process.
Extended active space CASSCF/MRSD CI calculations of the barrier height for the reaction O+H2→OH+H
Authors: Walch Stephen P.
Published: July 2002

The convergence of the barrier height for the O+H2→OH+H reaction has been studied as a function of the size of the active space and basis set completeness. The barrier height is rapidly convergent with respect to expansion of the active space. Addition of 2p→2p′ correlation terms to the active space lowers the barrier to the O+H2 reaction by about 2.0 kcal/mol, but addition of 3d and other terms has little additional effect. Multireference singles and doubles contracted CI plus Davidson’s correction calculations using a [5s5p3d2f1g/4s3p2d1f] basis set with a 5σ2π active space lead to a barrier height of 12.7 kcal/mol. Including an estimate of the CI contraction error and basis set superposition error leads to 12.4 kcal/mol as the best estimate of the barrier height.
Conformality in the Kirkwood–Buff solution theory of statistical mechanics
Authors: Hamad E. Z. , Mansoori G. A. , Ely J. F.
Published: July 2002

The Kirkwood–Buff solution theory of statistical mechanics is examined in the light of the conformal solution approximation of the mixture radial distribution functions. By joining the mixture compressibility equation of the Kirkwood–Buff solution theory with the mixture energy and virial equations of statistical mechanics, a set of density and temperature dependent mixing rules has been developed which are used here to calculate properties of molecular fluids with varying size and interaction energy differences. It is demonstrated that the conformality approximation in the compressibility equation produces mixture results with a deviation, from the exact mixture data, on the opposite side of the predictions of the van der Waals theory of mixtures. The Kirkwood–Buff relation for the composition derivative of the chemical potential is also integrated by combining it with the conformal solution approximation and compared with the simulation data.
Electronic structure and bond length dependence of the effective valence shell Hamiltonian of S2 as studied by quasidegenerate many-body perturbation theory
Authors: Wang Xiao-Chuan , Freed Karl F.
Published: July 2002

The effective valence shell Hamiltonian (Hv) of S2 is calculated as a function of internuclear distance using quasidegenerate many-body perturbation theory with the full valence space spanned by eight valence orbitals. Calculated potential curves and excitation energies for several valence states are in good agreement with experiment and are compared with configuration interaction calculations using the same primitive basis. In order to test assumptions of semiempirical theories, we also perform a more approximate calculation of Hv in which the valence space is constructed as the union of the atomic valence spaces with the atomic orbitals taken from atomic SCF calculations. A new and important feature of this approximate, correlated Hv is the use of optimized valence and excited orbitals as determined from a constrained SCF procedure. The matrix elements of this approximate, correlated Hv are transformed to the original nonorthogonal atomic valence basis, and their bond length dependences are fit with simple analytical functions. Some calculated Hv matrix elements agree with the forms commonly postulated for semiempirical integrals, while others display quite different behavior. An example of the latter are the one-center, two-electron integrals which depend significantly on bond length in marked contrast to semiempirical theories which assume them to be bond length independent.
Satellites and double ionization continua in the CS2 photoelectron spectrum
Authors: Roy P. , Nenner I. , Millié P. , Morin P. , Roy D.
Published: July 2002

The CS+2 valence satellites have been studied with photoelectron spectroscopy using synchrotron radiation in the 35–75 eV photon energy range. Partial cross sections (σ) and asymmetry parameters (β) for most bands have been measured as a function of photon energy. Below 28 eV binding energy, the symmetry of satellite bands has been identified on the basis of their β value and a comparison between experimental photoelectron spectra and new ab initio SCF-CI calculations of CS+2 corrected for transition moments. For the bands with binding energies above 28 eV no configuration interactions states of CS+2 are predicted with significant intensity. We suggest that they originate from the opening of direct double ionization continua. This is supported by the similarity of their cross sections with the total double ionization one.
Extension of the optimized RHNC equation to multicomponent liquids
Authors: Enciso E. , Lado F. , Lombardero M. , Abascal J. L. F. , Lago S.
Published: July 2002

We have extended the optimized reference-hypernetted chain formalism to multicomponent liquids. The reference system is constructed from a mixed hard spheres fluid with additive diameters whose structural and thermodynamic properties have been conveniently parametrized. The theory is applied to binary liquid mixtures interacting through a repulsive Lennard-Jones potential as well as the complete Lennard-Jones potential; calculated results are in excellent agreement with those of numerical simulations.
A periodically forced oscillator model of van der Waals fragmentation: Classical and quantum dynamics
Authors: Gray Stephen K.
Published: July 2002

A one degree of freedom, periodically forced Hamiltonian is shown to describe much of the relevant classical dynamics observed in earlier studies of a two degree-of-freedom model for HeI2 fragmentation. The simplicity of the forced oscillator model permits easy, but thorough numerical investigation of the quantum dynamics of fragmentation using an accurate wave packet propagation method. Quantum dynamics results are compared with classical dynamics results based on both the usual quasiclassical procedure for selecting initial conditions and on the Wigner distribution. The exact Wigner distribution as a function of time is obtained from the quantum dynamics results and very interesting, unexpected parallels with the classical phase space structure are evident.
Molecular theory of solvation and dielectric response in polar fluids
Authors: Loring Roger F. , Mukamel Shaul
Published: July 2002

The response of a polar fluid to an electric field of arbitrary spatial and temporal characteristics is considered. Microscopic expressions are derived for the transverse and longitudinal dielectric functions εT (k,ω) and εL (k,ω). The notions of ‘‘transverse relaxation time’’ and ‘‘longitudinal relaxation time’’ are discussed and clarified. We propose an algorithm, by which data from macroscopic dielectric relaxation measurements, which probe the dielectric response at long wavelengths, may be used to predict the dielectric response of a polar fluid on molecular length scales. We construct a partially resummed density expansion of the polarization response of a model system composed of interacting dipoles which undergo rotational Brownian motion. Numerical calculations are given of the temporal and spatial behavior of the polarization induced in this system by the sudden introduction of a point charge.
FTIR spectra of the HF−2 and H2F−3 anions isolated in solid argon and neon
Authors: Hunt Rodney D. , Andrews Lester
Published: July 2002

Argon diluted samples of HF were bombarded by low energy electrons (150–400 eV) from a thermionic electron source during condensation at 12 K. Several polyfluoride anions, which have extremely strong hydrogen bonds, were characterized using matrix infrared spectroscopy. Three principal ions were trapped in the argon matrix: HF−2 (1377 cm−1), H2F−3 (1815 cm−1), and ArnH+ (904 cm−1). Electron impact experiments performed with HF/DF mixtures gave counterpart absorptions: DF−2 (965 cm−1), HDF−3 (1707 cm−1), D2F−3 (1391 cm−1), and Arn D+ (644 cm−1). The 1377 cm−1 matrix band is assigned to ν3 of linear, centrosymmetric HF−2, based on agreement with solid state and matrix ion pair spectra, and ab initio calculations, and a proper H/D ratio (1.427) for a linear centrosymmetric species. However, this assignment challenges the recent 1848 cm−1 gas phase assignment to ν3 of HF−2 using laser spectroscopy; the 1848 cm−1 absorption exhibits an H/D ratio (1.323) that is too low for the ν3 vibration of centrosymmetric HF−2, and it should be reassigned to the ν1+ν3 combination band. The 1815 cm−1 matrix band is assigned to the out-of-phase H–F stretching fundamental of C2vH2F−3, which is in agreement with solid K+H2F−3 spectra.
On the structure of the ground state of H+6
Authors: Montgomery J. A. , Michels H. H.
Published: July 2002

The electronic structure of H+6 has been studied by ab initio calculations. It is found that there exist two stable structures, of Cs and D2d symmetries. Post-SCF calculations suggest that the ordering of these states is determined by electron correlation, with the D2d structure lying slightly lower than the Cs structure. It is suggested that the D2d structure is formed in the recent experiments of Kirchner and Bowers.
Hund’s rule and singlet–triplet energy differences for molecular systems
Authors: Darvesh Katherine Valenta , Boyd Russell J.
Published: July 2002

A study of the 1 B1 and 3 B1 states of CH2 has been carried out, with the aim of evaluating the singlet–triplet differences in the total energy and its components. These were examined in connection with the interpretation of Hund’s multiplicity rule. Although vertical results near equilibrium tend to vindicate the new explanation of Hund’s rule, the effect of the molecular virial theorem must be considered at large separations. Also, for adiabatic comparisons, the effect of bond lengths on the energy components must be considered. Correlation energy was found to have a significant effect on vertical energy component differences at certain geometries.
The vibrational relaxation of highly excited SF6 by Ar
Authors: Beck Kenneth M. , Gordon Robert J.
Published: July 2002

The method of time-resolved optoacoustics was used to measure the time constant τ for the vibrational relaxation of SF6 by Ar as a function of the average initial excitation 〈〈E〉〉0. It was found that pτ=210±30 μs Torr for 〈〈E〉〉0 between 4000 and 19 000 cm−1, implying nearly pure exponential decay. The average amount of energy change per collision varies linearly with 〈〈E〉〉, having a value of −8 cm−1/collision at 19 000 cm−1. Below 4000 cm−1 pτ increases, approaching the low energy value obtained from IR fluorescence measurements.
Theory of vibrational energy relaxation in liquids: Construction of the generalized Langevin equation for solute vibrational dynamics in monatomic solvents
Authors: Adelman Steven A. , Stote Roland H.
Published: July 2002

Algorithms which permit the explicit, albeit approximate, construction of a physically realistic generalized Langevin equation of motion for the energy relaxation dynamics of a specified solute normal mode coordinate y in a monatomic solvent are developed. These algorithms permit the construction, from equilibrium solute–solvent pair correlation functions, of the liquid state frequency ωl of the normal mode and of the Gaussian model approximation to the autocorrelation function 〈ℱ̃(t)ℱ̃〉0 of the fluctuating generalized force exerted by the solvent on the normal mode. From these quantities one may compute, from equilibrium solute–solvent pair correlation functions, the vibrational energy relaxation time T1 of the solute normal mode and also related quantities which permit one to assess the relative importance of direct [y coordinate→solvent] and indirect [y coordinate→solute translation–rotational coordinates→solvent] energy flow pathways in solute vibrational energy relaxation. The basis of the construction of T1 is the formula T1=β−1(ωl) where β(ω)=∫∞0 β(t)cos ω dt and where β(t)=[kBT]−1 〈ℱ̃(t)ℱ̃〉0 is the friction kernel of the solute normal mode. This formula is valid if T1≫T2=vibrational phase relaxation time. The approximate formulas for T1 are worked out in detail for diatomic solutes. The approximations are tested for this diatomic solute case by comparing with molecular dynamics results.
Anisotropic intermolecular potentials for HeC2H2, HeC2H4, and HeC2H6, and an effective spherical potential for HeCHF3 from multiproperty fits
Authors: Danielson Laurie J. , Keil Mark , Dunlop Peter J.
Published: July 2002

Differential cross section (DCS) measurements are reported for scattering of a He atomic beam by crossed beams of C2H2, C2H4, C2H6, and CHF3. In addition, interaction virial measurements and accurate limiting diffusion measurements are presented for these systems. Damping of the DCS diffraction oscillations is used to extract anisotropic intermolecular potentials, which are constrained in multiproperty fits to accurately reproduce the dilute gas data. The radial anisotropies determined are in the sequence C2H6>C2H4∼C2H2>CHF3, as sampled by the He probe.
Chemisorption of dioxygen on the Ag(110) surface
Authors: Upton Thomas H. , Stevens Paul , Madix Robert J.
Published: July 2002

We have explored the chemisorption of molecular O2 at the long bridge site on the Ag(110) surface using a 24 atom ‘‘supercell’’ cluster model for the Ag(110) surface. From first principles generalized valence bond and configuration interaction calculations, we find that for chemisorption at this site, orientation of the O2 molecular axis along a [11̄0] azimuth is greatly preferred. In this orientation, we find geometric and spectroscopic parameters that are in excellent agreement with available experimental measurements. The molecule is found to sit 0.94 Å above the surface, with an O–O bond distance of 1.55 Å, and vibrational frequencies of ν(O–O)=627 cm−1 and ν⊥=278 cm−1. The O2 π orbitals normal to the surface interact strongly with second layer Ag atoms, while π orbitals parallel to the surface interact with first layer Ag atoms. Charge transfer is extensive (≊1.0–1.5 electrons) at the equilibrium geometry. The O2 is found to bond to an excited state of the Ag24 cluster when in the preferred geometry, and as a result, is endothermic with respect to the cluster ground state. We estimate the true bond energy on an Ag(110) surface to be 11.1 kcal/mol.
On the evaluation of lifetimes for spin-forbidden radiative transitions originating in coupling to states embedded in a continuum. Application to CH−
Authors: Lengsfield Byron H. , Jensen James O. , Yarkony David R.
Published: July 2002

A recently developed method for treating spin-forbidden electronic transitions within the Breit–Pauli approximation is shown to provide a means for characterizing such processes when the transitions derive their intensity by coupling to states embedded in a continuum. The success of this approach is attributable principally to the need to specify only the configuration state function space, rather than its spectrum relative to Ĥ0, in order to obtain the first order perturbation contribution (Ψ1I) to the wave function. Here Ĥ0 is the nonrelativistic Born–Oppenheimer Hamiltonian and Ψ1I is the solution of (Ĥ0−E0I) Ψ1I=−ĤsoΨ0I where Ĥso is the full microscopic spin–orbit portion of the Breit–Pauli interaction. A method for improving the molecular orbital basis used to describe Ψ1I based on the iterative natural orbital (INO) procedure is introduced. The a 1Δ→X 3Σ− transition in CH− was considered. Using the INO procedure, it was found that the optimum orbital space for describing Ψ1I includes a molecular orbital with character intermediate between the compact valence orbital and the diffuse orbital obtained from two alternative MCSCF procedures. Equivalent INO orbitals were obtained from these two distinctly different starting points. Using the INO orbital set, a total radiative rate for the ground vibrational level of the a 1Δ state of 0.163 s−1 was obtained which gives a lifetime, τ=6.14(±1.2) s. This value is in excellent agreement with the experimental value τ=5.9(+0.8, −0.6) s reported by Okumura et al.
Resonance-like phenomena in activated dissociative adsorption: N+2 on Ni(100) and Ni(111)
Authors: Akazawa Housei , Murata Yoshitada
Published: July 2002

Atomic nitrogen adsorption on Ni(100) and Ni(111) surfaces was studied by using very low energy (1–20 eV) N+2 and N+ ion beams. The amount of adsorbed nitrogen was measured by Auger electron spectroscopy as a function of the incident energy of the ions. The adsorption rate for N+ is almost a constant, whereas the dissociative-adsorption rate for N+2 is strongly energy dependent. In the latter case, there exist two channels; one has a low apparent activation barrier of about 1 eV and the other of about 5 eV. A resonance-like feature in the low-activation-barrier reaction was observed at the incident energy of 2–3 eV on Ni(100) and ∼4 eV on Ni(111). The adsorption rate is independent of the angle of incidence. The resonance-like feature is explained by a model which assumes that electronically excited molecules produced by resonance neutralization of the incident ions are dissociatively adsorbed over the low activation barrier and the excited molecules are deexcited on the surface at higher incident energies.
Optical selection studies of electronic relaxation from the S1 state of jet-cooled anthracene derivatives
Authors: Amirav Aviv , Horwitz Chanan , Jortner Joshua
Published: July 2002

In this paper we explore the energy dependence of the interstate electronic relaxation rates knr from the S1 manifold of anthracene and seven of its isotopic and chemical derivatives, which were inferred from quantum yield data. Absolute fluorescence quantum yields Y from groups of rotational states within the electronic origin S1(0) and from vibrational states were obtained over the excess energy Ev=0–3000 cm−1 above S1(0) by the simultaneous interrogation of the fluorescence excitation spectra and of the absorption spectra in seeded, pulsed, planar supersonic jets of Ar. Additional information was obtained from quantum yield data of van der Waals (vdW) complexes of these molecules with Ar. The fluorescence quantum yields from the S1(0) of anthracene, 9-cyano-anthracene, and 9,10-dibromoanthracene were found to be independent of the rotational state, providing further evidence for the rotational independence of knr from a single doorway state. From the Y data of the electronic origins and from the Ev dependence of Y we conclude that intersystem crossing (ISC) dynamics of the S1 manifold is dominated by the interplay between two classes of nonreactive coupling and/or relaxation: (i) Interstate coupling, involving the superposition of direct S1→(T1) ISC together with S1→(Tx) →(T1) mediated ISC through a sparse or dense (Tx) manifold of a higher triplet state; (ii) Intrastate coupling within the S1 manifold, which sets in with increasing Ev and which results in intramolecular vibrational energy redistribution (IVR) at high Ev. The dominant role of mediated interstate coupling in ISC dynamics from S1(0) and from low Ev states was inferred from the inverse deuterium isotope effect on the ISC rates, the extreme sensitivity of knr of deuterated anthracene to a single H atom substitution, and to level shifts induced by complexing with Ar, as well as from the three-orders-of-magnitude difference between the knr values from the S1(0) of 9-bromoanthracene and of 9,10-dibromoanthracene.

The onset of the mediated ISC is documented by an abrupt drop of Y in the narrow (Ev =617–805 cm−1) energy range for 9,10-dichloroanthracene and by the oscillatory energy dependence of Y vs Ev and the extreme energy sensitivity of Y in the range Ev =157–800 cm−1 of 9,10-dibromoanthracene, which is attributed to near-degeneracies between S1 states and the mediating (Tx) states. These resonance effects can be drastically modified by dispersive level shifts induced by complexing with Ar. At high excess vibrational energies some universal features of the Ev dependence of knr are exhibited. These involve a gradual increase of

k n r with increasing Ev at medium energies (Ev =1000–1800 cm−1), which correspond to the intermediate level structure for intrastate coupling and a very weak Ev dependence of knr at high energies (Ev =1800–3000 cm−1), which manifest the effect of statistical intrastate IVR on interstate ISC.
Correlation-consistent configuration interaction: Accurate bond dissociation energies from simple wave functions
Authors: Carter Emily A. , Goddard William A.
Published: July 2002

We have developed a general method employing relatively small but well-defined CI expansions for calculating accurate bond energies [e.g., errors of 1.4 kcal/mol (1.3%) for the C–H bond energy in CH4 and 4.9 kcal/mol (2.7%) for the C=C bond energy in ethylene]. The approach includes in a systematic way all correlations involving orbitals that change significantly during bond breakage. The CI expansion truncates rapidly, enabling the application of this technique to polyatomic molecules for which normal correlation approaches would be prohibitively expensive. Thus the bond energy for BH is calculated to within 0.3 kcal/mol of the full CI value but incorporating less than 0.1% of the spin eigenfunctions. Smooth dissociaton to the correct adiabatic limit by the CCCI method is demonstrated for the C=C bond of ethylene. The advantage of CCCI is illustrated for C2F4, where a full CI would involve ∼7×1022 spatial configurations, but only 1719 are used in CCCI. Here we predict a C=C bond energy for C2F4 of De (F2C=CF2)=68.3±2.5 (D298=64.6±2.5) kcal/mol. Experimental values range from 53 to 76 kcal/mol.
ℒ2 amplitude density method for multichannel inelastic and rearrangement collisions
Authors: Zhang John Z. H. , Kouri Donald J. , Haug Kenneth , Schwenke David W. , Shima Yaakov , Truhlar Donald G.
Published: July 2002

A new method for quantum mechanical calculations of cross sections for molecular energy transfer and chemical reactions is presented, and it is applied to inelastic and reactive collisions of I, H, and D with H2. The method involves the expansion in a square-integrable basis set of the amplitude density due to the difference between the true interaction potential and a distortion potential and the solution of a large set of coupled equations for the basis function coefficients. The transition probabilities, which correspond to integrals over the amplitude density, are related straightforwardly to these coefficients.
Dissociation dynamics of low-lying electronic states of SiH2
Authors: Francisco J. S. , Barnes R. , Thoman J. W.
Published: July 2002

Essential features of the potential surfaces for low-lying electronic states of silylene, SiH2, have been characterized. Calculated transition energies between the X̃ 1A1, ã 3B1, and Ã 1B1 states are in agreement with previously published experimental and theoretical values. The reactions Si(1D)+H2 and SiH(2Π)+H2 leading to SiH2(X̃ 1A1) appear to be barrierless processes. On the ã 3B1 and Ã 1B1 surfaces, asymmetric transition states are found for SiH2 dissociation. The activation energy for dissociation, SiH2(ã 3B1)→Si(3P)+H2(1Σ+g), is calculated to be 44.7 kcal/mol, and the dissociation energy for SiH2(X̃ 1A1)→Si(1D)+H2(1Σ+g) is calculated to be 65.8 kcal/mol. Structures and vibrational frequencies are presented for the low-lying electronic states of SiH2 and for the associated transition states. Lifetimes of individual rovibronic levels of SiH2(Ã 1B1) are found to decrease dramatically in v′2 =8 as compared with v2 =7. This lifetime shortening is attributed to the opening of the dissociation channel to Si(1D)+H2(1Σ+g) which establishes the relative energies of SiH2 and Si+H2. From these relative energies ΔH○f(298) and SiH2 is determined to be 63.6±2.8 kcal/mol.
A Monte Carlo analysis of diffusion measurements in surface science systems that undergo phase transitions
Authors: Jiang Xue-Pei , Metiu Horia
Published: July 2002

We use Monte Carlo simulations to examine the diffusional behavior of adsorbed systems in a temperature range in which the adsorbed layer undergoes a phase transition. Our aim is to provide some theoretical background for the interpretation of recent laser induced desorption diffusion measurements. We find that in systems with attractive interactions the phase transition practically stops diffusion, while in those with repulsive interaction the effect is gradual.
Semiclassical treatment of the vibrational spectroscopy of OCS
Authors: Aubanel Eric E. , Wardlaw David M.
Published: July 2002

The utility of a primitive semiclassical method for the quantitative prediction of vibrational eigenvalues and electric dipole transition intensities in triatomic molecules is assessed for the particular case of rotationless OCS in its ground electronic state by comparison to exact quantum calculations. The semiclassical method is based on numerical integration of appropriately selected classical trajectories. The potential energy function determined by Foord, Smith, and Whiffen [Mol. Phys. 29, 1685 (1975)] and the electric dipole moment function determined by Tanaka, Tanaka, and Suzuki [J. Chem. Phys. 82, 2835 (1985)] provide the model of the OCS system. Eigenvalues are obtained by the method of adiabatic switching, the number of trajectories required for this purpose being minimized to four using an extension of Johnson’s Fourier series method [J. Chem. Phys. 83, 1204 (1985)]. The resulting semiclassical vibrational transition frequencies (with respect to the ground state) agree with the corresponding quantum frequencies to within 1 cm−1 (2 cm−1) for 112 (128) of the 145 converged quantum levels, with the largest discrepancy being 5.2 cm−1. The semiclassical frequencies are compared to the experimental results of Fayt [Ann. Soc. Sci. Brux. 86, 61 (1972)] and to the semiclassical results of Colwell [Chem. Phys. 46, 165 (1980)]. The sets of 176 semiclassical and 145 converged quantum transition frequencies reported here are the most extensive and complete to date, the highest energy level being ∼7500 cm−1 above the ground state. The methodology of Wardlaw, Noid, and Marcus [J. Phys. Chem. 88, 536 (1984)] for the determination of semiclassical transition intensities in 2D oscillator systems is herein extended to the vibrational degrees of freedom in triatomic molecules. For numerous transitions from the ground state and from several low-lying excited states, the semiclassical intensities agree with the quantum intensities to within 6% in the absence of resonances in the associated approximate eigentrajectories. When resonances are involved, the primitive semiclassical treatment is found to be far less accurate, as is expected. A

numerical determination of the classical actions, Fourier spectra of the coordinates, surfaces of section, and 2D slices through configuration space are presented for representative resonant and nonresonant approximate eigentrajectories. The inherent uncertainties in the semiclassical energy levels and in the transition intensities (if no resonant trajectories are involved) are found to provide a very reliable upper bound on the difference between the semiclassical and quantum results.
Development of a pure diffusion quantum Monte Carlo method using a full generalized Feynman–Kac formula. II. Applications to simple systems
Authors: Caffarel Michel , Claverie Pierre
Published: July 2002

We have described in part I of this work the theoretical basis of a quantum Monte Carlo method based on the use of a pure diffusion process and of the so-called full generalized Feynman–Kac (FGFK) formula. In this second part, we present a set of applications (one-dimensional oscillator, helium-like systems, hydrogen molecule) with the purpose of illustrating in a systematic way the various aspects pertaining to the practical implementation of this method. We thus show how energy and other observables can be obtained, and we discuss the various sources of biases occurring in the different procedures (notably the so-called short-time approximation pertaining to the generation of the sample trajectories of the diffusion process, and the numerical integration pertaining to the evaluation of the ‘‘Feynman–Kac factor’’). After having thus considered the case of the genuine ‘‘bosonic’’ ground state, we illustrate the various proposals for dealing with some ‘‘relative’’ ground state (namely the lowest state belonging to some prescribed symmetry), one of the most important cases being obviously the physical ground state of many-fermion systems (owing to the Pauli principle requirements). More specifically, we consider the so-called fixed-node approximation (FNA), on one hand, and two variants of a potentially exact procedure, the so-called simple projection (SP) and release-node projection (RNP) methods, on the other hand. Finally, some perspectives concerning future developments are outlined.
Determination of orientation of the ground state using two-photon nonresonant excitation
Authors: Kummel Andrew C. , Sitz Greg O. , Zare Richard N.
Published: July 2002

A method is presented for determining the population A{0}0+, the alignment factors A{2}q± and A{4}q ±, and the orientation factors A{1}q± and A{3}q± for a ground state distribution of diatomic (symmetric top) molecules probed by elliptically polarized two-photon nonresonant excitation. General expressions are developed for the O, P, Q, R, and S branch transitions as a function of the rotational quantum number, J. This treatment assumes that the resonant state reached by the two-photon transition is subsequently detected independent of its orientation and alignment. This can be achieved by 2+n multiphoton ionization in which the ionization steps are saturated, or by 2+1 laser induced fluorescence in which the fluorescence is collected independent of its polarization and spatial anisotropy. For the case where elliptically polarized light is created by passing linearly polarized light through a quarter-wave plate, the alignment and orientation moments can be independently determined using a single experimental excitation–detection geometry.