bluuebunny/crossref_metadata_2025 huggingface dataset:
Fixed-node quantum Monte Carlo for moleculesa) b)
Authors: Reynolds Peter J. , Ceperley David M. , Alder Berni J. , Lester William A.
Published: February 2003

The ground-state energies of H2, LiH, Li2, and H2O are calculated by a fixed-node quantum Monte Carlo method, which is presented in detail. For each molecule, relatively simple trial wave functions ΨT are chosen. Each ΨT consists of a single Slater determinant of molecular orbitals multiplied by a product of pair-correlation (Jastrow) functions. These wave functions are used as importance functions in a stochastic approach that solves the Schrödinger equation by treating it as a diffusion equation. In this approach, ΨT serves as a ‘‘guiding function’’ for a random walk of the electrons through configuration space. In the fixed-node approximation used here, the diffusion process is confined to connected regions of space, bounded by the nodes (zeros) of ΨT. This approximation simplifies the treatment of Fermi statistics, since within each region an electronic probability amplitude is obtained which does not change sign. Within these approximate boundaries, however, the Fermi problem is solved exactly. The energy obtained by this procedure is shown to be an upper bound to the true energy. For the molecular systems treated, at least as much of the correlation energy is accounted for with the relatively simple ΨT’s used here as by the best configuration interaction calculations presently available.
Semiempirical dipole-moment functions with correct asymptotic behavior: Application to CO
Authors: Tran Loc Binh , Huffaker J. N.
Published: February 2003

Model dipole-moment functions for diatomic molecules are presented, which can include the correct asymptotic behavior at both r=0 and r=∞. Procedures for evaluating the model parameters via the perturbed-Morse-oscillator approach are illustrated by an application to the ground state of the CO molecule.
Configuration interaction in a basis of biorthogonal states
Authors: Payne Philip W.
Published: February 2003

This article develops techniques for configuration interaction in a nonorthogonal basis. Hamiltonian operator matrix elements such as 〈φ̃u‖Ĥ‖φv〉 are easily evaluated when biorthogonal orbitals are used for the bra and ket configuration states. The ket configuration state ‖φv〉 is constructed from a basis of orbitals {Xi} and the bra configuration state 〈φ̃u‖ employs the dual orbitals {X̃i}. The dual basis is defined by contraction of the original basis with the inverse overlap matrix: X̃i(r)= ∑jXj(r)S−1ji. A configuration interaction wave function ‖ψ〉 then corresponds to a stationary point of the modified energy functional E′=〈ψ̃‖Ĥ‖ψ〉/〈ψ̃‖ψ〉. This energy functional becomes exact as either ‖ψ〉 or ‖ψ̃〉 approaches an exact eigenstate of the Schrödinger equation. The wave function ‖ψ̃〉 is expanded with respect to the dual basis, whereas the original configuration state basis is employed for expansion of ‖ψ〉. The energy E′ satisfies the variation theorem if and only if the dual basis is related to the original basis by a linear transformation. Such a transformation exists when a full CI is done with respect to the manifold defined by nonorthogonal active orbitals. However, one can retain nonorthogonality among those orbitals which are doubly occupied in all configurations without invalidating the variational stability. Configuration interaction in a nonorthogonal basis can be efficiently done in direct CI mode. Modification of the second-quantization formalism shows that the structure constants of the direct CI are identical with those that would pertain to a nonorthogonal basis. The dual basis that is used for the bra states modifies the integral transformation, but otherwise has no effect.
Analytic second derivatives in restricted Hartree–Fock theory. A method for high-spin open-shell molecular wave functions
Authors: Saxe Paul , Yamaguchi Yukio , Schaefer Henry F.
Published: February 2003

Theoretical methods have been developed for the analytic determination of second energy derivatives (i.e., force constants) from restricted Hartree–Fock wave functions for certain types of open-shell systems. Specifically treated are systems for which all electrons outside closed shells have their spins parallel. Although the open-shell formalism is somewhat complicated, its application once implemented, is not greatly more arduous than the closed-shell theory presented in 1979 by Pople and co-workers. Like previous procedures for the evaluation of the second derivatives of electrons repulsion integrals, the present method exploits the Rys polynomial concept. Beyond this general framework, however, significant departures appear, and these differences are described. Preliminary application of the new method has been to the two lowest triplet states of the formaldehyde molecule, for which both equilibrium geometry and harmonic vibrational frequencies have been evaluated.
Quantum dynamics of the van der Waals molecule (N2)2: An ab initio treatment
Authors: Tennyson Jonathan , van der Avoird Ad
Published: February 2003

Starting with an available ab initio N2–N2 potential, which favors a crossed equilibrium structure for the (N2)2 dimer with well depth De=122 cm−1, Re=3.46 Å, and barriers to internal rotations of 25 and 40 cm−1, we calculate the bound rovibrational states of this dimer for J=0, 1, and 2. This is done by solving a secular problem over the exact (rigid monomer) Hamiltonian including centrifugal distortions and Coriolis interactions, using a product basis of radial (Morse oscillator) functions and angular momentum eigenfunctions. The full permutation-inversion symmetry of the system, in relation to the nuclear spin coupling, is used in order to simplify the calculations and to derive selection rules for IR absorption. We find that the (N2)2 dimer has a large number of bound rovibrational states (92 already for J=0). These are analyzed by correlation with rigid molecule (harmonic oscillator/rigid rotor) results, on the one hand, and with the states of two freely rotating N2 monomers, on the other, and by plotting some characteristic vibrational wave functions. In the ground state, the vibrations are nearly harmonic, with a small tunneling splitting; the dissociation energy D0 ranges from 74.9 cm−1 for oN2–oN2 to 80.5 cm−1 for pN2–pN2, the mean distances 〈R〉 equal 3.79 and 3.76 Å, respectively. In the lower vibrationally excited states, the monomer rotations are still locked in, but strongly anharmonic and coupled, also with the dimer stretch. With increasing energy, the internal rotations become successively delocalized in the different angles, starting with the torsion (φ) about R. The resulting energy level diagram is so complex that it is hard to discover regularities. The results are compared with the experimental IR spectrum.
Electrostriction and the dielectric constant of a simple polar fluid
Authors: Rasaiah Jayendran C.
Published: February 2003

An exact statistical mechanical expression for electrostriction in an open system to O(E2) (where E is the electric field) is obtained by adding the appropriate bridge diagrams to the hypernetted chain (HNC) approximation for the wall-particle interactions between a dipolar fluid and an electrified wall. The expression for the relative change in bulk density is found to be independent of the inclination of the electric field (or the orientation of the ‘‘wall-dipole’’ which creates the field). Comparison with the thermodynamics of electrostriction leads to a differential equation for the dielectric constant ε whose solution is an equation first proposed by Ramshaw. The significance of the bridge diagram contributions to electrostriction suggests that they play an important role in determining the structure and properties of dipoles and ions near a charged wall.
Angle-resolved photoelectron spectroscopy of CCl4: The Cooper minimum in molecules
Authors: Carlson Thomas A. , Krause Manfred O. , Grimm Frederick A. , Keller Paul , Taylor James W.
Published: February 2003

Angle-resolved photoelectron spectra have been taken for the first five orbitals in CCl4 (2t1, 7t2, 2e, 6t2, and 6a1) using a variable photon source extracted from synchrotron radiation. From these data, the partial cross sections and angular distribution parameters β have been determined for photon energies from 16 to 80 eV. In the case of the lone pair orbitals (2t1, 7t2, and 2e) minima in both the cross sections and β values were noted experimentally in the photon energy range of 40 to 50 eV. The behavior is similar in appearance to that observed with the 3p subshell of argon and is believed to be the molecular counterpart of the Cooper minimum in atoms. Calculations of the cross sections and β values have been made for each of the different orbitals of CCl4 measured experimentally using the multiple scattering Xα theory. Agreement between theory and experiment for the lone-pair orbitals is qualitatively good and in the case of photoionization in the 2t1 orbital it has been possible to identify which continuum channels correspond to the atomic 3s and 3d counterparts. It was also observed that the nonlone-pair orbitals (6t2 and 6a1) show both experimentally and theoretically shallow minima in the vicinity of photon energies where the Cooper minimum is found. Finally, experimentally determined β values for the first three valence orbitals in CH3Br were obtained from 20 to 50 eV. The behavior of the lone-pair orbital of CH3 Br could be correlated with the Cooper minimum observed for the 4p orbital of Kr. The general nature of the Cooper minimum in molecules is discussed.
The laser magnetic resonance spectrum of the ν2 band of the methylene radical CH2
Authors: Sears Trevor J. , Bunker P. R. , McKellar A. R. W.
Published: February 2003

Ten rotation–vibration transitions in the ν2 (bending) fundamental band of CH2 in its 3B1 ground electronic state have been measured and assigned using the technique of intracavity CO2 laser magnetic resonance (LMR) spectroscopy in the region 880–1090 cm−1. Using the present data, and the microwave and far-infrared LMR measurements of pure rotational transitions, an effective bending potential energy curve has been derived for CH2 using the semirigid bender Hamiltonian. The bending frequency of CH2 is determined to be ν2=963.0987(4) cm−1, which is considerably lower than previous theoretical and experimental estimates.
The equilibrium geometry, potential function, and rotation-vibration energies of CH2 in the X̃ 3B1 ground state
Authors: Jensen Per , Bunker P. R. , Hoy A. R.
Published: February 2003

By fitting the eigenvalues of the nonrigid bender Hamiltonian to the 26 available experimental rotation-vibration energy level splittings of the 3B1 ground electronic state of the methylene radical CH2 we determine the equilibrium geometry and potential function; the data only involve rotational transitions and transitions in the ν2 bending fundamental band. From the fit we determine that the equilibrium bond length is 1.075 Å, the equilibrium bond angle is 133.9° and the height of the barrier to linearity is 1878 cm−1. We are able to use the nonrigid bender Hamiltonian, with this potential, to predict all the lower rotation-vibration energy levels, including the ν1 and ν3 band origins, of CH2 and its isotopes.
Electron impact excitation of SO2: Differential, integral, and momentum transfer cross sections
Authors: Vus̀ković L. , Trajmar S.
Published: February 2003

Electron impact excitation of the electronic states of SO2 was investigated. Differential, integral, and inelastic momentum transfer cross sections were obtained by normalizing the relative measurements to the elastic cross sections. The cross sections are given for seven spectral ranges of the energy-loss spectra extending from the lowest electronic state to near the first ionization limit. Most of the regions represent the overlap of several electronic transitions. No measurements for these cross sections have been reported previously.
Effect of disorder on the Li intercalation properties of LixTayTi1−yS2: A mean-field theory
Authors: Osório R. , Falicov L. M.
Published: February 2003

A mean-field approximation is applied to a triangular-lattice gas model with three different site energies randomly distributed and nearest-neighbor pairwise repulsion. The model is intended to represent intercalated Li in LixTayTi1−yS2. Ground-state diagrams show various sequences of ordered phases. Finite temperature results show minima in the ‘‘incremental capacity’’ curves due to nearly complete filling of sites of a given chemical type. The model is good for those alloys where the experimental results suggest a disordered distribution of Li atoms.
Voronoi polyhedra and solvent structure for aqueous solutions (III)
Authors: David Elaine Eisler , David Carl W.
Published: February 2003

We enlarge the prescription for Voronoi polyhedra associated with water solvating a solute molecule, to include protons as well as oxide anions. As a result of this variant scheme, different, potentially interesting structural properties of the solute/solvent interface can be uncovered and explored.
Dynamics of the two-photon photodissociation of NO2: A molecular beam multiphoton ionization study of NO photofragment internal energy distributions
Authors: Morrison Richard J. S. , Grant Edward R.
Published: February 2003

Two-photon photodissociation of NO2 is induced by the output of a pulsed dye laser tuned over the region from 455 to 425 nm. We characterize the dynamics of this process by recording the multiphoton ionization spectrum of the product NO: intensities of spectral features associated with Ã(2Σ+)←X̃(2π3/2,1/2) two-photon resonance enhanced, four-photon ionization of nascent NO reveal its distribution over accessible rovibronic states. A single laser pulse serves both as photodissociation source and probe. Over the wavelengths studied the dominant reaction pathway yields NO(X̃ 2π) and O(1D). Its dynamics in all regions of the photodissociation spectrum, save one, are comparable to those observed for the loss of O(3P) in one photon photolysis at comparable available excess energy. In the region of 427 nm, however, the photodissociation dynamics are dramatically different. Here we find that the photoproduct NO rotational distribution is anomalously cold, apparently limited by the rotational temperature of our supersonic molecular beam, and that a product spin-orbit state 2π1/2 is missing. We argue that this result suggests a linear or near-linear dissociation geometry which imparts very little torque to the departing NO photofragment and places strict symmetry requirements on its spin-orbit state. We offer an interpretation that traces the cause of this anomalous behavior to the participation at the two-photon level of a theoretically predicted linear state of NO2.
Post barrier channel effects during unimolecular decomposition: A trajectory study of energy term variations
Authors: Rynefors Kjell
Published: February 2003

Post barrier channel effects during the decomposition of a three-atomic molecular complex to an atom and a two-atom molecule have been investigated in a classical trajectory study. This was done by following the extent of energy exchange between the degrees of freedom outside the centrifugal barrier. An ideal dipole potential was used to model the potential energy between the atom and the molecule. In all runs the total energy used was approximately 4.66×10−20 J, corresponding to a temperature of close to 1000 K in a crossed molecular beam experiment, while total angular momentum Ltot has been varied in the runs. For large Ltot values the centrifugal barrier height can be a considerable fraction of the total energy. A recently developed statistical method (Holmlid, Rynefors 1981) has been used to generate the initial conditions at the top of the centrifugal energy barrier. The half-reaction KNaCl→K+NaCl was chosen as a model decomposition process but the effects of light and heavy product atom masses have also been investigated. When the magnitude of Ltot was close to the upper bound compatible with this total energy, considerable exchange occurred between the degrees of freedom. The molecular rotation energy shifted 11%, in the meanwhile changes for individual molecules of 40% were not uncommon. A redistribution between the rotational degrees of freedom is the primary effect when the atomic masses are approximately equal. For systems with other mass ratios large shifts in the translational energy can also occur.
Picosecond dynamics of twisted internal charge transfer phenomena. The role of the solvent
Authors: Wang Ying , Eisenthal K. B.
Published: February 2003

To probe the role of the solvent in intramolecular charge transfer processes, and in particular, the origin of the well-known dual fluorescence phenomena of p-dimethylamino benzonitrile (DMABM), picosecond laser studies in mixed polar/nonpolar solutions were undertaken. The anomalous long wavelength emission is attributed to a complex formed between excited DMABN and butanol with a rate constant of (9.7±1.5)×108 M−1 s−1. The dominant stabilization of the twisted intramolecular charge transfer state is therefore concluded to be due to a short range specific interaction with a polar solvent molecule. A secondary solvent effect arises from a further stabilization of the complex by long range polarization interactions with solvent molecules. Evidence on the existence of ground state complexes between DMABN and butanol are also presented. Excitation of these ground state complexes leads to the rapid formation of the excited state complexes in 30 ps, which we have interpreted to be the time required for the complex to relax intramolecularly, presumably a rotational motion along the C–N bond of DMABN, to achieve the final twisted charge transfer geometry.
Update methods in multiconfigurational self-consistent field calculations
Authors: Olsen Jeppe , Jo/rgensen Poul
Published: February 2003

We have demonstrated how Hessian update methods may easily be implemented into an MCSCF calculation. We have shown that the Hessian update methods constitute the most efficient iterative methods for obtaining local convergence of an MCSCF calculation and we have further shown that Hessian update methods, when implemented with step size and sign control, have very promising global convergence properties.
Structures and reactions of radical cations of some prototype alkanes in low temperature solids as studied by ESR spectroscopy
Authors: Toriyama Kazumi , Nunome Keichi , Iwasaki Machio
Published: February 2003

The structures and reactions of radical cations of prototype alkanes such as ethane, propane, isobutane, neopentane, etc. produced in SF6 and in other halocarbon matrices irradiated at 4 K have been extensively studied by ESR. The geometrical structures as well as the unpaired electron orbitals of these cations are unequivocally characterized based on the simple symmetry considerations as well as the INDO MO calculations. The ESR hyperfine coupling constants determined for these cations as well as their deuterated homologs give conclusive evidence for the interpretations. The unpaired electron in linear alkane cations is delocalized over the in-plane C–H and C–C σ bonds forming delocalized σ radicals, whereas that in highly branched alkane cations is more confined to one of the C–C bonds forming localized σ radicals. The H1s spin density, giving a large hyperfine coupling in the linear alkane cations, is largely contributed from a direct participation of the C–H bond in the unpaired electron orbital, whereas that in the highly branched alkane cations is due to mainly hyperconjugation. The alkane cations undergo two types of reactions: one is deprotonation to form neutral alkyl radicals in SF6 and in CFCl2CF2Cl and the other is the H2 and CH4 elimination to form olefinic π cations in CFCl3. Based on the matrix effects on the structure of propane and isobutane cations, the site preference in forming alkyl radicals, and other observations, it is concluded that the alkane cations undergo reactions through the breakage of the bond in which the unpaired electron is highly populated.
Interaction between molecular impurities trapped in rare gas crystals. I. Effective potential calculations
Authors: Girardet Claude , Maillard Daniel
Published: February 2003

The effective potential felt by a dipolar dopant trapped in a harmonic monatomic matrix is studied by including the field induced in the crystal by either another molecular impurity or point defects. This potential is built up from the pair potential generally used in molecular interaction problems and its analytical expression is separated into contributions which characterize both the direct interaction between the dopant and the perturber and the indirect one through the perturbed matrix relaxation. New results are obtained on the validity of the ideal trapping in monatomic matrices by determining the minimum distance between one dopant and another defect (impurity or vacancy), which allows one to neglect the interdefect interaction. A direct application of these results is the study of the translational-orientational dynamics of an hydracid monomer trapped in a realistic imperfect matrix in order to interpret experimental data on the Q-branch behavior in the near infrared spectrum of HX in rare gas matrices (cf. paper II).
Temperature dependence of the elastic constants of α-NiSO4⋅6H2O
Authors: Stojanoff V. , Missell F. P.
Published: February 2003

We have measured the adiabatic elastic constants of tetragonal α-NiSO4⋅6H2O in the temperature range 4.2–300 K using ultrasonic pulse–echo techniques. For T = 300 K, we obtained (in units of 1011 dyn/cm2) C11 = 4.68±0.03, C33 = 3.55±0.02, C44 = 1.155±0.004, C66 = 2.00±0.01, C12 = 3.82±0.06, and C13 = 2.07±0.03. These elastic constants give an adiabatic compressibility KS = (3.39±0.02)×10−12 cm2/dyn. The Debye temperature was calculated in several approximations and was found to be in reasonable agreement with calorimetric data, but in good agreement with values obtained for other compounds of similar composition.
Dynamics of a suspension of a random array of spheres in a Navier–Stokes fluid
Authors: Muthukumar M.
Published: February 2003

An effective medium theory of the screening of hydrodynamic interactions in a suspension of randomly distributed immobile spheres in a Navier–Stokes fluid is presented. We consider the shear viscosity for a stationary suspension of penetrable spheres, where the fluid flow is generated by the motion of a single labeled sphere with uniform velocity. We also derive the tracer friction coefficient at arbitrary filling fractions (φ) of the immobile spheres. By a self-consistent calculation, the shear velocity (η) of the suspension is found to diverge nonexponentially at a filling fraction of 0.49, much lower than the closest random packing filling fraction: η∼[(1/φ)−(1/0.49)]−2.12⋅⋅⋅. The friction coefficient of the labeled mobile sphere is also found to diverge similar to η.
Three-photon resonance ionization of H2
Authors: Ritchie Burke , McGuire E. J. , Peek J. M. , Hand C. W.
Published: February 2003

Calculations are performed for the ionization of para-H2 by absorption of three 6.6 eV photons. Two photons are absorbed to excite H2 to the E, F 1Σ+g state, and a third photon is absorbed to ionize this state. The two-photon Rabi rate and one-photon ionization rate are linear in the laser intensity (I) and are thus competitive. This competition produces a strong dependence of the three-photon ionization rate on the vibrational-rotational levels of the bound–bound electronic transition and on the electron ejection angle. For example, for I = 1 GW cm−2 the strong (maximum ionization rate 6×108 s−1/molecule) vJ = 00→v′J′ = 30 transition is 70% saturated at 0° and 100% saturated at 90° ejection angle relative to the linear polarization direction of the photon. The weak (maximum ionization rate 2×1 07 s−1/molecule) vJ = 00→v′J′ = 32 transition is 0.3% saturated at 0° and 80% saturated at 90° ejection angle. These calculations suggest use of multiphoton ionization data to study one-photon excited-state angular distributions. The latter occur for an excited state which has a nonstatistical distribution of rotational M′ sublevels belonging to the J′ level of the excited state (in contrast to the photoionization of a ground state having a statistical distribution of M sublevels belonging to J).
Observation of the infrared absorption spectra of 20NeH+ and 22NeH+ with a difference frequency laser
Authors: Wong M. , Bernath P. , Amano T.
Published: February 2003

The fundamental bands of 20NeH+ and 22NeH+ in the region around 3.7 μm have been observed in absorption. The wave numbers of 11 transitions of 20NeH+ and 8 transitions of 22NeH+ have been measured, from which we obtain the following molecular parameters (in cm−1, three standard deviations in parentheses): The equilibrium molecular constants Be, αe, De, βe, ωe, ωeXe, and re have also been derived.
Emission spectra and relaxation dynamics of excited 79Br2 in Ar and Kr matrices
Authors: Mandich Mary , Beeken Paul , Flynn George
Published: February 2003

The photochemical dynamics of Br2 trapped in Ar and Kr matrices have been studied. Following laser excitation into either the B 3Π(0+u) or 1Π(1u) states, emission occurs in the near infrared spectral region. Analysis of this emission indicates that it consists of three electronic transitions which have been assigned as B 3Π(0+u) v′ = 0→X 1Σ+g (previously reported,) A 3Π(1u) v′ = 0→X 1Σ+g and (tentatively) A′ 3Π(2u)→X 1Σ+g. The B→X emission lifetime was measured to be 8.0±0.5 μs in Ar matrices (5.3±0.5 μs in Kr) and is in good agreement with both earlier determinations in matrices and the gas phase B→X radiative lifetime. The observed A→X emission lifetime of 170±10 μs in Kr is approximately the same as the measured A→X gas phase radiative lifetime, but appears shortened in Ar matrices where the lifetime is 67±4 μs. The lifetime of the A′→X transition is 11±1 ms in Ar and 6±1 ms in Kr matrices. The spectral and temporal data are consistent with an overall energy transfer mechanism involving both vibration and electronic relaxation. The intramolecular electronic relaxation is somewhat restricted in that the highest lying B 3Π(0+u) state is populated only by direct absorption into its bound and continuum levels, whereas the two lower states A 3Π(1u) and A′ 3Π(2u) acquire population after either the 1Π(1u) or B 3Π(0+u) states are initially excited. Both the initial vibrational and electronic relaxation rapidly (< 20 ns) routes the excited population into the lowest v′ = 0 levels of the A and B states (and tentatively A′), where subsequent decay occurs on a 10−6–10−2 s timescale. All of the observed emission lifetimes are independent of temperature, laser power and Br2: matrix dilutions ranging from 1:500–1:10 000.
High-resolution laser spectroscopy of the ν3 vibration-rotation band of HCOOH
Authors: Kuze Hiroaki , Amano Takayoshi , Shimizu Tadao
Published: February 2003

The ν3 (C = O stretching) band of HCOOH has been studied by the techniques of CO laser Stark spectroscopy and tunable diode laser spectroscopy. The coincidence in frequency between the 13–12 P(15) CO laser line and the 110←211 transition of the ν3 band has been found by Stark spectroscopy and confirmed by laser-microwave double resonance. It is found that the Ka = 4 and Ka = 7 levels of the ν3 state are perturbed because of Coriolis interactions with the close-lying ν5+ν9 and ν6+ν9 states, respectively. The band origin, rotational and centrifugal distortion constants, and components of the dipole moment are determined for the ν3 state from the analysis of the unperturbed transitions. The Coriolis coupling constants and information on the interacting states are obtained from the analysis of perturbed transitions.
Nonadiabaticity in ion–molecule reactions: Coupling of proton and charge transfer in the H+2 and D+2+Ar system
Authors: Houle F. A. , Anderson S. L. , Gerlich D. , Turner T. , Lee Y. T.
Published: February 2003

An experimental study of proton and charge transfer in H+2+Ar and D+2+Ar has been carried out as a function of kinetic and vibrational energy. The data show that charge transfer is predominantly long range, having a weak translational, but strong vibrational dependence. The existence of near-resonant energy levels is very important in determining the magnitude of the cross sections, while favorable Franck–Condon factors between the states is found to be less critical. Undulations in the charge transfer cross sections for both H+2 and D+2(v = 0) as a function of kinetic energy have been observed for the first time. As kinetic energy increases, the resemblence between the vibrational dependences of the proton and charge transfer channels becomes quite pronounced. This indicates that the two channels are closely coupled, especially above ∼3 eV.
State selected ion–molecule reactions by a TESICO technique. V. N2+(v)+Ar→N2+Ar+
Authors: Kato Tatsuhisa , Tanaka Kenichiro , Koyano Inosuke
Published: February 2003

Charge transfer reactions N2+(v)+Ar→Ar++N2 (1) have been studied by selecting the vibrational states of N2+ using the threshold electron–secondary ion coincidence (TESICO) technique. Relative cross sections σ(v) for the individual vibrational states v = 0–3 have been determined at three collision energies, 0.3, 1.5, and 11.8 eV. Results show that Reaction (1), which is endoergic for v = 0, is considerably enhanced by the vibrational excitation of N2+ at all collision energies. While excitation of one vibrational quantum enhances the cross section substantially, excitation of additional quanta further increases the cross section up to v = 3. The ratios σ(2)/σ(1) and σ(3)/σ(2) are, however, much smaller than σ(1)/σ(0) and are significantly larger at the highest collision energy than at the other two collision energies. These results are discussed in conjunction with the calculated results based on the simple two-state theory of Rapp and Francis and the Franck–Condon factors.
Determination of thermal amplitude of surface atoms in a supported Pt catalyst by EXAFS spectroscopy
Authors: Marques E. C. , Sandstrom D. R. , Lytle F. W. , Greegor R. B.
Published: February 2003

A temperature sequence of EXAFS measurements was carried out on a highly dispersed Pt/SiO2 catalyst to determine the temperature dependence of atomic structure in the very small supported clusters. A one-shell EXAFS model was fit to the first neighbor oscillations in the catalyst data to determine bond lengths and relative thermal motion (disorder) for an average cluster environment. A Debye model of disorder was employed in the EXAFS analysis. The disorder measured for the catalyst was determined to be 1.3 to 2 times the value determined for bulk Pt over all temperatures. Asymmetry of the radial distributions of nearest neighbors in the largely surface coordinated clusters and the increased atomic disorder lead to underestimates of the nearest-neighbor distance R1 at higher temperatures. The apparent bond distance contraction with temperature is manifest in the data as a retardation in the phase of the first neighbor oscillations. It is demonstrated that model EXAFS functions employing asymmetric interatomic pair potentials (such as the Morse potential) exhibit very similar bond distance contractions to the catalyst data while maintaining similar estimates for the disorder. Finally, a two-shell EXAFS model was fit to the catalyst data to separately determine the motion of the surface coordinated atoms in the small metal clusters.
Electric field gradients in imidazole at 103 K from x-ray diffraction
Authors: Epstein Joel , Swanton David J.
Published: February 2003

The electric field gradients at the two distinct nitrogen nuclei of imidazole have been calculated from the charge density derived from Bragg diffraction data for crystalline inidazole at 103 K. The resulting 14N coupling constants and asymmetry parameters were N1: e2qQ/h = −2.6(0.3) MHz, η = 0.4(0.2); N3: e2qQ/h = −2.2(0.3) MHz, η = 0.4(0.2). In contrast with the corresponding results obtained from NQR experiments, these calculations could not distinguish between the two nitrogen sites. At each site the parameters were approximately equal to the mean of the NQR values for N1 and N3. Model calculations for the diatomic molecules N2 and BF suggest that the accuracy with which field gradients are obtained directly from the Bragg structure factors depends critically on the elimination of quadrupole components induced by vibrational averaging. Estimates of the coupling constants and asymmetry parameters for imidazole obtained directly from the Bragg structure factors were N1: e2qQ/h = −1.4(0.3) MHz, η = 0.2(0.3); N3: e2qQ/h = −0.9(0.3) MHz, η = 0.7(0.5). Detailed analysis showed that the magnitudes of the coupling constants estimated in this way are too small due to residual vibrationally induced quadrupole components.
Reply to Abe’s comments on ‘‘The effect of solvent environment on molecular electronic oscillator strengths’’ [A. B. Myers and R. R. Birge, J. Chem. Phys. 7
3, 5314 (1980)]
Authors: Birge Robert R.
Published: February 2003

This reply examines the various criticisms presented by Abe in his comment on our article [A. B. Myers and R. R. Birge, J. Chem. Phys. 73, 5314 (1980)]. We conclude that his criticisms of our theoretical treatment are unjustified. (AIP)
Parametrization of the CsNpF6 optical spectrum
Authors: Poon Y. M. , Newman D. J.
Published: February 2003

According to the work of Hecht, Varga, Lewis, and Boring [J. Chem. Phys. 70, 101 (1979)], the spectroscopic parameters of Np5+ are very different to those for the tri- and quadrivalent actinide ions. We have re-examined the experimental data, and find that an alternative fit can be obtained with parameters close to those obtained for other ions.
Monte Carlo studies of polymer chain dimensions in the melt
Authors: Mansfield Marc L.
Published: February 2003

A Monte Carlo procedure for studying polymer chains in concentrated solution or in the melt is described. The procedure generates new configurations by breaking and reforming chains. The procedure is used to examine the dependence of the rms end-to-end distance on chain length for chains on a simple cubic three-dimensional lattice with every lattice site filled. Chains with as many as 512 segments were examined. The rms end-to-end distance is proportional to (N−1)ν for N the number of polymer segments and for ν in the range 0.51–0.52, rather than the expected value of 1/2. However, the data seem to indicate a continuing decrease of ν with increasing N so the value 1/2 cannot yet be ruled out.
Mechanisms for electronic energy transfer between molecules and metal surfaces: A comparison of silver and nickel
Authors: Whitmore P. M. , Robota H. J. , Harris C. B.
Published: February 2003

The phosphorescence lifetime of pyrazine above a Ag (111) surface has been measured as a function of molecule–metal separation between 10 and 420 Å. The distance dependence of lifetime is in accord with the predictions of the classical point dipole theory over this range of distances. Using this classical model, the decay of electronically excited pyrazine is separated into contributions from radiative decay, resonant surface plasmon excitation, and lossy surface wave damping. The relative importance of these decay channels is calculated for silver and nickel in the near UV, and the dominant mechanism for energy transfer to these two metals in this distance regime is interpreted in terms of the electronic structure of the metal.
The interaction of methanol with a titanium(001) surface investigated using photon stimulated desorption and UV photoemission spectroscopy
Authors: Hanson David M. , Stockbauer Roger , Madey Theodore E.
Published: February 2003

Synchrotron radiation at NBS SURF II has been utilized to study the interaction of methanol with a stepped Ti (001) single crystal surface at about 90 and 300 K. The techniques of photon stimulated desorption (PSD) and UV photoemission spectroscopy (UPS) were employed. Exposure of the clean surface at 300 K to methanol initially results in dissociative adsorption with features indicative of C, O, and H appearing in UPS. At higher exposures, features that have been identified as being characteristic of a methoxy species appear. At 90 K, this species is found to be present even at low exposures, and there is little, if any, further dissociation. Multilayers of methanol are then formed at higher exposures. PSD of ions from the condensed methanol multilayers was observed using photon energies from 15 to 75 eV with a maximum yield at about 25 eV. Using electron stimulated desorption and time of flight mass spectroscopy, the ions were identified as H+. Partial photoionization yields as a function of photon energy were measured for comparison with the PSD yield spectrum. No PSD was detected from the methoxy species.
Dynamical spectroscopy of triplet state pyridine
Authors: Weisman R. Bruce , Holt Patrick L. , Selco Jodye I.
Published: February 2003

Transient absorption near 311 nm in pyridine was studied at room temperature. (AIP)
Picosecond excitation of jet-cooled hydrogen-bonded systems: Dispersed fluorescence and time-resolved studies of methyl salicylate
Authors: Felker P. M. , Lambert Wm. R. , Zewail A. H.
Published: February 2003

Long progressions involving frequency intervals of 180 cm−1 are observed in the fluoresence of MS for 3327.5 Å excitation. (AIP)
Experiments on ‘‘relative stabilities’’ in a chemical system
Authors: Stuchl Ivan , Marek Miloš
Published: February 2003

Experiments are performed to test the ’’dynamic stability’’ on pumped chemical systems based on mixing process. The Belousov–Ahabotinsky reaction is used. The results are discussed in terms of the relative stability in the chemical systems. (AIP)
Evaluation of higher-order semiclassical phase integrals: Application of Chebyshev series approximation and Taylor series expansion
Authors: Luppi Jussi , Pajunen Petri
Published: February 2003

A new approach for evaluating semiclassical phase integrals of the form ∮ dr f(r)/[E−U(r)]k+1/2 for any smooth potential U(r) is derived and tested. The new methods are based on a Chebyshev series or a Taylor series expansions of the integrand and do not explicitly involve handling weights required by existing forms of this procedure. In addition, they are much more flexible and easier to use and incorporate realistic error estimation.
The electronic structure and photodissociation of HCl
Authors: Stevens W. J. , Krauss M.
Published: February 2003

Continuous absorption is possible from the ground state of HCl to the repulsive states that also arise from the ground state asymptote. Under conditions where vibrational excitation of the HCl is possible, continuum absorption can occur from an onset near 345 to 100 nm in the ultraviolet. Since the two states X 1Σ+ and A 1Π, that are dipole coupled, correlate to the same asymptote, the transition moment varies rapidly with the internuclear distance. Using all-electron ab initio calculations based on multiconfiguration self-consistent field (MC-SCF) and first-order configuration interaction (FOCI), the energy curves and wave functions have been obtained for the X 1Σ+, a 3Π,A 1Π, b 3Σ+ states, and the first excited states of each symmetry. The electronic structure of the states are analyzed and the energy curves are compared favorably to experiment. Using the calculated A–X transition moment, the A–X absorption cross section has been obtained as a function of the ground state vibrational level. The possible impact of this process on the operation of the XeCl UV laser which uses HCl as a fuel would not be significant since the cross section at 310 nm never exceeds a few times 10−19 cm2.
Radiative lifetimes and dipole moments of the A 2Σ+, B 2Σ+, and C 2Σ+ states of OH
Authors: Langhoff S. R. , van Dishoeck Ewine F. , Wetmore R. , Dalgarno A.
Published: February 2003

Three independent theoretical studies of the eigenfunctions of the X 2Π and excited A 2Σ+, B 2Σ+, and C 2Σ+ states of OH are described, and results are presented for the transition dipole moments connecting each excited state to the others and to the ground state. The bound–bound and bound–free vibrational band transition probabilities are obtained, and the resulting radiative lifetimes are compared with the measured values. The calculations confirm the recent measurements for the A 2Σ+ and C 2Σ+ states, but are inconsistent with the lifetimes measured for the B 2Σ+ state.
Spatial correlations between charged colloidal particles: The mean spherical (dense point limit) approximation
Authors: Medina-Noyola Magdaleno
Published: February 2003

The static structure factor of a suspension of charged polystyrene spheres is calculated within a Debye–Hückel level of approximation. This is based on the mean spherical closure of the Ornstein–Zernike integral equation of the primitive model of the ionic solution constituted by the macroparticles and their counterions, in the limit negligible size of the latter. The explicit calculations in the dense point limit (charge of the counterions/charge of the macroparticles→0) are compared with the experimental results and it is found that this extremely simple theory reproduces the main features of the liquidlike structure of the suspension. Further quantitative refinements of the theory are naturally suggested by the method employed here, in which this Debye–Hückel approximation is the simplest nontrivial application.
Dynamic correlations between charged colloidal particles: Coupled macroion–counterion diffusion
Authors: Medina-Noyola Magdaleno
Published: February 2003

A simple theory is presented which predicts the main short-time and long-time features observed in the intermediate scattering function F(k,t) measured in experiments of quasielastic light scattering by suspensions of highly charged polystyrene macroparticles. This theory is based on the description of the coupled diffusion of the macroparticles and their counterions by the linear laws of irreversible thermodynamics, and on the use of spatially nonlocal thermodynamic relations to describe the spatial correlations between the diffusing particles.
Quantum manifestations of classical stochasticity. II. Dynamics of wave packets of bound states
Authors: Weissman Yitzhak , Jortner Joshua
Published: February 2003

In this paper we consider some of the consequences of classical stochasticity phenomena in the quantum mechanical Henon–Heiles and Barbanis systems. To explore classical–quantum analogies we have introduced a quantum mechanical phase space based on the coherent state representation. Quantum Poincaré maps (QPMs) were constructed from contour plots of the stationary phase-space densities. We have observed a correlation between the topological features of a QPM at an eigenenergy E and the sensitivity ‖dE/dε‖ of that energy to the strength ε of the nonlinear coupling. States with extreme values of ‖dE/dε‖, corresponding to both high and to low values of ‖dE/dε‖, show regular regions in the quantum phase space, while states with intermediate values of ‖dE/dε‖ span large parts of the quantum phase-space regions. These general features of the QPMs apply for almost each energy multiplet, and no qualitative change in the character of the QPMs was observed above the classical critical energy Ec for the stochastic transition. To investigate the quantum analog of classical trajectories, we have studied the dynamics of initially coherent Gaussian wave packets. We have discovered two limiting types of time evolution of intially coherent wave packets, which exhibit quasiperiodic time evolution or rapid dephasing. Quasiperiodic quantum dynamics is analogous to quasiperiodic classical trajectory dynamics, while rapid dephasing of the wave packet reflects the effects of spreading of the wave packet or/and stochastic classical dynamics. We were able to establish a correspondence between the topology of the quantum phase-space density and the wave packet dynamics. Quasiperiodic time evolution and rapid dephasing are exhibited by wave packets initially located in the regular and in the irregular regions of the quantum phase space, respectively.
Laser-induced excited state and ultrasonic wave gratings: Amplitude and phase grating contributions to diffraction
Authors: Nelson Keith A. , Casalegno Roger , Miller R. J. Dwayne , Fayer M. D.
Published: February 2003

A detailed analysis of diffraction from laser-induced gratings is presented. The changes which occur in both the real and imaginary parts of the index of refraction are accounted for when excited states are created. These lead to phase and amplitude grating contributions, respectively, to the diffraction from a laser-induced excited state grating. Experimental confirmation of the predicted wavelength dependence of these contributions is presented. Diffraction from laser-induced excited state gratings, ultrasonic wave gratings, and mixed excited state and acoustic gratings is analyzed with the phase and amplitude contributions to each accounted for. The results permit the interpretation of mixed grating data and predict conditions under which density-dependent absorption spectral shifts and excited state-phonon interactions can be measured.
Site selective spectroscopy of defect chemistry in CdF2:Eu
Authors: Mho Sun-Il , Wright John C.
Published: February 2003

The site distribution in CdF2:Eu3+ has been studied as a function of dopant concentration and annealing temperature by site selective spectroscopy. A total of 17 sites have been identified. Their behavior is very different from rare earth doped CaF2. The cubic site increases with Eu3+ concentration until clusters form after which it levels off. There are no sites attributed to associated rare earth–F′i. The anomalous behavior observed in CaF2 as a function of annealing temperature and dopant concentration is not observed.
The Kerr effect relaxation processes caused by a rapidly rotating field
Authors: Morita Akio , Watanabe Hiroshi
Published: February 2003

The electric birefringence is calculated in the case where a constant electric field E is switched on suddenly at time t = 0 after another electric field E0 for t<0 has been applied for a sufficiently long time from the direction perpendicular to the plane containing both directions of the field E and a light beam to be used for the birefringence experiments. It is shown that the difference of the birefringence at t = 0 and that at t = ∞ is greater than the two conventional methods, i.e., the transient rise and rapidly reversing field methods. The electric birefringence obtained from the three methods is calculated numerically and shown graphically for comparison.
Lifetimes and predissociation mechanisms of O+2 b 4Σg−(v = 5–8)
Authors: Hansen J. C. , Moseley J. T. , Roche A. L. , Cosby P. C.
Published: February 2003

Predissociation linewidths (lifetimes) of the O+2 b 4Σ−g state have been measured as a function of both rotational levels N and spin-orbit fine structure components F1, F2, F3, and F4 for vibrational levels 5 through 8. The lifetime variation is inconsistent with predissociation by either the d 4Σ+g state or the f 4Πg state alone, but can be explained by assuming both states participate in the dissociation. Analysis of the linewidth variation, using an experimentally determined f 4Πg potential curve, allows a separation of the effects of the 4Σ+g and 4Πg states on the predissociation. The experimental linewidths are compared with theoretical calculations and found to be about a factor of 3 larger than calculated. Relative transition moments from the a state to the d and f states are estimated using a single configuration interaction calculation.