bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoionization dynamics of H2 C 1Πu: Vibrational and rotational branching ratios
Authors: O’Halloran M. A. , Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

Angle-integrated, rotationally resolved photoelectron spectra were determined for three-photon resonant, four-photon (3+1) ionization of H2 via the C 1Πu, v′=0–4 intermediate levels using a magnetic bottle electron spectrometer. The results confirm and extend our previous angle-resolved results obtained using a hemispherical electron energy analyzer. As predicted by both Franck–Condon factors and recent ab initio Hartree–Fock level calculations, ionizing transitions in which the vibrational level of the resonant intermediate state is preserved in the ion are most probable; however, large deviations from theoretical expectations are observed for photoionization of all intermediate levels. These deviations take the form of a progressive broadening of the ionic vibrational distribution as the vibrational level of the resonant intermediate state is increased. The rotational branching ratios also exhibit systematic variations with both the vibrational level of the C 1Πu state and the vibrational level of the ion. Photoelectron signal from ionization of electronically excited atomic hydrogen produced in the photodissociation of H2 C 1Πu is also observed.
The pair-distribution function and light-scattered intensities for charged rod-like macromolecules in solution
Authors: Schneider J. , Karrer D. , Dhont J. K. G. , Klein R.
Published: July 2002

The pair-distribution function of charged rods in a dilute solution is studied for several rod lengths and charges. The interaction energy in the Monte Carlo simulations is a sum over screened Coulomb potentials of beads, which constitute the rod. Two or three beads per rod were used. For short rods a perturbation theory is given to obtain an analytic expression for the pair-distribution function. There is a good agreement between this perturbation theory and the Monte Carlo simulations. These results are then used to calculate the polarized and depolarized structure factors for thin charged rods with an anisotropic polarizability. It turns out that the static structure factors are essentially independent of the magnitude of the anisotropic polarizability in the range which is experimentally accessible.
A
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t
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o study of O, O2, and C2H4 interactions with Ag2 and Ag+2. A model for surface–adsorbate interactions
Authors: McKee Michael L.
Published: July 2002

A b initio calculations of silver metal–adsorbate interactions were carried out by using a 36-electron relativistic effective core potential (RECP) for the core electrons of Ag and a 3s3p4d→2s2p2d basis for the valence electrons. Unpromoted surface interactions were modeled by a silver dimer while promoted interactions were modeled by a silver dimer cation. Molecular oxygen is predicted to bind as a π complex while the di-σ and π complexes are predicted to bind similarly for ethylene. The binding energy of O2 decreases on a promoted surface while the binding energy of C2H4 increases compared to an unpromoted surface. The dissociative adsorption of O2 on a promoted surface is found to be inhibited by the formation of a high energy intermediate. The binding energy of O on a silver surface is well reproduced when corrections are made for the electron affinity of the adsorbate and the ionization energy of the dimer.
Collisional broadening of rotation–vibration lines for asymmetric-top molecules. III. Self-broadening case; application to H2O
Authors: Labani B. , Bonamy J. , Robert D. , Hartmann J. M.
Published: July 2002

A semiclassical theory of asymmetric-top molecule line broadening, developed in part I of this series of papers for linear perturbers, is extended to asymmetric-top perturbers. This model has been successfully applied in part II to the broadening of H2O lines by N2, O2, and Ar. The present applications to the self-broadening of H2O infrared and Raman lines in the 300–900 K range also show the quality of the model. Due to the high electrostatic forces involved in the H2O–H2O interaction, the short-range anisotropic potential has little influence. Nevertheless, the need for a modeling of the trajectory is demonstrated, mainly in the case of high rotational quantum number lines.
The direct correlation function in hard sphere fluids
Authors: Groot R. D. , van der Eerden J. P. , Faber N. M.
Published: July 2002

The pair correlation function in a homogeneous hard sphere fluid at various densities has been measured in a large system, using the Monte Carlo method. The corresponding direct correlation function C2(r) has been determined directly from these measurements, and will be given here in closed form. We conclude that density functional models that neglect the effect of C3 and higher order direct correlation functions, that are defined in bulk fluids, are not able to describe an inhomogeneous hard sphere fluid accurately.
On the theory of multiple scattering for a nonpolar fluid composed of chiral molecules
Authors: Lee Dong J. , Kim Shoon K.
Published: July 2002

A statistical theory of multiple scattering is given for a nonpolar fluid composed of chiral molecules. The theory describes through the correlation function the scattering intensities to all orders. The circular intensity differential due to the single scattering is discussed. The results show that the scattering intensities orthogonal to the direction of propagation of the incident light are slightly different when the light is right and left circularly polarized, while the intensities parallel to the direction of propagation are same.
Energy transfer from metastable electronically excited N2, Ar, Kr, and Xe to CH3OH, H2O2, CH3NH2, and N2H4
Authors: Tao Wen , Golde Michael F. , Ho Grace H. , Moyle Alfred M.
Published: July 2002

The products of the reactions of metastable N2(A 3Σ+u) with the isoelectronic molecules CH3OH, H2O2, CH3NH2, and N2H4 have been studied using the discharge-flow technique, coupled with atomic resonance fluorescence and laser induced fluorescence detection. In the reaction with H2O2, the weak central bond is broken efficiently, but in the other reactions, scission of the central bond is unimportant compared to that of the stronger terminal X–H bonds. The behavior for all four reagent molecules parallels their UV photochemistry, but contrasts with IR multiphoton dissociation of CH3OH and CH3NH2. The reactions of N2(A) appear to parallel those of the excited metastable noble gases Ar(3P0,2), Kr(3P2), and Xe(3P2), but scaled to the lower energy of N2(A).
Quenching of OD (A 2Σ+,v′=0 and 1) by various gases
Authors: Vaghjiani Ghanshyam L. , Ravishankara A. R.
Published: July 2002

A tunable pulsed dye laser was used to pump OD to the (A 2∑+,v′=1) level. The fluorescence excitation spectrum of OD in the wavelength region 287.26–290.03 nm in the A→X system was recorded. The rate coefficients for quenching of OD (A 2∑+) from v′=0 and v′=1 levels by O2, N2, air, and SF6 were measured. The rate coefficient for vibrational relaxation of OD (A 2∑+) from v′=1 to v′=0 level was separated from the overall rate coefficient for quenching of OD (A 2∑+) from v′=1 level to the ground (X 2Π) state for the above four gases. The rate coefficients for quenching of OD (A 2∑+) from v′=0 level to the ground (X 2Π) state by 15 common gases were measured by using excess SF6 to rapidly quench the OD (A 2∑+) from v′=1 to v′=0 level. All measurements were carried out in excess helium and are for rotationally thermalized OD (A 2∑+) radicals.
Transition probabilities for the driven harmonic oscillator
Authors: Fernández Francisco M.
Published: July 2002

A simple and straightforward method is presented for calculating transition probabilities for the driven harmonic oscillator. The procedure does not require the explicit calculation of neither the time-evolution operator nor the Green’s function. As a result, a recurrence relation for the transition probabilities is obtained.
Liquid GeBr4. II. Molecular packing and liquid structure
Authors: Ludwig K. F. , Wilson Lane , Warburton W. K. , Bienenstock A. I.
Published: July 2002

Anomalous scattering data from 1-GeBr4 are compared with theoretical predictions and with several models. Published hard-sphere RISM calculations of the pair correlation functions accurately predict the features of the experimental distribution functions, but discrepancies between theory and experiment are larger than the experimental uncertainty. New calculations with more realistic potentials are needed. The total structure factor of 1-GeBr4 contains a prepeak which is due largely, but not exclusively, to correlations between molecular centers (i.e., germanium atoms). Modeling results indicate that there is strong interlocking of molecules in the liquid and that the Br–Br structure factor is largely determined by the packing of the halogen atoms with the germanium atoms effectively fitting in tetrahedral interstices. Models based on the monoclinic crystal of SnBr4 agree better with the liquid data than models based on the cubic crystal of c-GeBr4, suggesting that GeBr4 undergoes a change in short-range molecular packing on melting.
Relativistic pseudopotential calculations for HBr+, HBr, HBr−, HI+, HI, and HI−
Authors: Schwerdtfeger P. , Szentpály L. V. , Stoll H. , Preuss H.
Published: July 2002

Valence SCF/CI calculations using nonrelativistic, relativistic, and semiempirical pseudopotentials have been carried out for the ground states of HBri and HIi (i=+1,0, −1). Autoionization of HBr− and HI− is characterized by the crossing points between the Born–Oppenheimer potential energy curves of the negative and neutral molecules. Relativistic and correlation effects are discussed for several molecular properties. Using semiempirical pseudopotentials+valence-CI, our calculated values for HX and HX+ (X=Br, I) are in good agreement with experiment. The crossing between the 1Σ+ (HX) and 2Σ+(HX−) curves is calculated to occur at 1.70 Å for HBr/HBr− and 1.84 Å for HI/HI−. Dissociative attachment energies for HX/HX− are compared with results from low-energy electron scattering experiments.
The dynamics and microscopic kinetics of the reactions of O(1D2) atoms with CHCl3 and CHF3
Authors: Aker P. M. , Niefer B. I. , Sloan J. J. , Heydtmann H.
Published: July 2002

The energy disposal and branching ratios in the reactions of O(1D2) with CHCl3 and CHF3 have been measured using an implementation of time-resolved Fourier transform spectroscopy in this laboratory. The infrared emission from the products of the reactions is measured as a function of time after the creation of the O(1D2) atom by UV photolysis of ozone. The reaction with CHCl3 produces OH, HCl, and CO as primary products. The OH vibrational excitation indicates simple abstraction dynamics. The HCl has much lower vibrational excitation, characteristic of a longer-lived insertion-elimination process, which also produces CO in the decomposition of the internally excited Cl2CO product. Only HF is observed in the reaction with CHF3. In this case the vibrational distribution is nonmonotonic, indicating contributions from two microscopic channels, possibly associated with the formation of F2CO in both the ground (X̃ 1A1) and first excited (Ã 1A2) states.
The Gaussian orthogonal ensemble with missing and spurious levels: A model for experimental level-spacing distributions
Authors: Lehmann Kevin K. , Coy Stephen L.
Published: July 2002

This paper reports analytical approximations to the level-spacing distributions for two types of eigenspectra: a Gaussian orthogonal ensemble (GOE) eigenspectrum, a fraction of whose levels are omitted, and the superposition of a fractional GOE distribution with levels from a random sequence. The combined distribution would be expected when a spectrum with a GOE spacing distribution is observed, but with insufficient dynamic range to observe all the lines, and with the possibility of contamination from other sequences or from noise identified as spectral lines. Berry recently demonstrated that GOE type spacing statistics are expected in the semiclassical limit of a classically chaotic Hamiltonian. Therefore, the present formula may prove useful in testing for quantum signatures of classical chaos in quantum molecular spectra.
Rotational transitions of N2(a 1Πg) induced by collisions with Ar/He and N2(a 1Πg)–N2(X 1Σ+g) rovibronic energy transfer studied by laser REMPI spectroscopy
Authors: Sha G. , Proch D. , Kompa K. L.
Published: July 2002

This paper reports the results of two related experiments: (A) The state-to-state rotational transition probabilities of N2(a 1Πg) in collisions with rare gas atoms (Ar or He) were measured by the technique of two-step multiphoton ionization. Results show that the selection rule antisymmetric–symmetric is obeyed. The transition probability drops rapidly with increasing ‖ΔJ‖. A propensity rule related to the Π+ or Π− symmetry conservation of the electronic wave function during the collision induced rotational transition holds. (B) The cross section for the rovibronic energy transfer between N2(1Πg) and N2(X 1Σ+g) is found to be ∼28 Å2. N2(a 1Πg) product populations show a Boltzmann-like distribution with a rotational temperature suggesting an equipartition of the available energy among the rotational and translational degrees of freedom of the products. A mechanism invoking an intermediate collision complex along with intermolecular electron exchange may explain the results.
The high-resolution far infrared spectrum of a van der Waals stretching vibration: The ν3 band of Ar–HCl
Authors: Robinson Ruth L. , Gwo Dz-Hung , Saykally Richard J.
Published: July 2002

The first high-resolution measurement of a van der Waals stretching vibration is reported. The ν3 fundamental stretching vibration in Ar–HCl has been measured by intracavity far infrared laser/microwave double resonance spectroscopy of a supersonic free jet. Coupling between the stretching state and the nearby bending states gives the spectrum an intensity which is greater than predicted. The molecular constants for the stretching state are in close agreement with calculations by Hutson for the M5 potential surface.
Dissociation energetics of SiF systems of relevance to etching reactions
Authors: Garrison Barbara J. , Goddard William A.
Published: July 2002

The bond energies of the 2Π and 4Σ− states of SiF, the 1Σ+ state of SiF+, the 3Σ− state of SiF−, the 1A1, 3B1, and 1B1 states of SiF2, the 2A1 state of SiF+2, and the ground states of SiH3–F, SiF3–F, SiH3–SiH3, and SiH3–SiF3 have been calculated using generalized valence bond and dissociation-consistent configuration interaction methods. Excellent bond energies are determined using only a relatively small number of configurations in the calculations. For example, the bond energy of the 2Π state of SiF is calculated to be 5.45 eV, in excellent agreement with the experimental value of 5.62 eV. For the inversion bend of SiF3 we predict that the transition state is a T-shaped configuration rather than one of D3h symmetry. Of note for the etching of solid silicon by reaction with F atoms is that unsaturated Si species react readily with F atoms to form strong bonds and that the bonding of a F atom to a lone pair of electrons on a Si atom is an activated process.
The effect of H/D substitution and pressure on the liquid–liquid equilibrium between cyclohexane and methanol
Authors: Singh Rajiv R. , Van Hook W. Alexander
Published: July 2002

Coexistence curves for the systems C6H12+CH3OH(i), C6D12+CH3OH(ii), C6H12+CH3OD(iii), and C6H12+CD3OD(iv) have been studied as a function of pressure [0.1<(P/MPa)<13] and reduced temperature t=(1−T/Tc) (0<t<1.3×10−2). A multiple sample technique was employed. The amplitudes and critical exponents and their pressure and isotope dependences are reported. The effect of isotopic dilution of each component on the critical solution temperature Tc has also been studied. The critical exponents show neither isotope nor pressure dependence. Critical temperatures show singnificant isotope dependence [Tc(i)−Tc(ii)]= i−ii=3.91 K, i−iii=−2.50 K, i−iv=0.23 K, and a significant pressure dependence dTc/dP=0.317 K/MPa, which over the range of conditions is independent of pressure and isotopic substitution. The amplitude factors, which carry larger experimental errors, show both isotope and pressure dependences. Isotopic dilution studies were carried out only at ambient pressure. Their interpretation leads to the conclusion that C6H12/C6D12 binary solutions are ideal within the experimental error of these measurements, but CH3OH/CH3OD mixtures show significant nonideality. The pressure and isotope dependence of the coexistence parameters for solutions i–iv are discussed in terms of thermodynamic and scaling theories of critical solution phenomena, and in terms of the statistical theory of isotope effects in condensed phases. The classical part of the analysis employed the Guggenheim theory of symmetrical mixtures, which quantitatively succeeded in rationalizing the isotope and pressure dependences of the effects.
The structures of c
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s-dichloroethenes adsorbed on Pt(111)
Authors: Grassian Vicki H. , Pimentel George C.
Published: July 2002

The structures of cis-1,2- and trans-1,2-dichloroethene (DCE) adsorbed on the Pt(111) surface have been examined by electron energy loss and thermal desorption spectroscopies over the temperature range 110 to 300 K. For monolayer and submonolayer coverages, the DCEs form sigma bonds to the platinum surface and assume staggered, alkane-like structures. Cis-DCE gives the gauche isomer with its most intense EEL peaks at 650 and 2985 cm−1, while the trans-DCE gives the trans conformer with its most intense peaks at 720 and 3000 cm−1. Isomerization does not occur. On warming to 160 K, the multilayer desorbs and above 200 K, dehalogenation occurs. A prominent EEL peak around 290 cm−1 shows that the chlorine atoms remain bonded to the surface. For both cis- and trans-DCE, the remainder of the EEL spectrum is that of C2H2 bonded to the surface in an olefinic structure signaled by prominent peaks at 770–785 and 3010–3020 cm−1.
Energy transfer to CO(v) in the O(1D)+CO(1Σ+g) reaction
Authors: Harding David R. , Weston Ralph E. , Flynn George W.
Published: July 2002

The O(1D)+CO quenching process has been reinvestigated using time-resolved diode laser absorption to probe the time dependence of the collisionally induced CO vibrational population distribution. The absolute electronic-to-vibrational energy transfer efficiency was determined to be 25+8, −5%, in good agreement with a previously reported value. Nascent vibrational populations for CO (v=0 to 4) were determined and a vibrational temperature of ∼5000 K was measured.
The core hole decay of N2O following core to bound state excitations
Authors: Larkins F. P. , Eberhardt W. , Lyo In-Whan , Murphy R. , Plummer E. W.
Published: July 2002

We present here a semiempirical assignment for the electronic decay spectra of core to bound state resonances in N2O. The electronic final states observed in these spectra can be assigned to participator and spectator decay processes, according to whether the initially excited core electron participates or acts as a spectator in the Auger decay of the core hole. The participator states are interpreted empirically using the known one electron binding energies. The assignment of the spectator lines is derived from the calculated Auger spectrum which is corrected for the Coulomb interaction between the spectator electron and the double hole valence configuration.
Variational–perturbational treatment for the polarizabilities of conjugated chains. II. Hyperpolarizabilities of polyenic chains
Authors: de Melo C. P. , Silbey R.
Published: July 2002

We use a perturbative density matrix treatment to investigate the behavior of the first and second static hyperpolarizabilities of linear conjugated chains CNHN+2 described by a Pariser–Parr–Pople Hamiltonian. We examine the evolution of the hyperpolarizabilities with increasing chain lengths, and the effect of different conformations (such as those corresponding to soliton and polaron defects) and of different charge states on the polarizability response of the chains. It is shown that charged soliton chains have large first hyperpolarizabilities β, and that the behavior of the different components of the second hyperpolarizability tensor γiiii is highly dependent on the geometry and charge of the conjugated chain. In all cases, the contribution of the longitudinal component is shown to dominate the orientationally averaged hyperpolarizability of the chains.
Chemisorption of fluorocarbon free radicals on silicon and SiO2
Authors: Joyce S. , Langan J. G. , Steinfeld J. I.
Published: July 2002

We have investigated the interaction of CF3 free radicals, produced by infrared multiple-photon dissociation of C2F6, with Si(100) and SiO2 surfaces. The CF3 radicals initially undergo dissociative chemisorption on the Si surface to produce surface carbides and fluorosilyl species. At higher coverages, the fluorine transfer reaction is inhibited. Significantly smaller amounts of the CF3 radical chemisorb on the oxide surfaces and those that do adsorb do not dissociate. Sputtering the oxide surface by argon ion bombardment increases the amount of radicals that can be adsorbed. Ion bombardment of the fluorocarbon overlayer on both surfaces results primarily in removal of the CF3 groups. The radical reactions cannot by themselves account for SiO2/Si selectivity in plasma reactive etching, but do appear to play an important role in anisotropic etching.
Potential energy surfaces of CH+4
Authors: Frey Regina F. , Davidson Ernest R.
Published: July 2002

The potential energy surfaces of the three lowest electronic states of CH+4 are calculated. CI calculations based on average natural orbitals from preliminary multireference singles and doubles CI calculations are found to provide a balanced description of the three states. The calculated points are fit to the eigenvalues of a 3×3 matrix whose elements are a power-series expansion in the normal-mode displacements from the SCF tetrahedral geometry. The equilibrium structures for the C2v, D2d, and C3v Jahn–Teller distortions are located and the vibrational frequencies for these structures are calculated. In addition, the transition state for the pseudorotation between equivalent C2v minima is located and the nature of the pseudorotation is discussed in detail.
A theoretical study of the hard sphere fluid–solid interface
Authors: McMullen William E. , Oxtoby David W.
Published: July 2002

We investigate in detail the fluid–solid interfaces of hard spheres using the density functional formalism of inhomogeneous systems. The interfacial profiles and surface free energy γ are determined variationally but we avoid the usual square-gradient approximation (SGA) for the nonlocal parts of the free energy. Our findings suggest a narrow interface of 3–4 σ (σ=a hard-sphere diameter) and an upper bound to γ of about 1.7 kBT/σ2. For the low-order crystal planes, γ is almost independent of the crystal orientation, varying less than a few tenths of a percent between the [111] and [110] planes. Our results generally agree with the SGA when a single, hyperbolic-tangent description is used for the inhomogeneous density. However, more realistic variations of the interfacial profiles produce greater disparity between the two methods. We discuss ways in which the SGA might be improved to account for these differences.
The effect of reagent rotation in the reaction OH( j)+H2( j′)→H2O+H
Authors: Harrison Judith A. , Mayne Howard R.
Published: July 2002

Classical trajectory calculations on the reaction OH( j)+H2( j′)→H2O+H have been carried out with j≤40 and j′≤15 on both the Schatz–Elgersma [Chem. Phys. Lett. 73, 21 (1980)] and the Rashed–Brown [J. Chem. Phys. 82, 5506 (1985)] potential energy surfaces. When there is no rotation in the OH, then a plot of reaction cross section, SR( j′) resembles that for an atom–diatom system: Just above threshold, rotation decreases reactivity for small j′, but increases it for high j′; at higher translational energies this trend is less obvious, but still present. When j′=0, then SR( j) is a complicated function, decreasing for low j, then climbing to a maximum, finally decreasing once more at very large values of j. We have also carried out calculations with isotopically substituted H in OH, and show that these effects scale as the mass of the hydrogen isotope. We show that this behavior is due to artifacts in both the potential surfaces. Using a simple model we are able to rationalize this behavior. Using this same model, together with a potential in which the artifacts are absent, we predict that rotation in OH will decrease reactivity rather less than was reported in previous trajectory studies [Schatz, J. Chem. Phys. 74, 1133 (1981); Rashed and Brown, J. Chem. Phys. 82, 5506 (1985)].
The electronic structure and stability of asymmetric dinitrogen dioxide (a-N2O2)
Authors: Michels H. H. , Montgomery J. A.
Published: July 2002

The electronic structure of asymmetric dinitrogen dioxide (a-N2O2) has been studied by ab initio calculations. A stable structure in Cs symmetry, of similar geometry to FN3, is predicted for the lowest 1A′ state. This structure is characterized by an IR frequency of 1206 cm−1 for the N–O stretch and exhibits a significant activation barrier (65 kJ/mol) for decomposition into N2+singlet O2. This asymmetric form of N2O2 has a heat of formation of approximately +500 kJ/mol and is potentially explosive.
Invariant expansions of the radial distribution function for fluids of hard convex bodies
Authors: Kumar Bijoy , James Corinne , Evans Glenn T.
Published: July 2002

The radial distribution functions (RDF) for systems consisting of a single nonspherical convex body in a fluid of spheres and for a pure fluid of nonspherical convex bodies are expressed in terms of the minimum surface-to-surface separation and a set of angles derived from the surface normal. The orthogonal function expansion of the anisotropic RDF, g, has the property that the first anisotropic contribution vanishes at zero density and is directly proportional to the nonsphericity of the particle; however, the resulting convex body orthogonal polynomials depend on the surface-to-surface separation. An invariant expansion of the product of the Jacobian and the RDF, S12g, has anisotropic expansion coefficients even at zero density, but the expansion functions are the spherical harmonics (independent of the surface-to-surface coordinate). Monte Carlo simulations are conducted on the two dimensional system consisting of a single convex ellipse in a bath of circular disks. A comparison is made between the center-to-center expansion of the RDF, an expansion based on the orthogonal function expansion, and the S12g (or Kabadi–Steele) expansion.
On the theory of homogeneous nucleation and spinodal decomposition in condensation from the vapor phase
Authors: Ruth Volker , Hirth John P. , Pound G. M.
Published: July 2002

On the basis of the assumptions of statistical theories, nonlinear expressions for some of the fundamental equations of nucleation theory have been derived. The results apply to any of the statistical theories but are specifically applied to the modification of the original Lothe–Pound theory. In the range of usual supersaturations the numerical results of the linearized Lothe–Pound theory (LLP) differ only slightly from those of the nonlinear Lothe–Pound theory (ELP), but the difference increases with degree of supersaturation. All fundamental equations of nucleation theory can be expressed in terms of a dimensionless energy parameter that also defines the condition of onset of spontaneous condensation without nucleation, expected to occur at higher supersaturations. Furthermore, under the premises of the present theory the growing clusters will pass an intermediate stable stage at a specific radius before reaching the size of a stable critical nucleus. This effect should result in changes in the relaxation time for establishing steady-state of nucleation.
Laser-induced desorption: Thermal and nonthermal pathways
Authors: Burgess D. , Cavanagh R. R. , King D. S.
Published: July 2002

Distributions of internal states and kinetic energies are reported for the laser-induced desorption of NO from a platinum foil. Two desorption channels are observed, one with internal-state populations which are well fit by Boltzmann distribution functions that correspond to the peak surface temperature. The second desorption channel displays population distributions which are non-Boltzmann: kinetic energies exceed the peak surface temperature by a factor of ≈5; kinetic energies depend on J; the spin–orbit states are inverted; and the vibrational population exceeds that associated with the maximum surface temperature by a factor of 50. The wavelength dependence of the non-Boltzmann desorption signal suggests that a substrate mediated desorption process is operative.
Near-infrared bound–free emission from the NaK molecule
Authors: Huennekens J. , Loza A. , Masters M. , Sando K. M.
Published: July 2002

We report the observation of bound–free emission corresponding to the lowest triplet transition 1 3Π→1 3Σ+ of the NaK molecule. In the experiment, specific levels of the upper triplet state were populated directly from the ground state by virtue of spin–orbit induced perturbations between the 1 3Π0 and 2 1Σ+ states. Oscillations in the fluorescence spectra between 1.1 and 1.6 μm reflect the probability distribution (wave function squared) in the bound upper state. We have also carried out quantum mechanical simulations of the fluorescence spectra, based on recent ab initio calculations of the relevant potential curves. These simulations verify the identification of this near-infrared emission and provide a critical test for the calculated potentials.
Electric-field-induced two-photon absorption
Authors: Andrews David L. , Hopkins Kevin P.
Published: July 2002

This paper provides a theoretical treatment of the electrochromic effect of a static electric field on molecular two-photon absorption, based on a full quantum electrodynamical formulation of the interactions. The results include a description of changes in the spectrum resulting both from electro-optical interactions and, in the case of polar fluids, from the anisotropy associated with energetically favored molecular orientations. A detailed consideration of the effect of varying laser polarization and field direction is included in the analysis. The results demonstrate that on application of the field, weak but potentially detectable new lines should appear in the two-photon spectrum. These lines are associated with transitions to vibronic states which would normally require three-photon excitation. The application of the static electric field also results in the selective enhancement of other lines normally present in the two-photon spectrum. The effect may thus offer scope for the accurate determination of the frequency and symmetry character of weak vibronic transitions when they occur under the wing of strongly allowed transitions.
Collisional relaxation of single rotational states in highly vibrationally excited acetylene
Authors: Carrasquillo E. M. , Utz A. L. , Crim F. F.
Published: July 2002

The combination of vibrational overtone excitation with time-resolved laser induced fluorescence detection of the excited molecule permits the study of collisional energy transfer in highly vibrationally excited molecules with single quantum state resolution. We apply this new technique to aceytlene excited in the region of the second CH stretching overtone transition (3νCH) (εvib =9640 cm−1) and probed via the Ã electronic state. Our results show that self-relaxation proceeds at essentially the gas kinetic collision rate while quenching by the rare gases He, Ar, and Xe is only about a factor of two slower. The insensitivity of the relaxation rate to the structure of the collision partner clearly points to rotational relaxation or intramolecular vibrational energy transfer as the mechanism for collisionally depopulating the initially prepared state.
An inversion procedure for oscillatory continuum spectra: Method and application to NaK
Authors: LeRoy Robert J. , Keogh William J. , Child Mark S.
Published: July 2002

An inversion procedure is developed for determining an attractive potential energy well from the structure of an oscillatory absorption or emission continuum originating in a single vibrational level of that well. It is successfully applied to synthetic data for the 1Π1u←X(1Σ+g) absorption continuum of Br2, and to experimental data for d(3Π1)→a(3Σ+) emission of NaK. The latter application succeeds in spite of the facts that (i) the repulsive wall of the final-state potential is also not known, and (ii) this final-state curve has an attractive well whose location causes the structured emission continuum to be truncated, making the vibrational assignment uncertain. This analysis yields potentials for the well of the d(3Π1) state and the repulsive wall of the a(3Σ+) state of NaK, together with the slope of the transition dipole function coupling these two states.
Vibron band structure in chlorinated benzene crystals: Lattice dynamics calculations and Raman spectra of 1,2,4,5-tetrachlorobenzene
Authors: Jongenelis A. P. J. M. , van den Berg T. H. M. , Jansen A. P. J. , Schmidt J. , van der Avoird A.
Published: July 2002

From a combined theoretical and experimental study of 1,2,4,5-tetrachlorobenzene (TCB) we conclude that this crystal shows many interesting effects. In agreement with earlier optical measurements, which probe the complete vibron band structure of TCB, we calculate that several of the vibron modes have the dispersion of a one-dimensional crystal with stacks of molecules along the a axis. The inclusion of fractional atomic charges in the atom–atom potential used in the calculations is absolutely necessary to obtain the correct vibron bandwidths. Also the sign of the vibrational coupling matrix elements, which is given correctly by the calculations, is determined by these charges. For other properties, such as the crystal stability, the phonon frequencies, the site splitting in the vibron bands, and also the dispersion of the lower frequency vibrations, the interactions between the ‘‘one-dimensional’’ stacks are essential, however. The calculations predict further, in qualitative agreement with the Raman spectra, that the splitting between the pairs of inequivalent vibron bands in α-TCB (the site splitting) is significantly larger than the factor group splitting in β-TCB. Finally we have found that, for those vibrations where the conditions are shown by the calculations to be favorable, the 35Cl/37Cl isotope effects are clearly visible in the Raman spectra.
A Langevin approach to the macroscopic stochasticity of chemical systems
Authors: Sagués F. , Sancho J. M.
Published: July 2002

A theoretical approach to the problem of the marked irreproducibility of certain chemical reactions studied by Epstein’s group at Brandeis University is presented. The model is based on the use of a Langevin equation with a multiplicative noise. Stirring and finite volume effects are incorporated in our approach. Analytical results are provided for the mean first passage time (MFPT), which is the appropriate quantity we associate here with the reaction time.
Emission spectroscopy of the predissociative Rydberg B state of CH3I and CD3I at 193.3 nm
Authors: Lao Kaiqin , Person Maria D. , Chou Tom , Butler Laurie J.
Published: July 2002

We measured the emission spectra of CH3I and CD3I excited to a predissociative Rydberg state (E,1){2} near 193.3 nm and tabulated emission line positions with their assignments. The emission spectrum of CH3I both provides previously unobserved energies of ground state combination bands in ν2 (CH3 umbrella) and ν3 (C–I stretch), and also suggests the excited level in the (E,1){2} Rydberg state is the previously unobserved 2132 state. These results are discussed in relation to recent comparisons of UV absorption spectra of monomeric and dimeric CH3I. The CD3I emission spectrum measured results from excitation of the combination mode of CD3 rocking (ν6), umbrella motion (ν2), and C–I stretching (ν3) in the (E,1){2} rydberg state of CD3I, followed by emission to a progression of 2n3161 combination modes in the ground electronic state. A simulation of the line shape of the peaks in the CH3I spectrum shows the width of the peaks results from the contribution of both inhomogeneous (K and J rotational structure) and homogeneous (lifetime) broadening. Emission from molecules after crossing to the repulsive surface was not observed.
Kinetics of the reaction of gallium arsenide with molecular chlorine
Authors: Ha J. H. , Ogryzlo E. A. , Polyhronopoulos S.
Published: July 2002

The reaction of Cl2 with the (100) face of a GaAs single crystal was studied in the temperature range from 25 to 150 °C. The reaction was found to be first order in Cl2 at low pressures with an activation energy of 23.6 kcal. At pressures above 10 Torr it was found to reach a limiting rate with an activation energy of 14.2 kcal, attributable to the enthalpy of desorption of the GaCl3 product from this surface.
Promotion of the proton transfer reaction by the intermolecular stretching mode: Application of the two-dimensional finite element method to the nuclear Schrödinger equation
Authors: Sato Nobuyuki , Iwata Suehiro
Published: July 2002

A model Hamiltonian is proposed to analyze the recently observed promotion of the proton transfer reaction by the intermolecular stretching vibrational mode. The Schrödinger equation was solved numerically with the two-dimensional finite element method. The contour maps of the wave functions clearly indicate that when the intermolecular stretching mode is excited, the proton transfers from one site to the other over the ridge, taking the detour path. Only when the intermolecular mode is in the lowest states, does the proton transfer under the barrier with the tunneling mechanism. It is demonstrated that the full two-dimensional analysis is necessary in the analysis of the large amplitude mode coupling and that the finite element method is a powerful tool to solve the Schrödinger equation for the nuclear motion in molecules.
Theoretical studies of the monoxides and monosulfides of Y, Zr, and Nb
Authors: Langhoff Stephen R. , Bauschlicher Charles W.
Published: July 2002

Spectroscopic constants (De, re, ωe, μe) are presented for the ground and low-lying states of the monoxides and monosulfides of yttrium, zirconium, and niobium using large valence basis sets in conjunction with relativistic effective core potentials. For states well described by a single configuration, electron correlation is incorporated using the modified coupled-pair functional method. When several molecular states of the same symmetry or transition moments are determined, we employ state-averaged CASSCF/MRCI methods. The theoretical spectroscopic constants for the oxides are generally in excellent agreement with experiment, so that the corresponding results for the sulfides should be helpful to spectroscopists attempting to characterize these band systems. The quartet manifold of states in YO is found to lie too high to significantly affect the spectroscopy of the low-lying doublet states. Both the A 2Π–X 2Σ+ and B 2Σ+–X 2Σ+ transitions in YO are very strong, resulting in radiative lifetimes of 21 and 17 ns for the A 2Π and B 2Σ+ states, respectively. The (1)3Φ–a3Δ band system of ZrS is identified as a likely carrier of the Keenan bands observed in the infrared spectra of S stars. The theoretical De values parallel experiment, but are systematically low, as the calculations do not fully account for the large correlation contribution to the bond energy.
Geminate recombination in proton-transfer reactions. IV. Ground-state yields
Authors: Agmon Noam
Published: July 2002

Our calculations of proton–anion distance distribution functions for HPTS dissociation recombination in the excited state are extended to the nanosecond time regime. When these functions are used for averaging the ground-state recombination probability, good agreement with experimental ground-state recombination yields is obtained. The relation with the theory of ‘‘delayed’’ geminate recombination is briefly discussed.
Theoretical studies of collisional energy transfer in highly excited molecules: The importance of intramolecular vibrational redistribution in successive collision modeling of He+CS2
Authors: Bruehl Margaret , Schatz George C.
Published: July 2002

In this paper we present a new method for studying the collisional relaxation of highly excited molecules in low density gases known as the redistributed successive collisions (RSC) method, and we apply it to the relaxation of CS2 by He at 300 K in the vibrational energy range E=32 640–3180 cm−1. The RSC method involves calculating sequences of collisions, subject to the assumption that rapid vibrational redistribution occurs between each collision. As a result, initial conditions for each trajectory in a sequence are sampled from a microcanonical ensemble that is defined by the final energy and angular momentum of the previous trajectory. The application to He+CS2 leads to 〈ΔE〉’s that vary linearly with E over the entire energy range considered. The agreement of these 〈ΔE〉’s with measured values is good, but there is a qualitative difference in the E dependence of 〈ΔE〉 over part of the range of E’s. We also examine a second successive collision method that is more appropriate for high-density gases in which the internal coordinates and momenta are conserved (i.e., not redistributed) between collisions (CSC method). We find that a substantial fraction of the CSC ensembles (∼50%) exhibit extremely slow relaxation which in some cases is not complete even after 80 000 collisions. This unphysical result appears to be a classical artifact, and it leads to very small 〈ΔE〉’s at medium to low E and a stronger dependence of 〈ΔE〉 on E (close to quadratic) at high E. Omission of these slowly relaxing ensembles from the CSC ensemble average leads to CSC 〈ΔE〉’s which are nearly identical to those from the RSC calculation. An analysis of the distribution of energy among vibrational modes in the CSC calculations indicates that the slow relaxation arises from energy becoming frozen in the asymmetric stretch of CS2. The influence of the CS2 intramolecular dynamics on the collisional relaxation is considered, and we find evidence of abrupt collision induced intramolecular energy redistribution due to nonlinear resonance formation.
Nonprimitive model of electrolytes: Analytical solution of the mean spherical approximation for an arbitrary mixture of sticky ions and dipoles
Authors: Wei Dongqing , Blum Lesser
Published: July 2002

A model for association in a nonprimitive ionic solution is presented. It consists of a mixture of hard sphere ions and dipoles with sticky interactions. This model can be solved analytically in the mean spherical approximation (MSA). We consider first the case in which the stickinesses between ions, the ion–dipole, and the dipole–dipole have spherical symmetry. In this case, the solution for the unrestricted case is given formally. Much simpler formulas can be obtained in the case of equal ionic sizes, which however are different from the size of the dipole. If the size of ions is equal, and also equal to the size of the dipole, then we get a cubic equation for the ion–dipole parameter b2 and a quadratic equation for ion–ion parameter b. We found that the spherical stickiness between the ion–dipole and the dipole–dipole has very little effect on the directional correlation of the ion–dipole and the dipole–dipole. So we further consider the case in which the stickiness is directional. In this case, the solution is obtained for all equal sizes. Finally we compute the thermodynamic properties.
Rotation–vibration interactions between the two lowest frequency modes in formaldehyde
Authors: Burleigh Darin C. , Mayrhofer Rudolph C. , Sibert Edwin L.
Published: July 2002

Rotation–vibration interactions between the two lowest frequency normal modes of H2CO, the out-of-plane bend and the in-plane wag, are studied using classical trajectories. The dynamics is investigated for a range of rotational angular momenta, J, and energy values. Vibrational energy flow is elucidated by examining trajectories in several different canonical representations. The a-axis Coriolis term, which is quadratic in the normal coordinates, accounts for most of the coupling, as seen by comparing plots in the normal mode representation and one in which the Coriolis term has been subsumed into the zero-order Hamiltonian. In the former, the modes are more strongly coupled as the projection of J onto the body-fixed z axis increases; in contrast, the Coriolis adapted normal modes are more decoupled. Making use of the observed decoupling, the rovibrational Hamiltonian is reduced to an effective one degree-of-freedom rotational Hamiltonian whose dynamics depends on the vibrational excitation. Model spectra have been obtained using the semiclassical method of Gaussian wave packet propagation of Heller [J. Chem. Phys. 62, 1544 (1975)]. Semiclassical and full quantum results analogous to the observed classical dynamics are presented.
Approximate calculation of the Jahn–Teller energy and Ham reduction factor for CH3S
Authors: Bent Gary D.
Published: July 2002

The difference in the spin–orbit splittings of CH3S and CD3S, the value of ν5 for CH3S, and the Teller–Redlich rule for CH3S/CD3S and CH3O/CD3O are used to make an approximate calculation of the Jahn–Teller energy and Ham reduction factor p for CH3S. The results are EJT=95 cm−1 and p=0.79. The results indicate that the unpaired electron in CH3S is localized on the sulfur atom.
Theoretical investigation involving electronic and vibrational calculations of the 2 2A1(3p)→1 2B2(3p) and the 3 2A1(4s)→1 2B2(3p) transitions in FH2 and FD2
Authors: Petsalakis I. D. , Theodorakopoulos G. , Wright J. S. , Hamilton I. P.
Published: July 2002

Theoretical calculations have been carried out on the X 2A1, 1 2B2(3p), 2 2A1(3p), and 3 2A1(4s) electronic states of FH2. Equilibrium geometries and rotational constants as well as the first few vibrational levels of the excited states have been calculated, in order to obtain theoretical information on the 2 2A1(3p)→1 2B2(3p) and the 3 2A1(4s)→1 2B2(3p) transitions in FH2, which might be relevant to the observed spectra at about 7500 and 8000 Å. The results show that the equilibrium geometry of the first excited state of FH2, 1 2B2(3p), is quite different from those of the other excited states. The estimated transition energies (ΔE0) in FH2 are 1.68 and 1.97 eV for the transitions 2 2A1(3p)→1 2B2(3p) and 3 2A1(4s)→1 2B2(3p), respectively, while in FD2 the corresponding quantities are 1.65 and 1.95 eV, respectively. A search for a minimum on the ground state surface of FH2, which has been carried out near two saddle point geometries, has not found one. Thus the present calculations do not find a metastable ground state species.
Interaction of halogen anions with some cations and inert gas atoms
Authors: Patil S. H.
Published: July 2002

We present a unified analysis of the interaction of halogen anions with some cations and inert gas atoms, within the framework of a perturbative approach. The Born–Mayer repulsion term is evaluated using asymptotic densities and correlation functions. Along with the long-range polarizability and van der Waals terms, this term gives a satisfactory description of the interaction energy for halogen ions interacting with some cations and inert-gas atoms.
Very low temperature spectroscopy: The pressure broadening coefficients for CH3F between 4.2 and 1.9 K
Authors: Willey Daniel R. , Crownover Richard L. , Bittner D. N. , De Lucia Frank C.
Published: July 2002

Pressure broadening parameters and cross sections for the CH3F–He system have been measured between 4.2 and 1.9 K by means of a recently developed technique based on collisional cooling in a cryogenic cell. The cross sections for the K=0 and K=1 components of the J=1–2 transition were found to be 75.2 and 66.6 Å2, respectively, at 4.2 K and 55.5 and 54.5 Å2, respectively, at 1.9 K. The cross sections at 4.2 K are about 50% larger than the 300 K values. These results are compared with earlier experimental and theoretical results obtained for the CO–He system.