bluuebunny/crossref_metadata_2025 huggingface dataset:
Tunneling frequencies of NH2D+2 and CH2D2 in crystalline fields of low symmetry
Authors: Smith David
Published: July 2002

Procedures for calculating the rotational energy levels of a hindered asymmetric top in crystalline fields of C3, C2, and C1 symmetry are outlined. These procedures are used to compute the splittings between tunneling states of NH2D+2 and CH2D2 in various crystalline environments. These splittings have been computed for NH2D+2 in (NH4)2SiF6, (NH4)2GeF6, (NH4)2TiF6, NH4F, NH4ReO4, NH4IO4, NH4HF2, and NH4ClO4. Large splittings between tunneling states are computed for this asymmetric top in these compounds, all of which have site symmetries for the ammonium ion that are less than tetrahedral. Two groups of tunneling states are found when the site symmetry of NH2D+2 is C3v and four groups of states are found when the site symmetry of NH2D+2 is S4, C2, or Cs. These large tunneling frequencies are similar to those computed for NH3D+ and NHD+3 in these solids. Tunneling frequencies have also been calculated for the asymmetric top CH2D2 in phase II of solid methane and for CH2D2 adsorbed on the surfaces of graphite and MgO. For the latter two environments, the splittings between the groups of tunneling states are larger than was found for CH3D, but the tunneling frequencies within a group of tunneling states is smaller than for CH3D in the same environment.
Bond-bias simulation of phase equilibria for strongly associating fluids
Authors: Tsangaris D. M. , de Pablo J. J.
Published: July 2002

In this work a novel Monte Carlo method is developed to simulate the equilibrium thermodynamic properties of strongly associating fluids. The highly anisotropic nature of intermolecular interactions in these fluids makes conventional simulation techniques of little use. By introducing biased sampling techniques we are able to explore configuration space efficiently, thereby obtaining reliable estimates for the thermodynamic properties, including phase equilibria, of model systems. The results of our simulations are used to assess the accuracy and validity of various theories for associating fluids.
Collisional energy transfer in gaseous xenon with vacuum ultraviolet laser excitation of the 5d[1/2]1 atomic level
Authors: Museur L. , Kanaev A. V. , Zheng W. Q. , Castex M. C.
Published: July 2002

Time-resolved analysis of the luminescence decay of gaseous xenon has been carried out with one-photon excitation of the 5d[1/2]1 atomic level for the first time. The one-photon selective excitation is realized with a coherent vacuum ultraviolet (VUV) light source generated via nonlinear processes in mercury vapor. Dominant three-body recombination of Xe(5d[1/2]1) atoms with a rate constant of 3.2(0.3)×10−31 cm6 s−1 has been found. Resonance radiation from this atomic level undergoes a self-trapping, which results in its dominant fluorescence decay in the IR with t=4.9(0.7) μs. Branching into two relaxation channels is found at low xenon pressure (5–100 mbar)—both avoiding the 6s[3/2]1 first resonance atomic level and terminating by VUV emission. At higher pressure, the relaxation kinetics changes displaying after 500 mbar the well known effect of ‘‘atomic reservoir’’ and radiation from the A1u/0−u molecular state with lifetime of 101(4) ns. The scheme of energy relaxation involving the 5d[1/2]1 atomic level is discussed.
Characterization of silicon–carbon clusters by infrared laser spectroscopy: The ν3(σu) band of linear Si2C3
Authors: Van Orden A. , Giesen T. F. , Provencal R. A. , Hwang H. J. , Saykally R. J.
Published: July 2002

The ν3(σu) fundamental vibration of 1Σ+g Si2C3 has been observed using a laser vaporization-supersonic cluster beam-diode laser spectrometer. Forty rovibrational transitions were measured in the range of 1965.8 to 1970.9 cm−1 with a rotational temperature of 10–15 K. A least-squares fit of these transitions yielded the following molecular constants: ν3(σu)=1968.188 31(18) cm−1, B″=0.031 575 1(60) cm−1, and B′=0.031 437 4(57) cm−1. These results are in excellent agreement with recent Fourier transform infrared (FTIR) measurements of Si2C3 trapped in a solid Ar matrix [J. Chem. Phys. 100, 181 (1994)] and with ab initio calculations [J. Chem. Phys. 100, 175 (1994)] which suggest cumulenic-like bonding for Si2C3, analogous to the isovalent C5 carbon cluster.
Calculation of rotational state-to-state ozone relaxation rates for O3–N2 and O3–O3 collisions
Authors: Boursier C. , Ménard-Bourcin F. , Boulet C.
Published: July 2002

Rotational state-to-state rate constants have been calculated for O3–N2 and O3–O3 by splitting global relaxation rates predicted by a semiclassical method accounting for both electrostatic and atom–atom interactions. Calculations are greatly sensitive to both the interaction potential and the splitting procedure. They also indicate a strong propensity for first-order quadrupolar transitions in the case of O3–N2 collisions whereas both first-order quadrupolar- and dipolar-type transitions are dominant for self-relaxation. The calculated rates have been introduced into a kinetic model describing rotational energy transfers. Although the results are in good agreement with most of the available experimental data, they show that changes in J larger than two for a single collision should be accounted for.
Competition between unimolecular C–Br–bond fission and Br2 elimination in vibrationally highly excited CF2Br2
Authors: Abel B. , Hippler H. , Lange N. , Schuppe J. , Troe J.
Published: July 2002

The competition between C–Br–bond fission and three-center elimination of molecular bromine (Br2) in highly excited CF2Br2 molecules has been studied under collision-free conditions. Transient resonantly enhanced multiphoton ionization (REMPI) was used to monitor Br(2P1/2) and Br(2P3/2) formation during and after infrared (IR) multiphoton excitation of CF2Br2; time-resolved laser-induced fluorescence (LIF) spectroscopy was employed for the detection of transient CF2 after Br2 elimination. Direct time-resolved measurements of the sum of afterpulse reaction rates, absolute product yields for the CF2 and Br(2P3/2) channels as well as absorbed energies per excitation pulse were used to characterize parts of the vibrational energy distribution P(E) established after IR multiphoton excitation and to determine rate coefficients and branching ratios for the elimination and dissociation reaction as a function of the average internal energy 〈E〉. The existence of both channels, the dissociation and the elimination channel, has been confirmed. A comparison of the experimental data with statistical adiabatic channel model calculations (SACM) enabled us to determine the threshold energies E0(J=0) for the unimolecular Br2 elimination [E0(J=0)=19 070±500 cm−1] and the C–Br bond fission [E0(J=0)=20 700±500 cm−1], the two possible pathways of the reaction.
Ab initio computations close to the one-particle basis set limit on the weakly bound van der Waals complexes benzene–neon and benzene–argon
Authors: Klopper W. , Lüthi H. P. , Brupbacher Th. , Bauder A.
Published: July 2002

The equilibrium geometries and binding energies of the van der Waals (vdW) complexes benzene–neon and benzene–argon have been calculated at the level of second-order Mo/ller–Plesset perturbation theory (MP2). Terms linear in the interelectronic distances r12 were used in the MP2 treatment to converge fast to the one-particle basis set limit. This new method, MP2-R12 as implemented in the sore program, was applied with high quality basis sets derived from Dunning’s aug-cc-pVXZ (X=D,T,Q,5) sets. In reward of the efforts to reach the basis set limit, it is found that the calculated binding energies for the vdW complexes were computed virtually free of a basis set superposition error (BSSE). The key MP2-R12 results are De=154 cm−1 and re=3.32 Å for benzene–neon and De=553 cm−1 and re=3.41 Å for benzene–argon. The permanent dipole moments of the vdW complexes have been computed by finite field perturbation theory. Coupled-cluster calculations of type CCSD(T), although performed with considerably smaller basis sets than the MP2-R12 calculations, show that, to improve the MP2-R12 results, it is inevitable to consider correlation effects due to connected triple excitations which go beyond the MP2 description.
Molecular integrals with Slater basis. V. Recurrence algorithm for the exchange integrals
Authors: Fernández Rico J. , López R. , Ramírez G. , Tablero C.
Published: July 2002

The numerical and analytical procedures, used for calculating exchange integrals with Slater functions, fail to give high accuracy for large quantum numbers and some values of the parameters. We propose here an algorithmic approach based on recurrence relations which start from some simple functions. An extensive study in which exponents, interatomic distances, and quantum numbers were varied proves that the new procedure is fully reliable. Finally, the cost of the procedure is analyzed, including the effect of the number of terms needed to attain a given accuracy.
The relaxation of structural fluctuations in a lattice model of a simple liquid
Authors: Moss Richard , Harrowell Peter
Published: July 2002

We report on dynamic Monte Carlo simulation studies of the extent and dynamics of structural fluctuations in the liquid phase of the fcc lattice gas with nearest-neighbor interactions. The structural relaxation exhibited nonexponential decay characterized by two time scales, a result in qualitative agreement with relaxation observed in colloidal suspensions. The time scale of the slow component increased more rapidly with decreasing temperature than that of the faster component. The origin of the slow component was traced to the presence of relatively long lived low-energy domains. The fluctuating cluster model was proposed to model the structural relaxation.
A single arrangement variational method for reactive scattering: Total and state-resolved reaction probabilities
Authors: Wu Xudong , Ramachandran B. , Wyatt Robert E.
Published: July 2002

A detailed discussion of an approximate, variational approach to atom–molecule reactive scattering is presented. This approach reduces the formally three arrangement atom–diatomic molecule reactive scattering problem to one of a single arrangement without the use of negative imaginary potentials at the exit channel boundaries. The method is based on applying the Kohn variational principle for the log-derivative matrix to a representation of the scattering wave function that spans just the reactant arrangement. For many reactive systems, the method yields impressively accurate results for total reaction probabilities from a specific initial state. In such cases, it is also possible to extract fairly accurate state-resolved reaction probabilities from the results of the variational calculation. The mathematical and practical aspects of accomplishing this are presented. We evaluate the advantages and the limitations of the method by numerical computations on the collinear H+H2 (and isotopes) and F+H2, and the three dimensional (J=0) F+H2 reactions.
Computational studies of the potential energy surface for O(1D)+H2S: Characterization of pathways involving H2SO, HOSH, and H2OS
Authors: Goumri A. , Rocha John-David R. , Laakso Dianna , Smith C. E. , Marshall Paul
Published: July 2002

Structures and vibrational frequencies for minima and transition states on the O(1D)+H2S potential energy surface have been characterized at the unrestricted second-order Mo/ller–Plesset (UMP2)=full/6-31G(d) level. The results for the thioperoxide HOSH agree with experimental IR spectra. Gaussian-2 theory was employed to calculate ΔHf,298 for HOSH of −119.3 kJ mol−1, −47.1 kJ mol−1 for the sulfoxide H2SO, and 47.0 kJ mol−1 for the thiooxonium ylide H2OS. We also derived ΔHf,0 for HOS and HSO of −2.7 and −17.0 kJ mol−1, respectively. Comparisons with ΔHf for known asymptotes on the potential energy surface gave good agreement, except in the case of HSO. Rice–Ramsperger–Kassel–Marcus (RRKM) analysis suggests that in most environments, except at low pressures and temperatures, H2OS will be short lived, and rate constants for isomerization of the three bound adducts under thermally equilibrated conditions are derived. The potential energy surface is discussed in the context of single-collision experiments, and pathways leading to SH+OH, 1SO+H2, HSO+H, HOS+H, and 1S+H2O have been analyzed.
Collisional intramolecular energy transfer CN(X 2Σ→A 2Π) in a beam
Authors: de Moor M. , Ottinger Ch. , Vilesov A. F. , Xu D. D.
Published: July 2002

The collision-induced intramolecular energy transfer from CN(X 2Σ+,v″) to CN(A 2Π) was observed in a thermal energy molecular beam through the CN(A→X) emission. In some preliminary experiments, CN(B→X) emission, resulting from collision-induced energy transfer from CN(X 2Σ+) to CN(B 2Σ+), was also observed. CN(X,v″) radicals were generated in a dc discharge burning in the expansion region of an Ar/CH3CN supersonic beam. Downstream from the source they interacted with a target gas in a collision cell. The relative formation rates of CN(A) with He, Ne, Ar, and Kr as collision gases were obtained. The relative vibrational population distribution of CN(A,v′=2–12) was also determined. It was found to be very similar for the four collision gases. In the wavelength region of the well-known perturbations between the A and X states, the rotational contours of the A→X emission bands show no pronounced enhancement. This demonstrates that in the spin-allowed collision-induced transition CN(A←X) the so-called gateway mechanism is much less important than in some spin-forbidden transitions [N2(B←A′) and NO(B←a)] studied previously by us.
Molecular dynamics simulation of the approach and withdrawal of a model crystalline metal to a silica glass surface
Authors: Webb Edmund B. , Garofalini Stephen H.
Published: July 2002

Room temperature interfacial atomistic behavior between a model Lennard-Jones Pt (111) crystalline surface and a silica glass surface was investigated using classical molecular dynamics simulations. The approach and pulloff of the crystalline surface to two silica glass surfaces was simulated. During approach, both simulated interfaces evolved from a state of tensile to compressive stress parallel to the direction of approach. Compression of both glass surfaces occurred with accompanying structural shifts that created coordination defects and small rings with strained siloxane bonds in the glasses. Upon pulloff, the system stress again went through a tensile region and, for both interfaces, the maximum tensile stress on pulloff exceeded that of the approach. In both glass surfaces, the relaxation accompanying pulloff of the crystal did not result in complete removal of the defects created during the cycle. The results have important implications with respect to the reactivity of glass surfaces during and after compressive contact with a crystalline phase.
EXAFS and Raman studies of dysprosium doped sodium sulfates
Authors: Dharmasena G. , Schlindwein W. S. , Frech R. , Linford R. G.
Published: July 2002

Results of Raman, conductivity and EXAFS studies of dysprosium doped sodium sulfates at concentrations of 0.5% and 3% of Dy3+ are presented. The Raman studies show that the lower dopant level (0.5% mole of Dy3+) produces the phase III structure whereas the higher doping (3% mole of Dy3+) results in a change to the phase I form. The conductivity increases with dopant level. The EXAFS results, which probe the immediate local environment of the dopant ion, show that the nearest neighbor shells for the two structures are almost identical and only subtle differences can be seen in the second nearest neighbor shells. This is consistent with the concept that it is the number rather than nature of the dopant ion that influences the phase stabilization process. The EXAFS studies also showed the absence of dysprosium clustering. These studies confirm the complementary nature of the Raman and EXAFS techniques, the former probing the host structure and the latter addressing the local dopant ion environment.
Absorption, fluorescence, and Raman spectra of mass-selected rhenium dimers in argon matrices
Authors: Hu Zhendong , Dong Jian-Guo , Lombardi John R. , Lindsay D. M. , Harbich W.
Published: July 2002

We report absorption, laser fluorescence, and Raman spectra for Re2 in an argon matrix prepared by the mass-selected ion deposition technique. The dirhenium absorption spectrum consists of seven band systems (A–G) extending from the near infrared into the ultraviolet region. For the A system (a simple vibrational progression), we find T0=10 817(1) cm−1, ωe=317.1(5) cm−1 and ωexe=1.0(1) cm−1. A Franck–Condon analysis of the A system intensities predicts that this state has a smaller equilibrium internuclear distance than the ground state (Δre=−0.073 Å), in violation of Badger’s rule. The B system starts at 13 250 cm−1 and consists of four overlapping (and possibly perturbed) subsystems, whose average vibrational spacing is 270(11) cm−1. The C, D, E, and F systems (vibrational spacings in parentheses) are centered at 22 300 cm−1 (210 cm−1), 24 500 cm−1 (195 cm−1), 29 150 cm−1 (175 cm−1), and 32 900 cm−1 (160 cm−1), respectively. Weak fluorescence spectra, obtained upon laser excitation into the A system, were characterized by vibrational progressions to the dimer ground (X) state and to a low lying (X′) state for which T0=357.6(5) cm−1 and ωe=332.3(2) cm−1. Raman and fluorescence progressions to the ground state were observed when the B system was excited. These data give ωe=337.9(49) cm−1 for the dimer ground state in good agreement with measurements from photodetachment spectra [J. Am. Chem. Soc. 108, 178 (1986)]. We propose likely assignments for the low lying electronic states of Re2 and discuss our results in terms of the bonding in the other group VIIB dimers, Mn2 and Tc2.
Grid method for the Wigner functions. Application to the van der Waals system Ar–H2O
Authors: Leforestier Claude
Published: July 2002

We present a method to switch back and forth between a basis set of Wigner functions and an associated three-dimensional grid of Euler angles. The grid-spectral transformation is not one to one as more grid points are used than Wigner functions, and thus departs from the Fourier method of Kosloff or the discrete variable representation method of Light and collaborators, but this extra number of grid points allows one to achieve a numerically exact integration of all the potential matrix elements in the Wigner basis set. As an example, we apply this method to the determination of the bound states of the H2O–Ar van der Waals system, already studied by Cohen and Saykally [J. Chem. Phys. 98, 6007 (1993)]. The calculation consists of coupling a Lanczos scheme with a split representation of the Hamiltonian. The iterative scheme is formulated entirely within the spectral representation in which the kinetic energy operator terms are analytic, the potential term being evaluated in the grid representation. Using the rigid rotor approximation for H2O all the J=0 bound states are obtained in a few seconds of computation time on a workstation.
Response to ‘‘Comment on ‘A multiconfiguration time-dependent Hartree approximation based on natural single-particle states’ ’’ [J. Chem. Phys. 101, 2652 (1994)]
Authors: Jansen A. P. J.
Published: July 2002

The relation between the MCTDH equations of motion derived from a variational principle and derived from the defining equations of natural single-particle states is clarified.
Shape asymmetry of star-branched random walks with many arms
Authors: Zifferer Gerhard
Published: July 2002

Star-branched (off-lattice) random walks with up to F=96 arms and a total chain-length of up to 15 361 segments have been produced by means of Monte Carlo simulation. Several quantities that characterize the shape asymmetry of molecules have been calculated. By suitable regression long-chain limits of these quantities were obtained and analyzed with respect to the number of arms. Shape factors as well as asphericities are described very well by polynomials in F−1 or F−1/2, respectively.
Three-dimensional quantum mechanical rate constants for the reaction O+O3→2O2, employing a six-dimensional potential energy surface
Authors: Szichman H. , Varandas A. J. C. , Baer M.
Published: July 2002

In this work is reported the first quantum mechanical treatment of the atmospheric atom–triatom O+O3→2O2 reaction. Temperature-dependent rate constants were calculated and were found to be in good agreement with the experiment.
Semiclassical behavior at a quantum avoided crossing
Authors: Joyeux Marc
Published: July 2002

For a polynomial potential with resonant fundamental frequencies (1:2 and 1:3 resonances), quantum avoided crossings can occur when quantum eigenvalues are plotted versus a parameter in the Hamiltonian. In the present paper, primitive (EBK) semiclassical behavior at the quantum avoided crossing is reinvestigated, using the exact analytical calculation of the action integrals, which was devised recently [Chem. Phys. 185, 263 (1994)] for an approximate resonance Hamiltonian that can be deduced from the exact polynomial Hamiltonian by low order perturbation theory. The previously reported behavior, that is semiclassical levels passing through the intersection instead of avoiding each other, is shown to happen if there exist two superimposed branches in the plot of the second action integral ℐ2 as a function of the energy. These results are interpreted in terms of semiclassical diabatic basis and of quantum dynamical tunneling. In contrast, if the semiclassical system enters the (anti)crossing region with semiclassical quantum numbers ℐ2 which do not lie on superimposed branches of the plot, it is shown that at least one, and possibly two, level(s) must cross the separatrix, that is pass from the inside to the outside of the resonance region (or conversely) in order to adapt to the quantum avoided crossing. This causes (i) corresponding semiclassical quantum number ℐ2 to change (ii) the close correspondence between quantum and semiclassical mechanics to break down.
Mixed quantum-classical calculations on the nonthermal desorption of physisorbed CO
Authors: Dzegilenko Fedor , Herbst Eric , Uzer Turgay
Published: July 2002

A combined quantum-classical approach has been used to study the nonthermal desorption of CO from a variety of model surfaces to which it is weakly adsorbed. Three degrees of freedom associated with the CO adsorbate (bond stretching, physisorption, libration) are treated quantum mechanically, while classical mechanics is applied to the lattice degrees of freedom, which have been included using the generalized Langevin approximation. Two sets of equations for the quantum and classical subsystems (coupled via the Ehrenfest theorem) are solved self-consistently using the discrete variable representation method for the propagation of the quantum wave function. Nonthermal amounts of energy have been put into both the CO stretching and physisorption-librational modes at time t=0. We find that for initial values of the stretching quantum number nr=0–4, desorption does not take place at all within 22.5 ps unless there is also significant librational excitation. The role of the surface is also explored; we find that the probability of desorption is a nonmonotonic function of the Debye frequency of the solid in the range 28–5000 cm−1, and is larger for ‘‘nonrigid’’ lattices with low Debye frequencies. Two different mechanisms for desorption (due to lattice effects and due to symmetry properties of the wave function) have been found and analyzed in detail.
Photoelectron spectroscopy of alkali metal tetramer anions: The anomalous spectrum of Li−4
Authors: Sarkas H. W. , Arnold S. T. , Hendricks J. H. , Bowen K. H.
Published: July 2002

We present the photoelectron spectrum of Li−4. This spectrum displays a spectral pattern that is strikingly different from that of the other alkali tetramer anions. Using the photoelectron spectrum of Li−4 along with our previously measured photoelectron spectra of Na−4, K−4, and Rb−4 plus other existing evidence, we find that Li−4 does not have a linear geometry, as do the tetramer anions of sodium, potassium, and rubidium. This observation indicates that for both anions and neutrals, lithium clusters appear to take on higher dimensional structures at smaller sizes than do sodium and probably other alkali clusters. By examining the clues found in its photoelectron spectrum, we then speculate as to what the structure of Li−4 may be and also summarize the present state of theoretical progress on this problem.
Vibrational line shapes of solvated molecules with a normal mode approach
Authors: Schvaneveldt Stephen J. , Loring Roger F.
Published: July 2002

We develop a theory of the vibrational absorption line shape of a solvated molecule. This approach is based on the instantaneous normal mode approximation, in which the fluid is taken to evolve on a harmonic potential surface whose curvature matches that of the true potential surface at the fluid’s initial configuration. We apply this method to the vibrational line shape of a harmonic diatomic molecule dissolved in an atomic solvent. The line shape is related to a configurationally averaged phonon Green’s function. A diagrammatic analysis of this Green’s function is shown to lead to a self-consistent approximation to the line shape. The only inputs to this calculation from other theory or simulation are the pair correlation functions for two solvent particles and for a solute atom and a solvent particle. The resulting spectra are compared with calculations for a similar model by Berne et al., based on the generalized Langevin equation [J. Chem. Phys. 93, 5084 (1990)].
Spin-adapted open-shell state-selective coupled cluster approach and doublet stability of its Hartree–Fock reference
Authors: Li Xiangzhu , Paldus Josef
Published: July 2002

The performance of the unitary group based state-selective coupled cluster approach, in both linear and quadratic approximations, is examined for the case of doublet ground states of the first two alkaline earth hydrides in the whole range of internuclear separation. It is shown that the doublet instability of the restricted open-shell Hartree–Fock wave functions is responsible for the singular behavior of the linear coupled cluster potential energy curves, as well as for slight discontinuity in these curves when the bilinear terms are included. The effect of using different types of orbitals is investigated and the results are compared with full or very precise limited configuration interaction results as well as with the coupled cluster results employing the unrestricted Hartree–Fock reference, which is free of the instability problems in open shell systems.
Multiconfigurational spin tensor electron propagator vertical ionization potentials for O2: Comparison to some other forefront methods using the same basis sets and geometries
Authors: Heryadi Dodi , Yeager Danny L. , Golab Joseph T. , Nichols Jeffrey A.
Published: July 2002

In a recent paper in The Journal of Chemical Physics, we showed the potential energy curves for several cation states of O2 obtained using the multiconfigurational spin tensor electron propagator method (MCSTEP) with a 〈5s4p3d〉 basis set. In this communication we present vertical ionization potential calculations to the same O2 states. However, for the results reported here, exactly the same basis sets and geometries are used that were used for two other forefront methods; the coupled-cluster reference electron propagator theory (CC-EPT) and the Fock space multireference coupled-cluster method (FSMRCC). Hence, more direct comparisons and contrasts among these methods are now available.
Final state-selected spectra in unimolecular reactions: A transition-state-based random matrix model for overlapping resonances
Authors: Peskin Uri , Miller William H. , Reisler Hanna
Published: July 2002

Final state-selected spectra in unimolecular decomposition are obtained by a random matrix version of Feshbach’s optical model. The number of final states which are independently coupled to the molecular quasibound states is identified with the number of states at the dividing surface of transition state theory (TST). The coupling of the transition state to the molecular complex is modeled via a universal random matrix effective Hamiltonian which is characterized by its resonance eigenstates and provides the correct average unimolecular decay rate. The transition from nonoverlapping resonances which are associated with isolated Lorentzian spectral peaks, to overlapping resonances, associated with more complex spectra, is characterized in terms of deviations from a χ2-like distribution of the resonance widths and the approach to a random phase-distribution of the resonance scattering amplitudes. The evolution of the system from a tight transition state to reaction products is treated explicitly as a scattering process where specific dynamics can be incorporated. Comparisons with recently measured final state-selected spectra and rotational distributions for the unimolecular reaction of NO2 show that the present model provides a useful new approach for understanding and interpreting experimental results which are dominated by overlapping resonances.
Unitary group based state-selective coupled-cluster method: Comparison of the first order interacting space and the full single and double excitation space approximations
Authors: Li Xiangzhu , Paldus Josef
Published: July 2002

A unitary group based state selective coupled-cluster method, truncated at the first order interacting space and at the full single and double excitation space levels, has been applied to examine a complete dissociation (atomization) of the OH, NH2, CH3, and CN radicals. These molecules were chosen to model various bond breaking processes, including the dissociation of one, two, and three single bonds as well as of a multiple (triple) bond. In all cases the coupled-cluster expansion employing the full single and double excitation space represents an improvement over the interacting space approximation. Although this improvement is more significant for geometries involving stretched bonds than for the equilibrium ones, it is found to be rather small in all cases that were examined.
A combined density functional theoretical and photoelectron spectroscopic study of Ge2O2
Authors: Nicholas John B. , Fan Jiawen , Wu Hongbin , Colson Steve D. , Wang Lai-Sheng
Published: July 2002

We study Ge2O2 and Ge2O−2 with density functional theory (DFT) and photoelectron spectroscopy (PES). We find that Ge2O2 is a rhombus (D2h), closed-shell molecule with a large HOMO-LUMO gap. The PES spectrum of Ge2O−2 is obtained at four detachment photon energies: 1064, 532, 355, and 266 nm. Vibrational structure is resolved at the lowest photon energy with a single progression and a frequency of 400 (60) cm−1. The experimental adiabatic electron affinity is obtained to be 0.625 (0.050) eV for Ge2O2. The calculated vertical and adiabatic electron affinities and the HOMO-LUMO gap are in good agreement with the experimental values. The calculated totally symmetric vibrational mode of Ge2O2 (335 cm−1) is in reasonable agreement with the observed vibration and represents a Ge–Ge breathing motion.
Quantum tunneling in an anharmonic classical bath. Enhanced kinetic isotope effects in an Arrhenius region
Authors: Dakhnovskii Yuri , Bursulaya Badry , Kim Hyung J.
Published: July 2002

Quantum tunneling reactions in a general classical bath are studied. By invoking the semiclassical approximation, a general theoretical framework for an arbitrary quantum double-well reactive system, coupled to the anharmonic classical modes is developed in a strong tunneling regime. For illustration, a simple two-dimensional model proton transfer system in solution at room temperature is considered. It is found that the bath anharmonicity strongly modulates the overall rate constant and kinetic isotope effect. For the positive anharmonicity, the reaction rate decreases compared to the harmonic case, while the associated kinetic isotope effect increases. By contrast, the negative anharmonicity reduces the kinetic isotope effect, while it enhances the overall rate. The temperature dependence of the rate constant and kinetic isotope effect is also analyzed. Despite tunneling (k(H)/k(D)=10–40), the Arrhenius behavior for the rate constant is found, regardless of the bath anharmonicity. This clearly indicates a strong interplay between the quantum and classical modes of the system.
Kinetic energy analysis of (HI)n cluster photofragments
Authors: Young Mark A.
Published: July 2002

We have employed a time-of-flight analysis to determine the velocity and spatial distribution of photofragments resulting from excitation of (HI)n cluster species at approximately 240 nm. The formation of clusters in the supersonic expansion broadens the I+ flight-time distribution and destroys the spatial anisotropy characteristic of HI monomer photodissociation, indicating that these fragments experience strong solvent cage effects when exiting the cluster environment. In addition, a high velocity component with an anisotropic distribution appears and is due to the photodissociation of I2, a product of cluster chemistry. Even with extensive clustering, the main features of the H+ flight-time spectrum do not change and cage effects are not as pervasive as for the I atom fragments. However, there is also a broad H+ component peaked near zero velocity and exhibiting an isotropic spatial distribution. Inelastic H atom collisions involving excitation of internal modes in HI cage species likely play a role in dissipating the excess photolysis energy and producing the low velocity distribution. Other pathways involving the reactive nature of H+HI collisions may be more important. Wavelength resolved spectra of the I+ flight-time features provide further insight into the origins of the observed behavior.
Dynamical angular momentum models for rotational transfer in polyatomic molecules
Authors: AlWahabi Zeyad T. , Besley Nicholas A. , McCaffery Anthony J. , Osborne Mark A. , Rawi Zaid
Published: July 2002

We propose a model for collision-induced rotational transfer (RT) in polyatomic molecules based on the angular momentum (AM) sphere, a classical representation of the dynamical motion of the rotational AM vector in the molecular frame. The model develops further that proposed by us [AlWahabi et al., J. Chem. Soc., Faraday Trans. 85, 1003 (1989)] in which RT probabilities are related to the AM gap linking initial and final Nkakc states. The AM sphere representation embodies the full internal motion of the molecule via its effect on the inertial axes and the trajectory of the individual rotational state vectors. In this representation there is no unique AM gap for a particular transition between states of nominally well-defined Nkakc and here we propose and test several models for obtaining the distance in AM space between initial and final trajectories. Models are evaluated from their ability to fit data on NH2–H collisions. We find that even the simplest approximations, such as shortest distance in AM space, give good fits to data sets but the best fits are obtained when both AM trajectory and molecular geometry are averaged over.
The energy-dependent transmission coefficient and the energy distribution of classical particles escaping from a metastable well
Authors: Bader Joel S. , Berne B. J.
Published: July 2002

We investigate the distribution of energies of thermally activated particles escaping from a metastable well. This energy distribution is connected by detailed balance to the energy-dependent transmission coefficient, the probability that a particle injected into a well will stick. Theoretical expressions for the energy-dependent transmission coefficient show good agreement with simulation results for a one-dimensional reaction coordinate coupled to a frictional bath. Slight deviations from theoretical predictions based on turnover theory [E. Pollak, H. Grabert, and P. Hänggi, J. Chem. Phys. 91, 4073 (1989)] are understood in light of the assumptions of turnover theory. Furthermore, the theoretical expressions for energy distributions also provide good fits for fully three-dimensional simulations of sticking and desorption of Ar and Xe on Pt(111) [J. C. Tully, Surf. Sci. 111, 461 (1981)]. Finally, we compare the theoretical efficiencies of several reactive flux sampling schemes, including a scheme designed to be optimal.
Extraction of dynamics from the resonance structure of HeH2+ spectra
Authors: Mandelshtam V. A. , Taylor H. S. , Jung Christof , Bowen H. F. , Kouri Donald J.
Published: July 2002

For the reaction of He with H2+, starting with accurate theoretically computed reactive, elastic, and inelastic scattering data that reveals many complex unassignable narrow resonances, the detailed motions governing the dynamics of the tight transition state are extracted. Methods ranging from scattering theory, the stabilization theory of dynamics, nonlinear dynamic periodic orbit theory, and hierarchical smoothing theory which was developed to study complex ‘‘chaotic’’ spectra, are all used in the analysis. Relationships between the doorway model of nuclear physics, aspects of transition state theory, and models of nonlinear chaotic dynamics are pointed out and used to shed light on the fact that the complex resonance structure observed is one quantum manifestation of classical transient chaos in scattering processes. The transition (or doorway) state corresponds to the only populous and robust periodic orbit or set of similar periodic orbits whose motion allows the types of energy transfers necessary to go from reactants to products. Wave packet motion and quantum eigenfunctions are influenced by these periodic orbits.
Jet-cooled fluorescence excitation spectrum, carbonyl wagging, and ring-puckering potential energy functions of 3-cyclopenten-1-one in its S1(n,π*) electronic excited state
Authors: Sagear Paul , Laane Jaan
Published: July 2002

The jet-cooled fluorescence excitation spectrum of 3-cyclopenten-1-one has been recorded in the 308–330 nm region, and the electronic origin for the S1(n,π*) state of A2 symmetry was observed at 30 229 cm−1. The observed spectrum consists of more than 80 bands involving primarily ν3 (carbonyl stretch), ν29 (carbonyl out-of-plane wagging), and ν30 (ring puckering). Bands were also assigned to combinations with seven other vibrational modes. The energies for the v=0 to 11 quantum states of ν29 were measured and used to determine a one-dimensional potential energy function. This function has energy minima at wagging angles of ±24° and a barrier to inversion of 939 cm−1. Four bands associated with ν30 were observed and were used to determine an asymmetric single-minimum one-dimensional ring-puckering potential energy function for the S1(n,π*) state. The ring-puckering energy levels in the ν29 vibrational excited states are little changed from the v=0 state indicating that there is little interaction between the carbonyl wagging and the ring-puckering motions.
On the analysis of the effective adhesion in colloidal dispersions in the sticky hard sphere model
Authors: Regnaut C. , Amokrane S. , Heno Y.
Published: July 2002

Structural trends in multicomponent mixtures of adhesive spheres are analyzed by using the Baxter formalism and the Percus–Yevick approximation (PYA). The Orsnstein–Zernike (OZ) equations in q space are cast in a form which allows a fully analytical expression of the effective adhesiveness coefficient of the large (solute) spheres in the asymptotic limit of vanishing size ratio and no solute self-stickiness. This allows a simple discussion of the factors which determine the effective solute adhesiveness and suspension stability: while the steric effect and like particles stickiness are found to favor suspension instability, hetero stickiness is found to act in one side or in the opposite depending on the concentration of the smaller species. These qualitative predictions are paralleled with studies on solvent effects in ordinary colloidal solute–solvent systems and with the behavior of pseudobinary systems such as colloid–polymer or bidisperse colloidal mixtures. Results from the literature for hard sphere mixtures and calculations in the PYA including solute self-stickiness at nonzero size ratio are finally used to discuss the reliability of the trends deduced from the asymptotic limit.
Coulombic fission and evaporation of antimony cluster ions
Authors: Bréchignac C. , Cahuzac Ph. , Carlier F. , de Frutos M. , Leygnier J. , Roux J. Ph.
Published: July 2002

The preferential dissociation channels of singly and doubly charged antimony clusters have been determined from the unimolecular dissociation of energy rich cluster ions, using an ion stopping technique. It is found that singly charged Sb+n clusters with 5≤n≤80 dissociate by loss of neutral molecules. Neutral dimer loss is observed for n=5, 6, 7 whereas for n≥8, Sb+n loses Sb4. The fragmentation of doubly charged Sb++n has been investigated above the critical size n++c=24 from which doubly charged clusters are detectable in mass spectra. On the time scale of the experiment, which is 1 μs≤t≤100 μs with respect to photoionization, the delayed Coulombic fission into two singly charged clusters competes with the evaporation of Sb4. It is shown that for the smaller Sb++n clusters with 26≤n≤36 the fission to two singly charged clusters is of relatively asymmetrical character, leading to the detachment of five and seven atom cationic fragments. In larger clusters n≥40 the fission is of more symmetrical character and the difference in the fission products is less than 30%. Such a behavior totally deviates from the drop model predictions.
Electronic structure of pristine and sodium-doped cyano-substituted poly(2,5-dihexyloxy-p-phenylenevinylene): A combined experimental and theoretical study
Authors: Fahlman M. , Bröms P. , dos Santos D. A. , Moratti S. C. , Johansson N. , Xing K. , Friend R. H. , Holmes A. B. , Brédas J. L. , Salaneck W. R.
Published: July 2002

The electronic structure of cyano-substituted poly(2,5-dihexyloxy-p-phenylene-vinylene), or CN-PPV, has been studied in both pristine and doped states. Ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS), as well as optical absorption spectroscopy have been carried out under ultrahigh vacuum (UHV) conditions, and the results have been interpreted with the help of quantum-chemical calculations. For the pristine polymer, the addition of cyano groups to the vinylene units does not affect the width of the π-bands closest to the Fermi level; however, the positions of the flat parts of the upper π-bands are shifted by approximately 0.4 eV towards higher binding energies relative to the Fermi energy, as compared with the corresponding bands of other alkoxy-substituted poly(p-phenylenevinylene)s. On the other hand, there are only marginal differences in the optical absorption spectra; the interband absorption onset is comparable to the values for alkoxy-substituted poly(p-phenylenevinylene)s. In the case of sodium doping, it is found experimentally that at saturation doping, there is about one sodium ion per phenylene vinylene unit; in that situation, two new states appear in the previously forbidden energy bandgap, which are consistent with the formation of bipolaron bands. These results are similar to those obtained for sodium-doping of poly(p-phenylenevinylene) (PPV). The peak-to-peak splitting of the bipolaron peaks in CN-PPV is 1.05 eV, compared with about 2.0 eV for sodium-doped PPV at saturation doping; this difference is related to the pinning of some of the transferred charges to the cyano vinylene groups and the phenylene rings that they are conjugated to in CN-PPV, causing a stronger confinement of the bipolaron charge carriers.
Femtosecond wave-packet dynamics studied by time-resolved zero-kinetic energy photoelectron spectroscopy
Authors: Fischer Ingo , Villeneuve D. M. , Vrakking Marc J. J. , Stolow Albert
Published: July 2002

Femtosecond pump–probe zero-kinetic-energy (ZEKE) photoelectron spectroscopy is studied using the known wave-packet dynamics of I2 (B state). The 340 fs wave-packet period, wave-packet dephasing and rephasing are observed in the ZEKE signal. The effect of various laser and ZEKE parameters on the wave-packet dynamics is discussed.
Classical trajectory studies of the reaction CH4+H→CH3+H2
Authors: Jordan Meredith J. T. , Gilbert Robert G.
Published: July 2002

Trajectory data are reported for the reaction CH4+H→CH3+H2, designed to provide information that can be used to test approximate quantitative theories for the dynamics of abstraction reactions. A potential function was devised which properly reflects the nuclear permutation symmetry of the process. Microscopic reaction rate coefficients were obtained as functions of fixed rotational and vibrational energy, and of the angular momentum. The data indicated significant uncoupling between the various modes although, at a minimum, the symmetric stretch is directly coupled to the reaction coordinate at the transition state. The data were used to test the assumption that the total angular momentum, J, may be approximated by the orbital angular momentum, L. L is approximately conserved from the reactant to the saddle point configuration in reactive and nonreactive collisions and may be well approximated by J. The angular momentum about the long axis of the reacting system (equivalent to the K quantum number) is not conserved in either reactive or nonreactive trajectories.
Prediction of gaseous diffusion coefficients from thermal diffusion measurements and other experimental data
Authors: Dunlop Peter J. , Bignell C. M.
Published: July 2002

Expressions, originally presented by Humphreys and Mason, are extended to indicate that accurate binary diffusion coefficients, 𝒟12, may be predicted over a large range of temperatures from one value of 𝒟12 and a value of the thermal diffusion factor, αT, at the same temperature. For those systems which have thermal diffusion factors which are relatively insensitive to temperature, the predicted diffusion coefficients are very accurate for a range of temperatures much greater than that used for the thermal diffusion measurements.
Photodissociative excitation processes of XeF2 in the vacuum ultraviolet region 105–180 nm
Authors: Kono Mitsuhiko , Shobatake Kosuke
Published: July 2002

Photodissociative excitation processes of XeF2 are studied by absorption and photofragment fluorescence spectroscopies in the vacuum UV region 105–180 nm. The quantum yields are measured along with the dispersed fluorescence spectra for emissions from XeF* (B, C, and D) excimers formed from XeF2 excited by linearly polarized, monochromatized synchrotron radiation. Fluorescence anisotropy, R, of XeF* fragment emission was also measured against excitation light wavelength in the region 105–165 nm. The quantum yields for XeF* excimer formation are found to be very high; especially in the valence band region shorter than 130 nm it reaches almost unity. From the fluorescence measurements we have observed new peaks assigned to 5πu→5dR Rydberg transitions. The intense 5πu3/2→6sR bands which appear around 143 nm exhibit no fluorescence excitation bands; even fluorescence dips are observed at 143.18 and 144.25 nm whereas very weak positive fluorescence bands are observed at 141.04 and 142.11 nm. The R values for the very broad valence bands which underlie intense Rydberg bands are found to be high. The 5πu series of Rydberg bands exhibit low R values. Based on the energetics and the results of quantum yields and fluorescence anisotropy values, the valence and Rydberg bands observed are assigned from the viewpoint of symmetry of the excited state, and the dynamics of the dissociation processes is discussed.
Thermal electron attachment to SO3
Authors: Miller Thomas M. , Viggiano A. A. , Arnold Susan T. , Jayne John T.
Published: July 2002

The rate constant for electron attachment to SO3 is 3±1×10−9 cm3 s−1 at 300 K, measured in helium gas at pressures from 53 to 160 Pa (0.4 to 1.2 torr). The sole product of attachment is SO−3 under these conditions. The same rate constant and ionic product were obtained at 400 and 505 K. The measurements were carried out using a flowing-afterglow Langmuir-probe apparatus.