bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational relaxation study of O3 in rare gas and nitrogen matrices by time resolved infrared–infrared double resonance spectroscopy
Authors: Dahoo P. R. , Jasmin D. , Brosset P. , Gauthier-Roy B. , Abouaf-Marguin L.
Published: July 2002

A time resolved infrared–infrared double resonance technique is used to study the vibrational relaxation of O3 in rare gas and nitrogen matrices. A tunable infrared (IR) pulsed source excites the ν1+ν3 level of O3 in the ground electronic state. A continuous wave (cw) CO2 laser probes the populations of the fundamental and v2=1 levels as a function of time. After minimization of thermal effects, the relaxation signal can be analyzed. At fixed probe frequency, the behavior of the rise time of the signals with the pump frequency shows spectral diffusion to occur inside the inhomogeneous profiles. At high concentration in argon (O3/Ar=1/250), intermolecular energy transfer is observed between the two sites. In xenon matrices, it has time to take place at concentrations 1/2000. The relaxation rates of the v2=1 level to the ground state are measured at different concentrations in rare gas and nitrogen matrices. At high dilution, a maximum relaxation time, called intrinsic relaxation time τi, is determined in the different matrices: it covers three orders of magnitude, from a few hundred nanoseconds in neon to 320 microseconds in xenon. The results are discussed and compared with literature data within the frame of the isolated binary collision model.
Metal ammonia solutions: A lattice model approach
Authors: Leung Kevin , Csajka Félix S.
Published: July 2002

A generalized Falicov–Kimball model is applied to study the phase diagram of metal ammonia solutions. The model includes a fluid–fluid interaction term and an electron–fluid interaction with a hard core and an attractive tail. Mean-field theories are derived for the localized and delocalized electron phases using ideas from the slave-boson approach to the Hubbard model. The attractive force stabilizes both the homogeneous delocalized electron phase and a regime where the electrons localize in cavities devoid of fluid particles. In its absence, the localized states are found to be unstable towards phase separation, in contrast to previous predictions. The Gaussian fluctuations and structure factors derived from this theory suggest the existence of bicontinuous channels percolating through the system. These findings agree qualitatively with experiments and computer simulations.
Hydrodynamic effects in three-dimensional microphase separation of block copolymers: Dynamic mean-field density functional approach
Authors: Maurits N. M. , Zvelindovsky A. V. , Sevink G. J. A. , van Vlimmeren B. A. C. , Fraaije J. G. E. M.
Published: July 2002

The dynamic mean-field density functional method is used to describe phase separation including hydrodynamic effects in specific three-dimensional (3D) compressible copolymer liquids. We show that it is justified to use Darcy’s approximation for the velocity field. This simple model enables us to reproduce both the increased domain growth and the faster removal of defects that are a result of hydrodynamics. We perform a simulation of a diblock copolymer melt to study the viscous effects in 3D. The free energy is shown to decrease twice as fast as in a simulation which only includes diffusion. The hydrodynamics are shown to become important only in the later stages of phase separation as is also predicted from theoretical analysis. The separation process proceeds faster because the growth of bulk domains is accelerated compared to the purely diffusive case.
The mean spherical model for a Lorentz-Berthelot mixture of sticky hard spheres
Authors: Tutschka Christian , Kahl Gerhard
Published: July 2002

We have analyzed the Percus-Yevick (PY) and the mean spherical model (MSM) equation for an N-component system of sticky hard spheres. The PY approximation leads to a set of N(N+1)/2 coupled quadratic equations for the unknown coefficients. While for this closure, the pair distribution functions have to be calculated numerically, we can proceed in the MSM one step further if we assume a Lorentz-Berthelot-type rule for the interactions: then the structure functions can be calculated analytically. We show that under these conditions in the limit N→∞ (stochastic limit) the analyticity of the solution is preserved. General expressions both for the discrete and continuous (polydisperse) case are presented.
Contributions of the two conformers to the microwave spectrum and scattering cross-section of the He–Cl2 van der Waals system, evaluated from an ab initio potential energy surface
Authors: Naumkin F. Y. , McCourt F. R. W.
Published: July 2002

A high-level ab initio PES has been calculated for the ground state He–Cl2 complex. The existence and relative depths of the two wells, one for each of the linear (L-well) and T-shaped (T-well) geometries, are shown to be correctly predicted with a simple atom-atom model using ab initio HeCl potentials of equivalent accuracy. Distortions of the He–Cl interaction due to perturbations of the electronic structure of the Cl atoms by strong intramolecular forces in Cl2 are suggested to be responsible for the remaining underestimation of the binding energy. Even though it has a deeper well, the linear configuration is less stable than the T-shaped configuration when zero-point vibrations are taken into account. Although the lowest rovibrational levels of each conformer lie above the potential barrier separating the two wells, the associated wavefunctions reflect the presence of the other well only weakly, as indicated by the low local maxima in the probability densities at the other well positions. The presence of the L-well is found to affect the order of the rotational levels associated with the T-well. The microwave spectra originating from each of the two conformers are simulated, and suggest a possibility for direct confirmation of the existence of the L-well. The positions of spectral lines associated with the T-shaped conformer are found to be quite sensitive to the binding energy for the linear conformer. The total differential cross section is found to be affected significantly by the L-well position in the PES, while the influence of both the well depths appears to be rather weak. The additional anisotropy of the He–Cl2 interaction, introduced with the L-well, gives rise to additional oscillations in the cross section behavior as a function of the scattering angle.
Hot-atom mechanism in photodesorption of molecular oxygen from a stepped platinum (113) surface
Authors: Sano M. , Ohno Y. , Yamanaka T. , Matsushima T. , Quinay E. B. , Jacobi K.
Published: July 2002

The photodesorption of oxygen admolecules was studied on a stepped Pt(113)=(s)2(111)×(001) surface with 193 nm irradiation at 110 K. Multidirectional desorptions were found to collimate at ±12–20° and ±45–49° off the surface normal and also along the surface normal in a plane along the trough. The first component is always dominant, and the weak second component only appears at higher oxygen coverages. The normally directed desorption is not significant. The translational energy of desorbing O2 peaks around 15–20° and 50°, confirming the inclined desorptions. It is proposed that these inclined components are due to the desorption induced by the impact of oxygen admolecules with hot oxygen atoms from the photodissociation of adsorbed molecular oxygen, emitted along the trough. A simple cosine distribution was found to fit the thermal desorption from oxygen admolecules and also the recombinative desorption of oxygen adatoms. The 193 nm irradiation also produces additional, less tightly bound oxygen adatoms, which yield a desorption component collimated at 15° from the surface normal in the step-down direction.
Interaction of gaseous D atoms with CH3I adsorbed on Pt(111), H/Pt(111), and C/Pt(111) surfaces: From hot-atom to Eley–Rideal phenomenology
Authors: Wehner S. , Küppers J.
Published: July 2002

The interaction of gaseous D atoms with methyl iodide molecules adsorbed on Pt(111), hydrogen saturated Pt(111), and graphite monolayer covered Pt(111) surfaces was studied. Direct product rate measurements were employed to determine the reaction kinetics. On all substrates, incoming D atoms abstract the methyl group from adsorbed CH3I via gaseous CH3D formation. In the monolayer regime of CH3I/Pt(111) pure hot-atom phenomenology was observed in the rates. With multilayers as targets, the fluence dependence of the rates get Eley–Rideal-type. With a coadsorbed H monolayer present, the CH3D rates at a CH3I monolayer on Pt(111) are affected by the suppression of hot-atom sticking. Accordingly, the rate curves exhibit similar features as expected for Eley–Rideal phenomenology. However, CH4 as a product and simultaneous abstraction of adsorbed H via gaseous HD and H2 formation clearly demonstrate that hot-atom reactions occur. With CH3I adsorbed on a graphite monolayer on Pt(111), the abstraction kinetics of methyl was found to agree with the operation of an Eley–Rideal mechanism. This observation is in line with the expectation that hot atoms do not exist on a C/Pt(111) surface.
Relativistic all-electron ab initio calculations of CsHg potential energy curves including spin-orbit effects
Authors: Polly Robert , Gruber Dieter , Windholz Laurentius , Gleichmann Matthias M. , Heß Bernd A.
Published: July 2002

We report a relativistic all electron ab initio calculation of the ground and excited state potential energy curves of the CsHg molecule along with a determination of the spectroscopic parameters. Spin-orbit and kinematical relativistic effects were taken into account using the Douglas–Kroll-transformed no-pair Hamiltonian.
Photofragmentation spectra and structures of Sr+Arn, n=2–8 clusters: Experiment and theory
Authors: Fanourgakis G. S. , Farantos S. C. , Lüder Ch. , Velegrakis M. , Xantheas S. S.
Published: July 2002

The total photofragmentation cross sections of size selected Sr+Arn,n=2–8, clusters have been recorded in a time of flight (TOF) mass spectrometer. In the energy range of 21 000–27 000 cm−1 three absorption bands are observed that are attributed to transitions from the ground to the three excited states that correlate to the 5p orbitals of strontium. No vibrational structure in the spectral bands is observed, even in regions where the spectrum is recorded with a resolution of 1 cm−1. The absorption spectra are calculated within a semiclassical approximation. Accurate ab initio calculations for the ground X 2∑ and the A 2∏ and B 2∑ excited states of Sr+Ar are combined with a perturbative one-electron model, which includes the spin–orbit coupling, to construct potential energy surfaces for the excited states of all clusters. The theoretical spectra obtained without using any adjustable parameters reproduce the patterns and trends of the experimental spectra, but they are shifted to higher frequencies. A better agreement between theoretical and experimental spectra is obtained by adjusting the depth of the potential wells of the ground X 2∑ and excited A 2∏ states of the Sr+Ar dimer to the experimentally estimated values. From both calculations we conclude that Sr+Ar3 is a trigonal pyramid of C3v symmetry and Sr+Ar6 has a C5v symmetry with five argon atoms forming a regular pentagon, one argon atom below and the cation above the pentagon plane. For these clusters, theory reproduces the characteristic blue shifts found in the absorption spectra and the splittings of the doubly degenerate states encountered in these symmetric complexes.
Theoretical investigations of the lifetime of SH and SD (Ã 2Σ+) in M⋯SH/D (M=Ne,Ar,Kr) complexes
Authors: McCoy Anne B.
Published: July 2002

The lifetimes of SH and SD in the Ã 2Σ+ electronic state are investigated as a function of the intermolecular vibrational states of Ne⋅⋅⋅SH, Ar⋅⋅⋅SH and Kr⋅⋅⋅SH. Model potentials are developed for these systems and the lifetimes, calculated from these potentials, agree well with those reported by Miller, and co-workers [Applegate, Yang and Miller, J. Chem. Phys. 109, 162 (1998)]. The calculated lifetimes are analyzed using an empirical ballistic model that assumes that the rare gas atom effectively blocks electronic predissociation for certain geometries. The results of this model are found to be in good agreement with the more demanding adiabatic calculations of the dynamics and with the experimental lifetimes.
Quenching of metastable states of antiprotonic helium atoms by collisions with H2 molecules
Authors: Ketzer B. , Hartmann F. J. , von Egidy T. , Maierl C. , Pohl R. , Eades J. , Widmann E. , Yamazaki T. , Kumakura M. , Morita N. , Hayano R. S. , Hori M. , Ishikawa T. , Torii H. A. , Sugai I. , Horváth D.
Published: July 2002

Laser resonance transitions between normally metastable states of antiprotonic helium atoms were induced making use of state dependent quenching effects caused by trace admixtures of H2 to the target helium gas. With this method of “H2-assisted inverse resonances” the decay rates of the states (n,l)=(39,l),l=36,37,38, and (38,l),l=35,36,37 of p̄He+ were determined as a function of the H2 admixture. The quenching cross sections at 30 K deduced therefrom for the states with n=39 were found to be of the order of the geometrical cross section for p̄He+–H2 collisons (2⋅10−15 cm2), with a moderate decrease with increasing l. Within a given cascade with constant v=n−l−1, the quenching cross sections for states with n=38 are smaller by a factor of 4–6 than those for states with n=39.
Treatment of core-valence correlation effects through difference-dedicated configuration interaction: Application to the lowest electronic states of K, Rb, KH, RbH, and K2
Authors: Garcı́a V. M. , Caballol R. , Malrieu J. P.
Published: July 2002

The iterative difference dedicated configuration interaction (IDDCI) method, recently proposed for calculating excited electronic states, is tested in systems where the core-valence correlation plays an important role. The calculated ionization potential, electron affinity, and electronic transition energies of K and Rb agree within 0.1 eV with experimental values. The three lowest Σ+ potential curves of KH and RbH and the six lowest states Σg+, Σu+, and Πu of K2 are calculated with the same accuracy.
The He++H2→HeH++H*(2s,2p) reaction: Study of the s and p contributions in a coincidence-correlation experiment
Authors: Dhuicq D. , Jugi B. , Benoit C. , Sidis V.
Published: July 2002

The HeH+ ions produced in the He++H2 reaction at Ecm=20 eV are analyzed in coincidence with the other reaction product: either an H*(2s) metastable atom or an H*(2p) atom. The measurements are restricted to laboratory scattering angles of the HeH+ ions in the range 5°<θ<15° encompassing the maximum of the differential cross section for their production. The H*(2s) atoms are observed at a corresponding fixed angle around −90° relative to the incident He+ direction. The Lα radiation which is the signature of the H*(2p) atom is detected at various angles in the collision plane; this provides polarization (in particular alignment) parameters describing the H*(2p) electron cloud. The HeH+ differential cross section maximum is composed of a few peaks associated with the H*(2p or 2s) species. The H*(2s) to H*(2p) cross section ratio is around 0.1. For the HeH+ peaks at small scattering angles, the Lα radiation patterns obtained for the nascent H*(2p) electron cloud are essentially of dipolar type. The measured alignment angle relative to the final HeH+ center of mass direction lies in the range 67°–81°. A simple model is devised to relate these observations to the H*(n=2) probability amplitudes formed right after the reaction, that is, before post-reaction Stark effect in the field of the receding HeH+ ion together with Coriolis coupling modify these amplitudes. The analysis shows that contribution of the 2p⊥ state lying perpendicular to the axis joining the H* atom to the HeH+ center of charge predominates. This is attributed to a dominant role played by the 2pπu crossing between the potential energy surfaces of the (He–H2)+ compound. It also implies, in agreement with earlier predictions of Dhuicq et al. [Chem. Phys. 206, 139 (1996)] that this compound is bent when going through the important {He++H2}→{He+H2+*}→{HeH++H*} stages of the reaction at the investigated energy. At the largest angles investigated, evidence is seen for the emergence of an additional mechanism attributed to the 2sσg crossing.
Electron attachment to PCl3 and POCl3, 296–552 K
Authors: Miller Thomas M. , Seeley John V. , Knighton W. B. , Meads Roger F. , Viggiano A. A. , Morris Robert A. , Van Doren Jane M. , Gu Jiande , Schaefer Henry F.
Published: July 2002

Rate constants for electron attachment to PCl3 and POCl3 have been measured over the temperature range 296–552 K in 135 Pa of helium gas, using a flowing-afterglow Langmuir-probe apparatus. Electron attachment to PCl3 is dissociative, producing only Cl− ions in this temperature range. The rate constant is 6.4±1.6×10−8 cm3 s−1 at 296 K and increases with temperature in a way that may be described by an activation energy of 43±10 meV. Electron attachment to POCl3 is a richer process in which a nondissociative channel (POCl3−) competes with two dissociative ones (POCl2− and Cl−). The rate constant for electron attachment to POCl3 is 1.8±0.4×10−7 cm3 s−1 at 296 K and is relatively temperature independent in our temperature range. POCl2− is the major product over the entire temperature range. Ab initio MP2 and MP4 calculations have been carried out on ground-state neutral and anionic PCln and POCln for n=1–3. The calculated adiabatic electron affinities agree with experimental estimates where available. The calculations yield C3v structural symmetries for PCl3 and POCl3, and Cs symmetries for PCl3− and POCl3−. The degree of distortion between the respective neutrals and anions is explored in the calculations, and the implications for electron attachment reactions are outlined.
Thermalization after photoexcitation to the S2 state of trans-azobenzene in solution
Authors: Terazima Masahide , Takezaki Makoto , Yamaguchi Shinji , Hirota Noboru
Published: July 2002

A simple method to measure a rate of the translational temperature increase after the nonradiative transition with a few ps time resolution is presented. This method uses an acoustic peak shift of the transient grating signal, which can be determined accurately, and is very sensitive to rates and the relative amount of the thermal energy from the nonradiative transition. Using this method, the thermalization rate after the photoexcitation of trans-azobenzene to the S2(ππ*) state is measured. The acoustic peak shift indicates that the temperature of the solvents rises within less than ∼3 ps after the nonradiative transition of the S2→S1 process. The thermalization after the S1→S0 transition completes faster than the decay of the S1 state (16 ps in acetonitrile and 18 ps in ethanol). The fast energy transfer from the solute to the solvent is discussed.
Phase relations of camphene at high pressure up to 9.5 GPa
Authors: Suito Kaichi , Ohta Akihiro , Sakurai Noriko , Onodera Akifumi , Motoyama Muneyuki , Yamada Kazutoshi
Published: July 2002

Camphene has been studied over ranges of pressure up to 9.5 GPa and temperature up to 1300 °C by in situ resist-volumetric, x-ray diffraction, and Raman scattering measurements as well as by postcompression analyses on retrieved samples. In the solid state, polymerized and amorphous phases appeared. The melting temperature was found to increase with increasing pressure and the melting line showed a small deflection at about 4 GPa, which was attributed to a triple point among the two solid phases and the liquid. Another amorphous phase was retrieved from a region right above the melting line. Heat treatment at temperatures beyond the melting line by about 200 °C and higher yielded turbostratic and graphitic carbons.
Collision induced desorption and dissociation of O2 on Pt(111)
Authors: Åkerlund C. , Zorić I. , Kasemo B.
Published: July 2002

We have studied desorption and dissociation of O2 on Pt(111) induced by incident energetic Xe atoms. The collision induced (CI) desorption rate was measured directly with a quadrupole mass spectrometer whereas the CI dissociation was investigated indirectly in post-Xe exposure TPD. We report the kinetics together with the energy, coverage, Xe incidence angle, and surface temperature dependences of the two processes. The cross sections for the two CI processes are determined via kinetic modeling, based on observed first-order CI desorption kinetics. There is a threshold in Xe kinetic energy at about 1.1 eV, common for both CI desorption and dissociation at normal Xe incidence. The Xe energy dependence of the cross sections for the two processes are well described by σdes=2.46⋅(EXe−1.1) (Å2) and σdiss=0.75⋅(EXe−1.1) (Å2), respectively, above the threshold at normal Xe incidence. A scenario where the O2–Pt bond is mechanically excited by an incident Xe atom, and where energy is transferred to the intramolecular bond via electronic coupling is presented. Implications of the results for the O2/Pt(111) PES are also considered.
Multiple-overtone resonance Raman scattering and fluorescence from I2 species adsorbed on silver surfaces
Authors: Sibbald Morgan S. , Chumanov George , Small Gerald , Cotton Therese M.
Published: July 2002

A detailed excitation profile of a Raman progression consisting of up to six overtones and a fundamental band at 123 cm−1 observed from iodide adsorbed on an electrochemically roughened silver surface at 20 K is analyzed. The excitation profile was constructed from 77 spectra obtained by tuning the laser wavelength in ∼0.25 nm steps through the spectral range 409 nm–433 nm. The shift between resonances in the excitation profile, corresponding to the spacing between vibronic levels in the excited state, is also equal to 123 cm−1 indicating that the ground state and excited state potential energy surfaces have the same shape. Only two distinct resonances spaced three vibrational quanta apart were evident in the profile for each band in the progression. Curve fitting of the Raman band shapes indicates that each overtone is composed of one sharp and one broad band, whereas the fundamental contains only one sharp component. The measured width of the fundamental was less than 2.5 cm−1 FWHM, limited by the instrument function. It is proposed that the sharp Raman bands represent a normal vibrational mode of a surface-adsorbed I2-like species with the width determined by the intrinsic vibrational dephasing in the ground state. On the other hand, the broad Raman bands reflect vibronic coupling between different I2-like species adsorbed on the same Ag cluster. The broad bandwidths result from both dephasing associated with the vibronic coupling and the intrinsic vibrational dephasing. Other weak emission bands are attributed to resonance fluorescence corresponding to direct transitions from higher vibronic levels of the excited state to the ground state. An emission at 429.9 nm is assigned to exciton recombination in small silver iodide clusters which are formed after spontaneous oxidation of the iodide-modified silver surface.
The structure and thermodynamics of energetically and structurally asymmetric polymer blends
Authors: Tillman Patrick A. , Rottach Dana R. , McCoy John D. , Plimpton Steve J. , Curro John G.
Published: July 2002

Simple polymer blends were studied by simulations with the intent of probing the response of structurally asymmetric chains to progressively stronger attractive interactions strengths. It was found, for miscible blends, that the intermolecular pair correlation functions, the g(r)’s, varied by less than 10% from those of the associated blend with strictly repulsive interactions. When used for the purposes of calculating changes in pressure due to the attractive strength, the g(r)’s from either the repulsive or attractive systems could be used interchangeably in the perturbation theory. On the other hand, calculations of the enthalpic χ parameter were sensitive to small variations in the g(r)’s, although, at least for the systems studied here, the associated predictions of the phase boundaries were relatively insensitive.
All the adiabatic bound states of NO2
Authors: Salzgeber R. F. , Mandelshtam V. , Schlier Ch. , Taylor H. S.
Published: July 2002

We calculated all 2967 even and odd bound states of the adiabatic ground state of NO2, using a modification of the ab initio potential energy surface of Leonardi et al. [J. Chem. Phys. 105, 9051 (1996)]. The calculation was performed by harmonic inversion of the Chebyshev correlation function generated by a DVR Hamiltonian in Radau coordinates. The relative error for the computed eigenenergies (measured from the potential minimum), is 10−4 or better, corresponding to an absolute error of less than about 2.5 cm−1. Near the dissociation threshold the average density of states is about 0.2/cm−1 for each symmetry. Statistical analysis of the states shows some interesting structure of the rigidity parameter Δ3 as a function of energy.
New final state in the autoionization decay detected for N2/(2×2)K/graphite: Relevance for the affinity level of NO on a K monolayer
Authors: Puglia C. , Brühwiler P. A. , Hasselström J. , Bennich P. , Nilsson A. , Mårtensson N.
Published: July 2002

We have observed a new type of charge transfer state in the autoionization spectrum for N2/(2×2)K/graphite. It can be viewed as a consequence of an important covalent component to the bonding with the surface in the core hole excited state for N2/(2×2)K/graphite or equivalently for the Z+1 system, “NO”/(2×2)K/graphite.
High-resolution pulsed field ionization photoelectron study of O2: Predissociation lifetimes and high-n Rydberg lifetimes converging to O2+(c 4Σu−, v+=0,1)
Authors: Evans M. , Stimson S. , Ng C. Y. , Hsu C.-W.
Published: July 2002

We have measured the pulsed field ionization photoelectron (PFI-PE) spectrum of O2 in the energy range of 24.53–25.0 eV at a PFI-PE resolution of 11 cm−1 (full width at half maximum, FWHM). The PFI-PE bands for O2+(c 4Σu−, v+=0 and 1) obtained at O2 rotational temperatures of 35 and 298 K have been simulated using the Buckingham–Orr–Sichel model. Only the ΔN=−3, −1, +1, and +3 (or N, P, R, and T) rotational branches are observed, indicating that the outgoing electron continuum channels with angular momenta l=0, 2, and 4 dominate in the threshold ionization transitions O2+(c 4Σu−, v+=0 to 1, N+)←O2(X 3Σg−, v″=0, N″). The simulation yields natural rotational linewidths of 19.6±2.0 and 77±8 cm−1 (FWHM) for the respective v+=0 and 1 PFI-PE bands of the O2+(c 4Σu−) state. These linewidths make possible the determination of the predissociation lifetimes for the v+=0 and 1 levels of O2+(c 4Σu−) to be (2.7±0.3)×10−13 and (6.9±0.7)×10−14 s, respectively. This experiment also provides accurate ionization energies of 24.56227±0.0005 and 24.75445±0.0005 eV for transitions to O2+(c 4Σu−, v+=0, N+=0) and O2+(c 4Σu−, v+=1, N+=0) from O2(X 3Σg−, v″, N″=1), respectively. The rotational constants of 1.58±0.02 and 1.54±0.04 cm−1 obtained here for the O2+(c 4Σu−, v+=0) and O2+(c 4Σu−, v+=1) states allow the calculation of their corresponding equilibrium bond distances to be 1.155±0.011 and 1.170±0.015 Å. The (nominal) effective lifetimes for high-n Rydberg states converging to the O2+(c 4Σu−, v+=0 and 1) states are measured to be ≈0.33 μs, which are significantly shorter than the values of ≈1.9 μs measured for the O2+(b 4Σg−, v+=0–5) states. The shorter (nominal) effective lifetimes for high-n Rydberg states converging to O2+(c 4Σg−, v+=0 and 1) observed are attributed to the higher kinetic energy releases (or velocities) of O+ fragments resulting from predissociation of the O2+ ion cores.
Identifying collective dynamical observables bearing on local features of potential surfaces
Authors: Lazarides A. A. , Rabitz H. , Chang J. , Brown N. J.
Published: July 2002

A singular value decomposition of dynamical sensitivities provides insight into the relationship between a data set and the potential which is often not evident from the sensitivities of individual observables. An illustration is treated consisting of data sets drawn from reactive transition probabilities as a function of energy for the collinear H+H2 system. While the sensitivities of individual reactive transition probabilities to the two-dimensional potential are highly structured functions of the potential coordinates, a set of reactive transition probabilities is identified which collectively has localized sensitivity primarily to the saddle point region and secondarily to the slope along the H3 symmetric stretch line in the outer corner tunneling region and to the width of the barrier. Information of this type garnered from a principal component sensitivity analysis can be especially valuable when attempting to use dynamics data to refine potential surfaces.
Inflection spacing symmetry of diatomic potential curves
Authors: Herrick David R. , O’Connor Sean
Published: July 2002

Molecular ground states are found to have an approximate symmetry related to equally spaced inflection points from djV(R)/dRj=0. Morse, Kratzer-Coulomb, Rydberg, (n+1,n), exp–exp, and cubic-anharmonic potentials turn out to have exact equal spacing of all inflection points out to dissociation. Equal spacing near equilibrium is consistent with the rule (R0+R2)/2=Re, connecting the hard-sphere radius and the point of maximum attractive bonding force to the equilibrium bond length. In theoretical and experimental molecular curves, the rule tends to be exact at high reduced force constant ke, with symmetry breaking over ke=4–81 related to covalent, ionic, and van der Waals bonding character. Scaling preserves spacing symmetry, and maps two-term potentials into a universal exp–exp limit, including the (2n,n) potential into the Morse potential. Scaled spacing parameters for different molecules are nearly constant. Anharmonic shape parameters for “tilt” and “width” of the well are linked to empirical correlations of Dunham constants [J. L. Graves and R. G. Parr, Phys. Rev. A 31, 1 (1985)], and RKR analysis suggests correlations induced by equal-spacing constraints. The inflection structure is linked to threshold singularities in the inverse Born–Oppenheimer potential R(V), which predicts the (2n,n) potential as a first approximation.
Gustavson’s procedure and the dynamics of highly excited vibrational states
Authors: Joyeux Marc
Published: July 2002

The well-known Birkhoff–Gustavson canonical perturbation theory has been used so far to obtain a reasonable approximation of model systems near the bottom of the well. It is argued in the present work that Gustavson’s calculation procedure is also a powerful tool for the study of the dynamics of highly excited vibrational states, as soon as the requirement that the transformed Hamiltonians be in Birkhoff’s normal form is dropped. Mathematically, this amounts to modifying the content of Gustavson’s null space. Physically, the transformed Hamiltonians are of the single or multiresonance type instead of just trivial Dunham expansions, even though no exact resonance condition is fulfilled. This idea is checked against 361 recently calculated levels of HCP up to 22 000 cm−1 above the bottom of the well and involving up to 30 quanta in the bending degree of freedom. Convergence up to 13th order of perturbation theory and an average absolute error as low as 2.2 cm−1 are reported for a two-resonance Hamiltonian, whereas the Dunham expansion converges only up to 4th order at an average error of 215 cm−1. The principal advantages of the resonance Hamiltonians compared to the exact one rely on its remaining good quantum numbers and classical action integrals. Discussions of the limitations of the method and of the connections to other canonical perturbation theories, like Van Vleck or Lie transforms, are also presented.
A j-walking algorithm for microcanonical simulations: Applications to Lennard-Jones clusters
Authors: Curotto E. , Freeman David L. , Doll J. D.
Published: July 2002

The j-walking method, previously developed to solve quasiergodicity problems in canonical simulations, is extended to simulations in the microcanonical ensemble. The implementation of the method in the microcanonical ensemble parallels that in the canonical case. Applications are presented in the microcanonical ensemble to cluster melting phenomena for Lennard-Jones clusters containing 7 and 13 particles. Significant difficulties are encountered in achieving ergodicity when Metropolis Monte Carlo methods are applied to these systems, and the difficulties are removed by the j-walking method.
State-resolved two-photon laser induced fluorescence detection of BrO
Authors: Delmdahl R. F. , Gericke K.-H.
Published: July 2002

In the present work we report state-resolved detection of BrO(X 2Π) radicals generated in reactive collision processes using two-photon excitation with subsequent monitoring of the vacuum-ultraviolet fluorescence. BrO(X 2Π) product excitation spectra extending in the spectral range between 348 and 363 nm were obtained from both the reaction O(1D)+CF3Br and Br(2P3/2)+O3. The spectra indicate the existence of a hitherto unobserved high-lying radiative C state at an energy of around 56 000 cm−1.
Non-Arrhenius modes in the relaxation of model proteins
Authors: Skorobogatiy Maksim , Guo Hong , Zuckermann Martin
Published: July 2002

We have investigated the relaxational dynamics for a protein model at various temperatures. Theoretical analysis of this model in conjunction with numerical simulations suggests several relaxation regimes, including a single exponential, a power law, and a logarithmic time dependence. Even though a stretched exponential form gives a good fit to the simulation results in the crossover regime between a single exponential and a power-law decay, we have not been able to directly deduce this form from the theoretical analysis.
Constant pressure and temperature molecular-dynamics simulation of the hydrated diphytanolphosphatidylcholine lipid bilayer
Authors: Husslein Thomas , Newns Dennis M. , Pattnaik Pratap C. , Zhong Qingfeng , Moore Preston B. , Klein Michael L.
Published: July 2002

Diphytanolphosphatidylcholine (DPhPC) is a lipid widely used in the study of membrane channel activity. Herein we report the results of a constant temperature (T=25 °C) and constant pressure (p=1 atm) molecular-dynamics (MD) simulation of a hydrated liquid crystal phase DPhPC bilayer. The simulated system consisted of a periodically replicated cell containing 64 lipid and 1792 water molecules. The system was monitored during a trajectory spanning more than one nanosecond. The resulting unconstrained area density per lipid agreed quantitatively with experimental data. The calculated bilayer profile and acyl chain order parameters also compared favorably with x-ray scattering and nuclear magnetic resonance (NMR) data.
A theoretical study of photofragmentation and geminate recombination of ICN in solid Ar
Authors: Fernandez Alberti S. , Halberstadt N. , Beswick J. A. , Echave J.
Published: July 2002

Photodissociation of ICN in an Ar matrix is studied by molecular dynamics with quantum transitions (MDQT) with the motion of the nuclei treated classically and the electronic motion quantum mechanically. Four electronic surfaces and their corresponding couplings are included in the calculations. The coupling between electronic states at large I-CN internuclear distances is modeled using a diatomic in molecules (DIM) treatment of the mixing of the different spin-orbit states of iodine induced by the Ar atoms. For a total propagation time of 3 ps, no cage exit is found and 44% of the trajectories recombine to the ground electronic state. The principal mechanism for geminate recombination involves the reaction path 3Π0+→1Π1→1Σ0++.
Photodissociation of ozone in the Chappuis band. III. Product state distributions
Authors: Flöthmann Heiner , Schinke Reinhard , Woywod Clemens , Domcke Wolfgang
Published: July 2002

We discuss the vibrational and rotational state distributions of ground-state O2 following the photodissociation of O3 in the Chappuis band. They are obtained from time-dependent wave packet calculations employing ab initio potential energy surfaces for the 1 1A″ and 2 1A″ electronic states and the nonadiabatic elements, which couple these states. The satisfying agreement with experimental results underlines that the essential mechanisms of this two-state process are well described.
Deuterium dissociation on ordered Sn/Pt(111) surface alloys
Authors: Samson P. , Nesbitt A. , Koel B. E. , Hodgson A.
Published: July 2002

We have explored the effect of alloying an unreactive metal, Sn, on the dynamics of D2 dissociative chemisorption at Pt(111). By comparing D2 sticking and recombinative desorption on Pt(111) with that on the ordered p(2×2) Sn/Pt(111) and (∛×∛)R30° Sn/Pt(111) surface alloys, we examine the influence of the local surface composition on reactivity. The energy dependence of D2 sticking S(E) has been measured for all three surfaces using a hyperthermal beam. We find that the activation barrier for dissociative chemisorption is low on the p(2×2) alloy, but the sticking probability is reduced, compared to Pt(111), by an increase in the steric constraint on the dissociation site. Sticking on the (∛×∛)R30° alloy is inefficient at thermal energies with a threshold of ∼280 meV, below which the sticking probability falls exponentially. The increase in the barrier to D2 dissociation occurs as the stable, high coordination Pt3–D binding sites are lost by formation of the (∛×∛)R30° alloy. Despite the large activation barrier, sticking is dominated by the vibrational ground state with the barrier occurring in the entrance channel, before the D2 bond has stretched. Departures from a normal energy scaling indicate that the dissociation site is localized in the unit cell and we suggest favorable dissociation sites on the alloy surfaces. Estimates for the heats of adsorption, obtained by comparing activation energies to adsorption and desorption, indicate an abrupt decrease in the D binding energy as the Pt3 sites are lost. We show that sticking and desorption parameters are consistent with an increasing steric constraint for adsorption/desorption on the alloy surfaces as the Sn content is increased and an increase in the barrier to dissociation as the stable Pt3 sites are lost by alloying.
Surface diffusion near the points corresponding to continuous phase transitions
Authors: Uebing C. , Zhdanov V. P.
Published: July 2002

Employing the scaling arguments, we show that the temperature dependence of the chemical diffusion coefficient near the points corresponding to continuous phase transitions has power-law or inverse logarithmic singularities, Dc∝|ΔT|α/(1−α) for α>0, ∝|ΔT|−α for α<0, or ∝1/|ln|ΔT|| for α=0, where α is the specific heat exponent. Monte Carlo simulations, executed with the parameters corresponding to the O/W(110) system, indicate that these singularities, occurring in a very narrow temperature interval, can be reproduced only if the lattice size is large (L>500). Outside the critical region, the temperature dependence of Dc is regular and the deviations from the ideal Arrhenius behavior are relatively weak. In particular, at appreciable coverages, the variations of the activation energy for chemical diffusion are about 10 kJ/mol (this value amounts to ≈10% of the activation energy).
Polarization effects in C60 fullerene complexes of alkali ions
Authors: Ruiz A. , Hernández-Rojas J. , Bretón J. , Gomez Llorente J. M.
Published: July 2002

We introduce a secular semiempirical model of the Pariser–Parr–Pople type to reproduce the electronic structure and polarizability of the C60 fullerene. The model is then used to simulate the response of this molecule to an electric charge and estimate its polarization energy. By expressing the charge potential at the C60-cage surface as a multipole expansion, an analytical form is obtained for the polarization energy. Application of these results to endo- and exohedral complexes of alkali ions gives data in rather good agreement with recent ab initio calculations [Hira and Ray, Phys. Rev. A 52, 141 (1995)].
Effect of an external electric field on diffusion-controlled bulk electron-ion recombination in high-mobility systems
Authors: Wojcik Mariusz , Tachiya M.
Published: July 2002

The dependence of the rate constant of electron-ion recombination on the external electric field in systems characterized by high electron mobility is calculated by means of computer simulation. Two simulation methods are proposed, applicable for high and low electric fields, respectively. The rate constant is found to decrease with increasing electric field, the effect becomes stronger as the electron mean free time increases. Results obtained with the energy and the space criterion of recombination are discussed and a comparison of the simulation results with experimental data is included.
UV photodissociation dynamics of allyl radical by photofragment translational spectroscopy
Authors: Stranges Domenico , Stemmler Martin , Yang Xueming , Chesko James D. , Suits Arthur G. , Lee Yuan T.
Published: July 2002

Photodissociation of the allyl radical, CH2CHCH2, has been studied using the method of molecular beam photofragment translational spectroscopy following excitation to the C̃(2 2B1) and Ã(1 2B1) states by 248 and 351 nm photons. Two different primary channels have been detected following 248 nm excitation: H-atom loss (84%) and CH3 elimination (16%). From the product translational energy distribution and polarization dependence studies, dissociation processes from the ground-state C3H5 potential energy surface are inferred for both wavelengths. At 248 nm there may also be a contribution to the H-atom loss channel from predissociation by a higher electronically excited state. Rice–Ramsperger–Kassel–Marcus (RRKM) calculations show that the formation of cyclopropene is not important, while formation of allene and methylacetylene from dissociation of 1- and 2-propenyl radicals are important reaction pathways at both wavelengths. Translational energy distributions peaking well away from zero provide evidence for CH3 elimination directly from an allylic structure through a four-member cyclic transition state.
Nonlinear optical response of molecules in a nonequilibrium solvation model
Authors: Sylvester-Hvid Kristian O. , Mikkelsen Kurt V. , Jonsson Dan , Norman Patrick , Ågren Hans
Published: July 2002

The response equations governing nonequilibrium solvation in the continuum approach are derived and implemented for second order in response theory. Applications to a simple benchmark system consisting of a solvated water molecule are presented. We consider the role of nonequilibrium solvent configurations on the first hyperpolarizability. Effects of solvation parameters, such as optical and static dielectric constants, and frequency dispersion are discussed.
Interaction-induced polarizability invariants and the interatomic potential of the mercury diatom
Authors: Bonechi Alessandra , Moraldi Massimo , Frommhold Lothar
Published: July 2002

Analytical models of the invariants (trace and anisotropy) of the diatom polarizability tensor are formulated that differ from existing models by the suppression of the classical long-range components at close range (“damping”). Such damping is imperative in diatomic systems where the long-range polarizability terms are relatively large and/or the collision energies are high so that substantial penetration of the electronic shells of the colliding atoms takes place, e.g., for collision-induced light scattering by mercury vapor at high temperatures. When a small number of physically meaningful parameters of the polarizability models are properly adjusted, the binary trace and anisotropy collision-induced spectra of mercury vapor are closely reproduced from theory in all details. An improved model of the Hg–Hg interaction potential, which reproduces the existing bound mercury dimer states as well as the viscosity data, includes a similar damping term for suppression of the dispersion part at close range.
A multilayer theory for interfacial properties of systems containing hydrogen bonding molecules
Authors: Suresh S. J. , Naik V. M.
Published: July 2002

We propose a generalized, lattice-based statistical thermodynamic theory for understanding the interfacial phenomena in systems containing hydrogen bonding molecules (often termed as associating molecules), such as water, amphiphiles, block copolymers and associating solid surfaces. The basic assumption is that the configurational partition function (Q) can be factored into two parts: (i) one term [Q(phys)] arising from the presence of nonassociating, or the “physical,” interactions, for which we adopt the self-consistent-field theory [Scheutjens and Fleer, J. Phys. Chem. 84, 178 (1980)], (ii) the other term [Q(hbond)] arising from the presence of hydrogen bond interactions, for which we propose a new association theory. The focus of the proposed association theory is on the correct counting of the number of H bonds that are formed between various types of donor and acceptor sites that satisfy the proximity and orientational requirements for bond formation. The expression for Q(hbond) is evaluated by accounting for the entropic loss and energy released upon the formation of each hydrogen bond, and the transient nature of hydrogen bonds. The equilibrium criteria for H bonding is satisfied by minimizing the free energy of the system with respect to the number of H bonds formed between each type of donor site present in each layer z and each type of acceptor site present in each layer z′, where z′=z, or z±1. It turns out that the final expression for Q(hbond), at equilibrium, depends only on the fraction of unbonded association sites of all types that are located at various distances from the interface, which are themselves related to the equilibrium constant of formation of H bond between various donor-acceptor pairs, temperature of the fluid and the concentration profile in the interfacial region. For systems containing pure, spherical, associating molecules in the fluid phase, our expression for Q(hbond) is found to be identical to that of the density functional theory [Segura et al., Mol. Phys. 90, 759 (1997)], except for the inherent differences existing between continuum and lattice treatments. We present the results of the proposed theory in two parts. First, we verify the thermodynamic consistency of our approach with the Gibbs adsorption rule. Second, to clearly elucidate the role of hydrogen bonding on interfacial properties, we provide results for systems containing a binary fluid mixture, which comprises of an associating monomeric solvent and an amphiphilic, di-block, chain molecule, against an associating solid surface.
Comment on “Assessment of complete basis set methods for calculation of enthalpies of formation” [J. Chem. Phys. 108, 692 (1998)]
Authors: Montgomery J. A. , Frisch M. J. , Ochterski J. W. , Petersson G. A. , Raghavachari K. , Zakrzewski V. G.
Published: July 2002

Large errors recently reported in the calculated CBS-Q, CBS-q, and CBS-4 heats of formation of two-butyne and naphthalene are shown to be an artifact of near linear dependence in the one-electron basis set. The corrected results are in much better agreement with experiment.
Vibrational spectrum of (CO)2 on Cu(100): Quantum calculations with 18 coupled modes
Authors: Dzegilenko Fedor , Bowman Joel M. , Carter Stuart
Published: July 2002

We report calculations of the vibrational frequencies of CO dimer on Cu(100) using the recently developed vibrational self-consistent field code. Eighteen modes are treated explicitly within a three-mode representation of the potential. Nine symmetry-distinct doublets are observed and the corresponding frequencies are computed. The thermally broadened spectrum of the CO-stretch fundamental is calculated at various temperatures. Both the temperature and coverage dependence of both the average CO-stretch frequency and the corresponding linewidth are consistent with experimentally observed trends.