bluuebunny/crossref_metadata_2025 huggingface dataset:
Reactions of Ar+ with H2, N2, O2, and CO at 20, 30, and 70 K
Authors: Rebrion C. , Rowe B. R. , Marquette J. B.
Published: July 2002

Reactions of Ar+(2P3/2) ions with H2, N2, O2, and CO have been studied with the CRESU apparatus at 30 K in argon buffer gas and with the newly developed mass-selected ion injector (CRESUS configuration) at 20 and 70 K in helium buffer gas. The atom exchange reaction with H2 is rather fast, with a rate coefficient k showing a small energy dependence well represented by k=1.5×10−9E0.16 cm3 s−1 with the collision energy in the center-of-mass frame, E, in the range 2.5×10−3−0.65 eV. This result is more compatible with a frozen-rotor capture model rather than with a threshold model for endoergic reactions. Concerning the reaction with N2, a competition between the two exit channels involving the two first vibrational levels of the N+2 product ion explains the minimum of the rate constant suggested at about 140 K by previous SIFT experiments [D. Smith and N. G. Adams, Phys. Rev. A 23, 2327 (1981)] and presently confirmed. The rate coefficients for the charge transfer reactions with O2 and CO increase with decreasing energy, i.e., k=9.6×10−12E−0.51 and k=1.1×10−11E−0.47 cm3 s−1, respectively, for E in the range 2.5×10−3−0.3 eV. Both these reactions proceed via an intimate collision mechanism.
Reactive dynamics in a deterministic thermal bath
Authors: Baratham Sanjaya , L’Heureux Ivan , Kapral Raymond
Published: July 2002

A study of barrier crossing dynamics is presented for a simple two-degree-of-freedom system coupled to a Nosé–Hoover heat bath. The characteristics of the deterministic heat bath are found to give rise to a variety of rate processes. Examination of the microscopic dynamics indicates how bath fluctuations drive the reactive dynamics and gives insight into how different deterministic heat baths can be constructed to model specific bath fluctuation effects.
Anisotropic reorientation and internal chain dynamics in liquid crystals: A deuterium nuclear magnetic resonance study
Authors: Dong Ronald Y. , Richards G. M.
Published: July 2002

The spectral densities of motion J1(ω) and J2(2ω) were measured at 15.3 MHz in the nematic phase of p-methoxy-d3-benzylidene-d1-p-n-butyl-d9-aniline for the methine and butyl chain deuterons. The present study concentrates on measurements near the clearing temperature as well as extends measurements to temperatures lower than those of an earlier report. We propose to interpret these spectral density parameters using a new model of molecular reorientation suggested by Vold and Vold. The reorientation motion of the molecules in a Maier–Saupe potential is governed by three rotational diffusion rate constants Dα, Dβ, and Dγ. The subscripts in the rate constants are the Euler angles ΩL,M which are involved in time-dependent transformation between the molecule fixed frame and the laboratory (director) frame. Dα and Dβ are the principal components of a rotational diffusion tensor in the director frame. The motion about the molecular ZM axis (γ motion) is assumed to be independent of the α,β motions. The internal chain dynamics is treated using the superimposed-rotations model in which rotation about each C–C bond can either be free or jump among t (trans) and g± (gauche) sites. The diffusion constants Dα, Dβ, and Dγ are found to show the Arrhenius temperature dependence. Similar behaviors are observed in the derived rotational diffusion constants for internal rotation of the C–C bonds in the butyl chain of MBBA.
Charge transfer and radiative dissociation dynamics in fluorine-doped solid krypton and argon
Authors: Kunttu H. , Feld J. , Alimi R. , Becker A. , Apkarian V. A.
Published: July 2002

The photodissociation of F2 in crystalline Kr is subject to only a minor cage effect—quantum yield of 0.5 at 308 nm, at T=12 K. Two-photon-induced harpooning in the same system leads to dissociation with near unit quantum efficiency; it is shown that this absorption is coherent with a giant cross section of 3(±2)×10−45 cm4  s at 275 nm. Excitation and emission spectra of charge-transfer transitions in solid Kr doped with F atoms, and solid Ar multiply doped with F/Kr are reported. The vertical excitation in F/Kr leads to a state delocalized over immediate neighbors—assigned to Kr+6F−. This state localizes on a subpicosecond time scale to the triatomic Kr+2F− configuration. The latter relaxes radiatively (τr=140 ns), to the repulsive wall of the ground state (∼1 eV above ground). The excitation in F:Kr/Ar solids shows the diatomic KrF(B←X) resonances. From the analysis of the spectrum, a harmonic frequency ω=75(±3) cm−1 is extracted for F atoms in the ground state, consistent with molecular-dynamics simulations of an octahedrally trapped F atom [ω=70(±1) cm−1]. The diatomic KrF(B) state relaxes on a time scale of 20 fs<t<500 fs to the mixed triatomic configuration, (ArKr)+F−, which then relaxes radiatively (τr=60 ns). The kinetic energy released upon the radiative dissociation of triatomic exciplexes generates acoustic waves, and defects, and leads to mass transport. The modes of propagation of the acoustic waves, their subsequent thermalization, and self-annealing of the solid, are directly monitored by spatially and temporally resolved transient scattering experiments. A transient loss (σ≂10−17 cm2), the origin of which is not fully understood, is observed for Kr2F isolated in solid Kr. The mass transport, and long-range migration of F atoms (∼10 lattice sites), is directly monitored in multiply doped Ar solids in which the shuttle of F atoms between Xe and Kr centers can be demonstrated.
One- and two-color polarization effects induced in emission and absorption spectra of monomeric and dimeric chlorophyll a in solution
Authors: Desjardins S. J. , Yixian Liu , Myslinski P. , Yizhong Shen , Koningstein J. A.
Published: July 2002

The results of polarization measurements are reported for a chlorophyll a dimer in solution. The experiments were conducted with a single laser beam (depolarization measurements) and two laser beams. In the latter case, the wavelength of one of the lasers is tuned to an absorption band of one of the constituent molecules of the dimer, that of the other is tuned to the absorption band of the other molecule. One of these beams produces effective population saturation of an excited state of one of the molecules of the dimer which has an appropriate transition moment parallel to the electric vector of the pump beam. The other, but weaker, laser beam probes either the resonance Raman or the absorption spectrum of the other molecule of the loosely bonded dimer. As a direct result of the slow rotational relaxation time of the dimer in comparison to the duration of the laser pulse, an anisotropy is created in the absorption as well as the resonance Raman spectrum probed with the weaker laser beam. The anisotropy measurements yield information on the direction of the principal axes of polarizability and transition moments of the constituent molecules of the dimer. In turn, this information can be used to predict structural properties of the dimer.
Application of hyperspherical coordinates to four-atom reactive scattering: H2+CN→H+HCN
Authors: Brooks A. N. , Clary D. C.
Published: July 2002

We develop the use of Delves’ hyperspherical coordinates to study the reactive scattering of four-atom systems within the collinear approximation. We present quantum mechanical calculations of reaction probabilities for the collinear exothermic reaction H2+CN →H+HCN. We use a potential energy surface which reproduces the essential characteristics of the reaction. The effect of freezing the CN bondlength to its equilibrium value during the reaction is also investigated and is found to be a good approximation. It is found that HCN product vibrational states with the C–H stretch excited are produced preferentially in the reaction.
Multiphoton ionization vs multiphoton dissociation: Competition between photoionization and dissociation in CO
Authors: Hill W. T. , Turner B. P. , Lefebvre-Brion H. , Yang S. , Zhu J.
Published: July 2002

The competition between photoionization and spontaneous dissociation involving the 3Π states of CO has been investigated both experimentally and theoretically. In this study we analyzed the mass and kinetic energy spectra of the photofragments generated after irradiating CO with a line narrowed (≊0.75 cm−1) tunable ArF* laser (λ≊193 nm) and calculated the cross section (3.2 Mb) for photoionizing the c 3Π Rydberg state of CO at the excitation energy. The basic conclusions of this study are that the interaction between the 3Π Rydberg states and the 3Π valence states near 12.8 eV causes rapid (picosecond time scale or faster) dissociation of CO which impedes photoionization and that multiphoton dissociation completely dominates multiphoton ionization at laser power densities up to at least 30 GW/cm2.
Spectroscopic properties of 34 electronic states of zirconium dimer (Zr2)
Authors: Balasubramanian K. , Ravimohan Ch.
Published: July 2002

State-averaged complete active space multiconfiguration self-consistent field followed by multireference singles + doubles configuration interaction calculations are carried out on low-lying electronic states of zirconium dimer (Zr2). In addition relativistic configuration interaction calculations are carried out including spin–orbit effects. Thirty-four bound electronic states are found for Zr2. Three very low-lying states of 7∑+u, 3∑+u, and 1∑+g symmetries are found as candidates for the ground state. Higher-order correlation effects and extension of basis sets tend to favor the 1∑+g state as the ground state of Zr2 exhibiting approximately a quadruple bond (bond order =3.6). Complete potential energy curves of seven states of Zr2 are presented. The nature of low-lying electronic states is analyzed through Mulliken populations, natural orbitals and their occupations, spin–orbit effects and weights of various configurations.
Anisotropic effects in x-ray Raman scattering from graphite
Authors: Tohji Kazuyuki , Udagawa Yasuo , Matsushita Tadashi , Nomura Masaharu , Ishikawa Tetsuya
Published: July 2002

X-ray Raman scattering spectra from single crystal graphite were recorded. By making use of the relative directions of the scattering vector (s) and an axis of graphite crystal (c), anisotropic effects in the x-ray Raman scattering spectrum are observed; the onset of the scattering shifts by about 7 eV and the extended fine structures show significant differences in the spectra obtained for s∥c and s⊥c. Analyses by the uses of the equation employed for EXAFS show that interlayer or intralayer C–C distances can be obtained separately from the spectra for s∥c and s⊥c, respectively. This is the first observation of the anisotropy effect in the extended structure in x-ray Raman spectra.
Negative binding capacity and bistability in one-component systems
Authors: Di Cera Enrico
Published: July 2002

The apparent violation of the second law at the level of individual site binding phenomena in a biological macromolecule working as an enzyme is shown to induce bistability in a simple one-component system.
Quantum mechanics of highly excited states of the H+3 molecular ion: A numerical study of the two degree of freedom C2v subspace
Authors: Tennyson Jonathan , Brass Otto , Pollak Eli
Published: July 2002

Two degrees of freedom quantum mechanical calculations on the bound states of H+3 are presented. Two different potential energy surfaces are employed. The effect of rotational excitation is analyzed. For J=0, the high energy region is composed largely of states that cannot be assigned. However, two regularly spaced series are observed, corresponding to ‘‘horseshoe’’ states predicted previously by classical calculations. In addition we find a new assignable series of inverted hyperspherical states. Conversely, for high orbiting angular momentum (l=20) in which the proton rotates about the diatom, assignable normal mode states persist up to dissociation. Semiclassical periodic orbit quantization is found to give excellent agreement for the regular quantum states. The significance of these results for the interpretation of the H+3 photodissociation spectrum is discussed.
Photoionization studies of KH2O, KNH3, K2NH2, and K2OH molecular complexes
Authors: Kuan T.-C. , Jiang R.-C. , Su T.-M.
Published: July 2002

Photoionization efficiency measurements of KH2O, KNH3, K2NH2, and K2OH molecular complexes are reported. Using a quadrupole mass spectrometer, a conventional light source, and a flow reactor, the molecular complexes between potassium atoms and water or ammonia molecules were generated and then detected by photoionization mass spectrometry. The photoionization efficiency curves were deconvoluted and the photoionization threshold energies of the complexes were determined. The photoionization threshold energies of KH2O, KNH3, K2NH2, and K2OH are measured to be 3.92±0.04, 3.87±0.04, 3.94±0.08, and 3.55±0.08 eV, respectively. The bond dissociation energies of K–H2O, K–NH3, and K–KOH are determined to be 0.25±0.08, 0.33±0.08, and 0.86±0.19 eV, respectively. Comparison is made between the experimental results and the theoretical values.
Thermodynamic and stochastic theory for nonequilibrium systems with more than one reactive intermediate: Nonautocatalytic or equilibrating systems
Authors: Hunt Paul M. , Hunt Katharine L. C. , Ross John
Published: July 2002

We consider chemical reactions occurring in a compartment separated by semipermeable membranes from reservoirs of reactant and product, both held at constant pressure. In earlier work, we have developed a nonequilibrium thermodynamic theory applicable to systems with a single reactive intermediate, and we have established a link between the thermodynamic and stochastic analyses of such systems. Here we show that our results generalize directly to cases with two or more reactive intermediates, if the reaction mechanism is nonautocatalytic, or if the system is evolving toward an equilibrium steady state in the reaction compartment without first exhausting the reactant or product reservoir. Starting with nonautocatalytic mechanisms, we identify effective driving forces for each intermediate; we then obtain the driving force for an arbitrary system by mapping to an instantaneously equivalent nonautocatalytic system. The driving force can be cast thermodynamically in terms of the difference between the actual chemical potential of the intermediate and its chemical potential at a reference state (the steady state of the instantaneously equivalent nonautocatalytic system); it can also be cast kinetically in terms of reactive fluxes in the instantaneously equivalent system. Taking the product of the driving force and the net flux of each intermediate and then summing over the species gives a term in the dissipation that is specific to the intermediates. This term is minimized at nonequilibrium steady states, unlike the total dissipation (or entropy production). For the nonautocatalytic or equilibrating systems, an integral of the driving forces yields a Liapunov function for the evolution of the reaction chamber toward the steady state. The same integral also determines the stationary solution of the birth–death master equation for the species numbers of intermediates in the reaction compartment; this generalizes the Einstein relation for the probability of equilibrium fluctuations to far-from-equilibrium conditions.
A semiclassical study of gas–solid energy transfer: He, Ne, and Ar on metal surfaces
Authors: Jackson Bret
Published: July 2002

A recently developed fully quantum theory of gas–surface energy transfer has been modified to treat systems where the wave function of the scattering molecule is written in terms of time dependent Gaussian wave packet basis functions. This allows for the inclusion of surface temperature effects in the systems studied using these techniques. The treatment is fully multiphonon, and can be applied to systems where the coupling of the gas particle to the vibrations of the solid is anywhere from weak to strong. Equations of motion are derived for the parameters which described each wave packet. In the limit we ignore the widths and phases of these wave packets, the remaining equations for the average position and momentum describe a classical trajectory coupled to a bath of quantum mechanical oscillators. Unlike earlier forms of this theory, our molecular trajectory has proper temperature dependence. Expressions are derived for both the sticking probability, and P(ΔE), the probability that a scattering particle exchanges and amount of energy ΔE with the solid. The dependence of these probabilities on the surface temperature, particle mass, particle energy, angle of incidence, and the interaction potential are examined for He, Ne, and Ar scattering from Cu(100). Comparison is also made with recent experimental studies of Ar on Ni and Ir. Reasonable agreement is found for average scattered beam energies and trapping probabilities.
Two dimensional nuclear magnetic resonance relaxation spectroscopy of molecular solids
Authors: Schleicher Andreas , Müller Klaus , Kothe Gerd
Published: July 2002

Molecular motions in solids cover a broad dynamic range, extending from the fast rotational to the ultraslow motional regime. Two dimensional (2D) NMR relaxation spectroscopy is designed to follow these motions and to differentiate the various motional modes. The method employs the pronounced anisotropy of the nuclear spin relaxation times, observed for polycrystalline or multidomain samples. Generally, 2D NMR relaxation spectra are obtained by recording the time signals S(t2) after the last pulse as a function of successive incremented time intervals t1, corresponding to the relaxation period of the particular sequence. A Fourier transformation in both time domains transforms S(t1,t2) into a 2D representation S(ω1,ω2) of the relevant relaxation experiment.

The normalized contour plot then displays the change of the corresponding relaxation rate 1/Ti along the frequency spectrum. It turns out that this variation is very dependent upon the character of the molecular motion. Model calculations for deuterons, involved in planar motions, demonstrate the potential of 2D NMR relaxation techniques. Generally, the type of motion can reliably be deduced from the shape of the contour plots. A model independent analysis provides the geometrical parameters of the dynamic process, including the jump angle ΔψK and the orientation ϑK of the rotation axis in the magnetic frame. In addition, from the separation of the contour lines the motional correlation times can be determined. The techniques are employed in the dynamical characterization of L-alanine, specifically deuteriated at the methyl group. From an analysis of 2D quadrupole echo spectra geometrical parameters of ΔψK=(120±1)° and ϑK=(70.5±1)° have been determined. Apparently, methyl group reorientation in L-alanine occurs via three-site jumps about a rotation axis, tilted by an angle of ϑK=70.5° relative to the C–2H bond direction. Computer simulations of 2D quadrupole echo and inversion recovery experiments provide the correlation times for this motion. The values range from τJ=5×10−10 s at T=353 K to τJ=3×10−5 s at T=140 K. An Arrhenius plot for these correlation times is linear over the entire dynamic range. From the slope of the straight line an activation of Ea=20 kJ/mol has been determined.
Photodissociation of CO−3: Product kinetic energy measurements as a probe of excited state potential surfaces and dissociation dynamics
Authors: Snodgrass Joseph T. , Roehl Coleen M. , van Koppen Petra A. M. , Palke William E. , Bowers Michael T.
Published: July 2002

The photodissociation process CO−3 +hν→O−+CO2 has been investigated at photon energies of 2.41, 2.50, 2.54, 2.60, and 2.71 eV. Experiments were conducted by crossing a mass-selected, 8 keV ion beam with a linearly polarized laser beam, and measuring the kinetic energy distributions of the charged photodissociation products. By varying the angle between the ion beam and laser polarization, angular distributions were obtained at photon energies of 2.41 and 2.54 eV. The photon energy dependence of the average photofragment kinetic energies shows conclusively that photodissociation at these photon energies does not proceed by a direct dissociation process on a repulsive potential surface, or by a statistical vibrational predissociation process on a bound surface. The photofragment angular distributions are isotropic, providing further evidence that precludes direct photodissociation on a repulsive potential surface. Ab initio calculations were performed using the gaussian86 programs. These calculations indicate that ground state CO−3 has a planar D3h geometry, and 2A′2 electronic symmetry. This ground state correlates adiabatically to the CO−2 +O dissociation asymptote, not the lower energy O−+CO2 asymptote. Taken together, these new experimental and theoretical results suggest that the photodissociation of CO−3 at these energies occurs via the interaction of bound and repulsive excited state potential surfaces. A new model of the potential surfaces of CO−3 is proposed.
Comparison of self-consistent calculations of the static polarizability of atoms and molecules
Authors: Moullet I. , Martins José Luís
Published: July 2002

The static dipole polarizabilities and other ground-state properties of H, H2, He, Na, and Na2 are calculated using five different self-consistent schemes: Hartree–Fock, local spin density approximation, Hartree–Fock plus local density correlation, self-interaction-corrected local spin density approximation, and Hartree–Fock plus self-interaction-corrected local density correlation. The inclusion of the self-interaction corrected local spin density approximation in the Hartree–Fock method improves dramatically the calculated dissociation energies of molecules but has a small effect on the calculated polarizabilities. Correcting the local spin density calculations for self-interaction effects improves the calculated polarizability in the cases where the local spin density results are mediocre, and has only a small effect in the cases where the local spin density values are in reasonable agreement with experiment.
Diffusion, adsorption, and reaction in pillared clays. I. Rod-like molecules in a regular pore space
Authors: Sahimi Muhammad
Published: July 2002

We report the results of the first computer simulation of transport, adsorption, and reaction processes in pillared clays, which are a class of catalytic materials with high catalytic activities. These materials have a very restricted pore structure which gives rise to the phenomenon of hindered diffusion in their pore space. We develop a dynamic Monte Carlo method and study diffusion, adsorption, and reaction phenomena in such systems. The pore space of the pillared clays is represented by parallel silicate layers connected to one another by pillars of various sizes, and the molecules are in the form of long, needlelike objects. Diffusion is represented by a random walk process, the adsorption of the molecules on the surface of the pillars takes place with a probability proportional to a Boltzmann factor, and the efficiency of the reaction properties of the pillared clays is investigated by measuring the average distance that a molecule has to travel in the pore space in order to reach a reactive site. Our results indicate that the structure of the pore space of the clays, the intermolecular interaction, the size of the molecules, and their adsorption on the surface of the pillars strongly affect their effective diffusivity and, even if the pore space of the clays is very regular and homogeneous, they can give rise to anomalous diffusion in which the effective diffusivity of the molecules varies slowly with time. Moreover, if the size of the molecules is comparable to the effective size of the pores, the irreversible adsorption of the molecules gives rise to a percolationlike phenomenon, in which the effective diffusivity decreases as the number of adsorbed molecules increases. The effective diffusivity would ultimately vanish if enough molecules are adsorbed so that a sample-spanning path of open pores would no longer exist.
The effects of core (3d) correlation on chemisorption
Authors: Pettersson Lars G. M. , Åkeby Hans , Siegbahn Per , Wahlgren Ulf
Published: July 2002

Large-scale calculations using atomic natural orbital (ANO) basis sets have been performed on Cu5O to establish the effects of correlation of the 3d shell on oxygen chemisorption. The largest calculation performed correlated 51 electrons in a basis set containing 205 ANO’s. Correlation of the 3d shell is found to increase the chemisorption energy by 13(16±2) kcal/mol and decrease the height above the surface of the oxygen by 0.23(0.38) a0 with CI superposition error corrections included. The 2E state of the bare cluster is found to be stabilized by 10(8) kcal/mol relative to the 4A2 state as a result of 3d correlation. The values in parentheses were obtained using an approximate core–valence correlation operator which thus gives results in very good agreement with the core correlated calculations.
Temperature dependence of state-to-state vibrational relaxation from the 441(1B2u) state of naphthalene induced by very low energy collisions with argon
Authors: Kable Scott H. , Knight Alan E. W.
Published: July 2002

State-to-state vibrational relaxation rate coefficients have been obtained for naphthalene colliding with argon in the very low energy collision range. A single vibronic level v′44=1(ν44 is an in-plane ring distortion) is prepared by laser excitation of naphthalene expanded in a supersonic free jet of argon. Relaxation to specific destination states is monitored using time resolved dispersed fluorescence spectroscopy. The observed state-to-state relaxation rate coefficients vary substantially among the available pathways but are explained satisfactorily by simple propensity rules when the destination state involves only out-of-plane vibrational motion. Transfer to the sole in-plane destination state (v24=1) is found to be less facile by more than an order of magnitude relative to that expected from simple propensity rules. Measured propensities are found to be relatively independent of collision energy for the temperature range studied here (3–20 K). This suggests that the translational energy dependence of the state-to-state cross section σif is similar to that for the total inelastic cross section σi, where from previous studies of polyatomic vibrational relaxation in this temperature range it has been demonstrated that σi scales with the Lennard-Jones cross section.
Molecular angular momentum reorientation of electronically excited hydrogen (B 1∑+u)
Authors: Pibel Charles D. , Moore C. Bradley
Published: July 2002

The room temperature rate constants for molecular angular momentum reorientation of H2, HD, and D2 (B 1∑+u, v′=0, J′=1, MJ′=0) in collisions with He, Ne, Ar and H2 (X 1∑+g) have been measured. The effective cross sections for changing MJ′ in collisions of H2, HD, D2 with He and Ne were found to be about 30 Å2 and were nearly the same for each isotope and with He and Ne as collision partners. The measured He–H2(B) reorientation cross section is about 50% larger than the cross section calculated with a simple semiclassical model using a potential that approximates the ab initio data for the H2(B)–He potential energy surface. The cross sections for reorientation of HD and D2 in collisions with Ar were found to be 10.6±2.0 and 13.9±3.0 Å2, respectively. The smaller cross section is due to the dominant role played by quenching of the electronic energy of molecular hydrogen in collisions with Ar. The reorientation of D2(B) in collisions with room temperature H2(X) occurs with a 7.6±3.4 Å2 cross section. The small cross section for reorientation of the angular momentum is again due to the dominance of quenching in the collision dynamics.
The poly-3-hexylthiophene/NOPF6 system: A photoelectron spectroscopy study of electronic structural changes induced by the charge transfer in the solid state
Authors: Lazzaroni R. , Lögdlund M. , Stafström S. , Salaneck W. R. , Brédas J. L.
Published: July 2002

The electronic structure of poly-3-hexylthiophene (P3HT) is studied in the solid state with photoelectron spectroscopy, as the polymer is gradually doped from NOPF6. Solubility and processability of P3HT allow for the preparation of very clean and very thin films, which are then doped without air-exposure. The evolution of the core level binding energies is related to the modification of the electron density on the conjugated backbone, due to the creation of polaron and bipolaron defects. Upon doping, valence spectra show a shift in the Fermi level of the system, and at saturation doping a finite density of states at the Fermi level is observed unambiguously for the first time in electrically conducting polymers with photoelectron spectroscopy. The experimental data on core and valence levels can be interpreted in terms of the formation of a polaron lattice at high doping.
Adsorption and desorption kinetics for SiCl4 on Si(111)7×7
Authors: Gupta P. , Coon P. A. , Koehler B. G. , George S. M.
Published: July 2002

The adsorption and desorption kinetics for SiCl4 on Si(111)7×7 were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. TPD experiments monitored SiCl2 as the desorption product at approximately 950 K using a heating rate of β=9 K/s. SiCl2 was also observed as the desorption product in the LITD yield at all surface coverages. LITD measurements determined the initial reactive sticking coefficient (S0) of SiCl4 on Si(111)7×7 versus surface temperature. The sticking coefficient was observed to decrease with surface temperature from S0≊0.18 at 160 K to S0≊0.03 at 600 K. TPD experiments revealed that the chlorine surface coverage saturated after large SiCl4 exposures and the saturation coverage was independent of surface temperature. Isothermal LITD studies enabled the surface chlorine coverage to be monitored as a function of time during SiCl2 desorption. These studies revealed second-order desorption kinetics for SiCl2 with a desorption activation barrier of Ed=67±5 kcal/mol and preexponential of νd=3.2×100±0.1 cm2/s. Experiments with preadsorbed hydrogen demonstrated that the initial sticking coefficient of SiCl4 was reduced as a function of increasing hydrogen coverage. This behavior indicated that the adsorption of SiCl4 on Si(111)7×7 requires free surface dangling bonds. LITD techniques were also used to study the surface diffusion of the chlorine on Si(111)7×7 after SiCl4 adsorption. No evidence of significant surface chlorine mobility (D≤2×10−11 cm2/s) was found for surface temperatures as high as 825 K.
The influence of potential energy surface topologies on the dissociation of H2
Authors: Halstead D. , Holloway S.
Published: July 2002

In this work we present a theoretical study of the dissociative adsorption of hydrogen molecules from a series of model potential energy surfaces. The aim is to discover those particular topological features in the potential surface which are responsible for determining the vibrational state-to-state cross sections in both the dissociated and the scattered flux. The potential energy surface is two-dimensional, and is chosen to be deliberately simple; a combination of Morse potentials and a Gaussian barrier. A quantum wave packet is chosen to represent the molecule and the dynamics are solved by a spectral grid method. Results show that the location of the barrier influences the scattering cross sections markedly. Early barriers result in vibrationally excited adsorbed species while late barriers produce translationally hot atoms. The individual state distributions resulting from the two model potentials are quite different. In addition, results are given for a potential where the activation barrier is deep in the exit channel. For this case, results show that molecules can trap near the barrier for significant times without invoking substrate degrees of freedom. This is explained in terms of trapping in dynamic wells. Finally, we assess the effect on dissociation probability following vibrational excitation of the hydrogen molecule.
Distributed complex Gaussian basis sets: A useful function space for the solution of predissociation problems via the complex eigenvalue Schrödinger equation. Application to the isotope effect of NeH, NeD
Authors: Petsalakis Ioannis D. , Mercouris Theodoros , Theodorakopoulos Giannoula , Nicolaides Cleanthes A.
Published: July 2002

A method is presented for the solution of the complex eigenvalue Schrödinger equation for predissociation resonances, employing a basis set of complex Gaussians, with centers distributed along the internuclear axis. Application to the predissociation of the v=0 level of the A2Σ+ state of HeH, v=0–4 level of the A2Σ+ state of NeH and all the vibrational levels of he A2Σ+ state of NeD have yielded widths that are in very good agreement with the available experimental values and the recent observations [Devynck, Graham, and Peterson, J. Chem. Phys. 91, 6880 (1989)] of the strong suppression of predissociation in NeD as compared to that of NeH.
Rotational spectrum and potential surface for Ar2–HCN: A T-shaped cluster with internal rotation
Authors: Gutowsky H. S. , Klots T. D. , Dykstra C. E.
Published: July 2002

The rotational spectrum of Ar2–HCN has been observed between 2.5 and 11.5 GHz with the pulsed nozzle, Fourier transform, Balle/Flygare Mark II microwave spectrometer. Eighteen transitions were found and their 14N quadrupole hyperfine structure analyzed. The line centers were fitted with the Watson Hamiltonian giving ground state rotational constants of 1769.366, 1743.854, and 857.600 MHz. The effective geometry of the cluster is found to be T-shaped with C2v symmetry and the H end of the HCN closest to the Ar2. However, the rms deviation of the fit is poor (300 kHz), the centrifugal distortion and inertial defect are huge, the Ar to HCN c.m. distance is nearly 0.2 Å shorter than in the Ar–HCN dimer, and the average angular displacement of the HCN from the C2 axis is both large (39°) and highly anisotropic (10°). In contrast, the Ar2 subunit exhibits an in-plane, average angular displacement of only 6°. These anomalies led us to calculate potential surfaces for Ar2–HCN and Ar2–HF using the molecular mechanics for clusters scheme. A comparison of the surfaces and the rotational properties of the two species prompts us to propose that in Ar2–HCN the HCN axis rotates about the C2 axis maintaining an angle of ∼40° between them for the m=0 internal rotation state. Such internal rotation accounts at least qualitatively for the otherwise anomalous rotational behavior of the Ar2–HCN cluster.
Raman Q-branch line shapes as a test of the H2–Ar intermolecular potential
Authors: Green Sheldon
Published: July 2002

Using an intermolecular potential determined from spectra of van der Waals complexes [R. J. Le Roy and J. M. Hutson, J. Chem. Phys. 86, 837 (1987)] and accurate close coupling scattering matrices, line-shape cross sections were calculated for vibrational Raman Q-branch spectra of H2 and D2 in Ar to compare with recent experimental values. While predicted room temperature linewidths are in moderate (≊25%) agreement with experiment, predicted line shifts are small by nearly a factor of 2. The calculations show a strong collision speed dependence in the shift cross sections, in qualitative if not quantitative accord with the model developed by R. L. Farrow et al. [Phys. Rev. Lett. 63, 746 (1989)] to explain their H2 data. However, the calculations predict a strong temperature dependence in the linewidth cross sections as well, whereas the data show little variation between room temperature and 1000 K. It is suggested that these discrepancies point to inadequacies in the vibrational dependence of the intermolecular potential at close intermolecular separations (approximately 2.8 to 3.1 Å).
Application of the stabilization method to the N−2(1 2Πg) and Mg−(1 2P) temporary anion states
Authors: Chao J. S.-Y. , Falcetta M. F. , Jordan K. D.
Published: July 2002

A new variant of the stabilization method is described and used in conjunction with an analytic continuation procedure to calculate the energies and lifetimes of the lowest energy temporary anion states of N2 and Mg. With this approach excellent results are obtained for the resonance parameters even when moderate size basis sets are employed.
Application of the log-derivative method to variational calculations for inelastic and reactive scattering
Authors: Mrugal/a Felicja
Published: July 2002

The log-derivative algorithm of Johnson is further generalized to evaluate transition amplitudes of orders up to third between states of free or bound character. These quantities appear in particular as constituents of a variety of low-order variational expressions for the reactance matrix which are based on the Lippmann–Schwinger type equations of scattering theory. The new algorithm is exploited to investigate relative accuracy of a number of these expressions on simple inelastic scattering test problems. Some findings of previous investigations, e.g., that of superior convergence of the expressions involving expansions of the amplitude density over the expressions based on expansions of the wave function, are revised. Superiority of the symmetric expressions over the asymmetric ones is demonstrated. The features of the new algorithm, such as relatively high efficiency and low storage requirements, make it well suited to variational calculations for reactive scattering. An exemplary implementation is presented to solving the Baer–Kouri–Levin–Tobocman (BKLT) equations for the collinear H+H2⇄H2+H reaction. Two new elements which improve the previous numerical treatment of these equations are exposed: the use of the Schwinger variational expression for the reactance matrix instead of the expression of the method of moments for the amplitude density and the use of distortion potentials producing inelastic transitions.
Thermoinduced and photoinduced reactions of NO on Si(111)7×7. III. Photoreaction mechanisms
Authors: Ying Z. C. , Ho W.
Published: July 2002

The relative yield of photoinduced desorption from NO-exposed Si(111)7×7 has been measured as a function of photon power, wavelength, polarization, incident angle, and coverage of coadsorbed potassium. The results are analyzed in terms of two possible mechanisms: direct photoelectronic excitation of the NO-surface complex and interaction of hot carriers photogenerated in the substrate with the NO-surface complex. The substrate-mediated mechanism is found to be principally responsible for the photoreactions.
Chemiluminescence measurements of the N+2, N++H2O charge transfer systems at suprathermal energies: Direct probe of the dynamics of large cross section charge transfer processes
Authors: Dressler Rainer A. , Gardner James A. , Lishawa C. Randal , Salter Richard H. , Murad Edmond
Published: July 2002

The chemiluminescence from suprathermal N+2–H2O and N+–H2O collisions is studied using a new experiment. Intense H2O+Ã 2A1–X̃ 2B1 emission is observed from the N+2 +H2O charge transfer whereas no emissions are detected from the N+–H2O system. OH A 2Σ+–X 2Π emission originating from the N+2+H2O atom pickup channel is also observed.
Photodissociation of methyl hydroperoxide at 266 nm: Vector correlations and energy distribution in the OH fragment
Authors: Novicki S. W. , Vasudev R.
Published: July 2002

The energy content and correlation of vectorial properties of the OH fragment photoejected by methyl hydroperoxide (CH3OOH) at 266 nm are probed by laser excitation, polarization, and high-resolution sub-Doppler spectroscopy. The nascent OH is found to be vibrationally cold and the rotational distribution peaks at ∼N=6. Polarization dependence of the fragment Doppler profiles and nonequilibrium Λ-doublet populations show that, at high rotational energy, the velocity vector of the OH fragment has a preference to be perpendicular to the planes of rotation of the nuclei and the half-filled 2pπ orbital. The fragment angular distribution indicates that dissociation of the parent excited state is fairly prompt, with a lifetime of ∼250 fs.
Molecular dynamics study of the temperature dependence of the interfacial thickness in two-dimensional fluid phases
Authors: Chen L.-J. , Robert M. , Shukla K. P.
Published: July 2002

The thickness of the interface between two-dimensional fluid phases is determined as a function of temperature by using the method of molecular dynamics. The model fluid is one of particles interacting via a truncated Lennard-Jones potential. Periodic boundary conditions are used and no macroscopic external field such as gravity is imposed. The temperature is varied between that of the triple point to values as close to the critical point as is consistent with the finiteness of the system studied on the computer. Because of the relatively small interfacial area probed in these simulations, long-wavelength capillary waves are strongly suppressed and within statistical uncertainty, the results for the interfacial thickness are found to be consistent with the prediction of the nonclassical van der Waals theory of the intrinsic interface.
Energy level statistics for the regular energy spectrum of nonlinear triatomics: Nongeneric aspects and quantum chaos
Authors: Hamilton Ian
Published: July 2002

We examine the regular energy spectrum of a nonlinear triatomic using model systems for which there is no coupling between the vibrational modes. We calculate the second and third moments of the successive spacing distribution and show that these may differ from those found for a generic regular energy spectrum in an energy range above the potential energy barrier for going through the linear geometry. Consequently, for nonlinear triatomics, caution must be exercised in using energy level statistics as evidence for the existence and degree of quantum chaos.
Raman studies on molecular and ionic forms in solid layers of nitrogen dioxide: Temperature and light induced effects
Authors: Givan A. , Loewenschuss A.
Published: July 2002

Raman spectra of zero pressure formed N2O4 solid layers are reported. Sample composition is extremely dependent upon deposition conditions. For ordered and pure solid N2O4(D2h), produced by slow NO2 deposition, temperature cycling over the range in which the solid is stable shows no significant spectral changes and does not result in autoionization, as argued in a previous Raman study. Fast and low temperature deposited layers are amorphous and multicomponent, showing bands of disordered and isomeric molecular N2O4 and of ionic NO+NO−3, nitrosonium nitrate. For nitrosonium nitrate, three solid modifications (two crystalline, one amorphous) can be characterized spectroscopically. In the amorphous phase, a light induced, temperature dependent, reversible transition between molecular and ionic nitrogen tetroxide is observed below 150 K. The paths leading to nitrosonium nitrate formation are examined.
Nonadiabatic effects in the photodissociation of H2S
Authors: Weide K. , Staemmler V. , Schinke R.
Published: July 2002

The photodissociation of H2 S in the first absorption band is studied by time-dependent wave packets evolving in two electronic states; the lower state is dissociative and the upper one is bound. The adiabatic potential energy surfaces and transition dipole functions are constructed from ab initio calculations while the nonadiabatic coupling is adjusted. The diffuse structure superimposed on the broad absorption spectrum is due to symmetric stretch motion in the upper (bound) electronic state which is strongly quenched by nonadiabatic coupling. This is different from the photodissociation of water in the first band.
Shear stabilization of critical fluctuations in bulk polymer blends studied by small angle neutron scattering
Authors: Nakatani Alan I. , Kim Hongdoo , Takahashi Yoshiaki , Matsushita Yushu , Takano Atsushi , Bauer Barry J. , Han Charles C.
Published: July 2002

The small angle neutron scattering (SANS) technique has been used to study the concentration fluctuations of binary polymer mixtures under shear. Two different polymer systems, deuterated polystyrene/poly(vinylmethylether) and deuterated polystyrene/polybutadiene, have been studied as a function of temperature and shear rate. Due to the small wavelength of the incident neutron radiation compared with light, the shear dependence of concentration fluctuations in the one-phase region and in the strong shear limit has been obtained from the q dependence of the scattering structure factor for the first time. From a detailed analysis of the scattering structure factor S(q) a crossover value of the wave number qs has been obtained as a function of temperature and shear rate. This crossover wave number represents the inverse of the lowest fluctuation mode which is not affected by shear. The temperature, viscosity, and shear rate dependence of this experimentally determined qs agree well with a simple rotatory diffusion model and also the dynamic mode–mode coupling analysis of Kawasaki and Ferrell. The apparent spinodal temperature as a function of shear rate is shown to be consistent with the prediction of Onuki.
Infrared absorption spectroscopy of gas-phase N2O–HX (X=F, Cl, Br) weakly bonded complexes utilizing the N2O ν3 chromophore
Authors: Zeng Y. P. , Sharpe S. W. , Reifschneider D. , Wittig C. , Beaudet R. A.
Published: July 2002

Rovibrational absorption spectra of weakly bonded complexes of N2O with HF, DF, HCl, and HBr were recorded in the ν3 region of N2O by using pulsed, slotted nozzle expansions and tunable diode lasers. A fast-scan technique was used that takes advantage of the rapid tuning capabilities of diode lasers; i.e., 4000 resolution elements were recorded with a single opening of the nozzle. Of the two known NH- and OH-bonded isomers of N2O–HF, we detected only linear ONN–HF; the ground-state rotational constants are in excellent agreement with previous microwave and IR results. Deuteration resulted in ONN–DF linewidths that are much narrower than those of ONN–HF, as observed previously in studies of the analogous CO2–H(D)F system. Vibrational band origins for ONN–HF and ONN–DF are blue shifted 21.8 and 23.4 cm−1, respectively, relative to uncomplexed N2O. The additional blue shift upon deuteration is attributed to enhanced hydrogen bonding in a highly anharmonic potential. High-resolution spectra of NNO–HCl and NNO–HBr are presented for the first time. The average NNO–HCl geometry is asymmetric, with the separation between the N2O and HCl centers-of-mass Rcm equal to 3.51 Å. The angle between Rcm and the NNO principal axis θ1 is 72°–76°. NNO–HBr complexes are also asymmetric (θ1=75°–82°) with Rcm =3.62 Å. Linear ONN–HCl(Br) isomers were not observed. Blue shifts in the NNO–HCl and NNO–HBr band origins are 2.44 and 1.86 cm−1, relative to uncomplexed N2O. The qualitative changes observed in the NNO–HX geometries and force fields are attributed to competing effects arising from hydrogen–bonding and dispersion forces, as were observed with CO2–HF(Cl) and CO2–HBr. The experimental geometries and vibrational frequencies are compared to ab initio calculations; agreement with N2O–HF is good, CO2–HCl less so. Although the H atom position cannot be determined experimentally with NNO–HCl(Br), ab initio estimates suggest it is localized near the O atom. Implications for photoinitiated reactions in weakly bonded complexes are discussed.