Diffuse LEED intensities of disordered crystal surfaces: II. Multiple scattering on disordered overlayers

The diffraction of low energy electrons from disordered overlayers adsorbed on ordered substrates is treated theoretically by an extension of Beeby's multiple scattering method. A lattice gas model is assumed for the disordered adsorbate layer. Multiple scattering within a certain area around each atom — each atom of the overlayer and within the ordered substrate — is treated self-consistently, the remaining contributions to the total scattering amplitude being averaged. The theory can be used in the limiting cases of random distribution and of long range order within the adsorbate layer.     

Resonance scattering of electrons from orientated O2 physisorbed on graphite

We have studied inelastic electron scattering from a saturated monolayer of O₂ physisorbed on graphite at 25K. The cross-section for excitation of the v = 0–1 vibrational transition in O₂ peaks at 8.5–9eV, and is attributed to the ⁴Σ_u negative ion resonance. The angular distribution of electrons inelastically ejected out of the resonance orbital has been measured, and is found to peak at 15° from the normal to the crystal for several different incident electron beam angles, in accord with the theory of resonant electron scattering by an orientated molecule. We discuss the implications of this measurement for a determination of the orientation of the O₂ molecule on the surface, taking account of possible multiple scattering effects.     

Reactive ion scattering study of physisorbed adsorbates: experiment and theory

We have studied reactive ion scattering (RIS) of hyperthermal (1–100 eV) Cs⁺ projectiles from physisorbed surfaces. RIS experiments from physisorbed water on Pt(1 1 1) reveal scattering products of Cs(H₂O)_n⁺ (n=1–3) cluster ions. The yields for RIS products are extremely high compared to those with chemisorbed species. Classical molecular dynamics simulations provide a new mechanism that explains the enhanced RIS yields with physisorbed species. Slow Cs⁺ projectiles pick up physisorbed molecules via an ion–surface abstraction reaction, preferably without direct collisions between projectile and adsorbate. This RIS process is very efficient and mechanistically different from the RIS process responsible for chemisorbed species that occurs through direct collision-induced desorption.     

Dynamics of atom scattering from 4He nanoclusters

We present a microscopic theory and results of atom scattering calculations to determine the dispersion of surface modes (ripplons) of superfluid helium-4 nanodroplets, expanding previous work [J. Chem. Phys. 115, 10161 (2001)]. A quantum transport formalism is adapted to the many-body scattering problem, yielding both elastic and inelastic fluxes. We demonstrate that, in analogy to the dynamic structure function S(k,ω) obtained from neutron scattering, a dynamic structure function σ(k,ω) can be obtained from ³He scattering. The ³He dynamic structure function σ(k,ω) is sensitive to surface dynamics, whereas the neutron dynamic structure function S(k,ω) is dominated by bulk-like excitations, in particular by rotons. Unlike for neutron-scattering, the total inelastic cross section for atom-scattering on ⁴He nanodroplets is large which we believe makes experimental detection feasible. We also show that scattering identical particles, i.e. ⁴He atoms, does not provide information about the dispersion of surface modes. Instead, inelastically scattered ⁴He atoms preferably lose roughly half their energy.     

Hydrogen atom abstraction from Piperazine by hydroxyl radical: a theoretical investigation

Dual level of quantum mechanical calculations have been carried out for hydrogen abstraction from Piperazine [HN(CH₂CH₂)₂NH] initiated by OH radical. Geometry optimisation and frequency calculations of all species involved in the titled reaction have been performed at M06-2X/6-31+G(d,p) level of theory. For the accuracy in the thermochemistry and kinetics data, single-point energy calculations have been further carried out at coupled cluster CCSD(T) method along with 6-311G(d,p) basis set. An energy profile diagram for the reaction has been plotted along with pre-reactive and post-reactive complexes at entrance and exit channels. Intrinsic reaction coordinates (IRCs) calculations have been performed for identification of real transition states that connect it via reactant to product. Our result shows that the H-atom abstraction takes place from the C–H position of Piperazine. The rate constant is calculated using canonical transition state theory (CTST) is found to be 2.86 × 10^?10 cm³ molecule^?1 s^?1 which is in good agreement with the reported experimental rate constant (2.38 ± 0.28) × 10^?10 cm³ molecule^?1 s^?1 at 298 K. We have also reported rate constant for the temperature range 300–500 K. Using group-balance isodesmic reaction, the standard enthalpies of formation for Piperazine and product radicals generated by hydrogen abstraction are reported. The branching ratios for both reaction channel (i.e. H-abstraction from –CH₂ and –NH position of Piperazine) are found to be 93% and 7%, respectively. The calculated atmospheric life time of Piperazine is found to be 0.97 hour.     

Exchange-degenerate Regge trajectories: A fresh look from resonance and forward scattering regions

The exchange degeneracy of the mesonic and Regge trajectories, dominant at moderate and high energies in hadron elastic scattering, is analyzed from two viewpoints. The first one concerns the masses of the resonances lying on these trajectories; the second one deals with the total cross sections and the ratios of the real to the imaginary parts of the forward amplitudes of hadron and photon induced reactions. Neither set of data supports exact exchange degeneracy. Received: 23 June 2000 / Revised version: 13 November 2000 / Published online: 23 January 2001     

Effects of radial relaxation and intershell correlations during resonance inelastic photon scattering by an atom

The absolute value and shape of the double differential cross section of the resonance inelastic scattering of a linearly polarized photon in the ionization threshold energy range of the subvalence s shell of the free neon or argon atom are calculated in the nonrelativistic approximation for one-electron wavefunctions and in the dipole approximation for the anomalously dispersive scattering probability amplitude. The effects of radial relaxation, intershell correlations, bremsstrahlung, spin-orbit splitting, and a finite decay width of s vacancies are taken into account. The effects of radial relaxation and intershell correlations substantially affect the near-threshold scattering intensity: they decrease the contribution of the leading Compton anomalously dispersive component of the total cross section calculated in the one-electron approximation by several times. The calculation results have a predictive character.     

Detection of excess crystalline As in GaAs: Nation oxide overlayers by Raman scattering

Laser Raman spectroscopy was employed as a non-destructive probe for the detection and monitoring of crystalline arsenic in the native oxide films formed during heating of GaAs in air at various temperatures. Spectroscopy of oxide films formed after successive heating and etching treatments could confirm the location of arsenic to be near the top of the GaAs: native oxide overlayer.     

A unified model for diffractive and inelastic scattering of a light atom from a solid surface

A simple model for gas-surface scattering is presented which permits treatment of inelastic effects in diffractive systems. This model, founded on an impulsive collision assumption, leads to an intensity distribution which is just a sum of contributions from n-phonon scattering events. Furthermore, by using a convenient form for the repulsive interaction potential, analytic expressions are obtained for the elastic and one-phonon intensities that are in qualitative agreement with experimental results.     

A phenomenological theory of light scattering by surfaces: II. Rough surfaces

The theory developed in a previous paper is applied to the special case of rough surfaces. An existing controversy is discussed and resolved within the context of our theory. Formulae for the frequency dispersion of the scattered light due to motion of the interface are also given which makes the theory applicable to e.g. a fluid-fluid interface.     

On the theory of multiple scattering II. Critical scattering

The systematic theory of multiple scattering which we gave in a previous paper is further elaborated for critical scattering. It is shown that in each order the multiple-scattering intensity near the critical point is in essence a contraction of consecutive uncorrelated single-scattering intensities. The anomaly of the critical depolarization factor is calculated and is found to be in quantitative agreement with recent experimental results. Double scattering corrections to the Ornstein-Zernike plot are discussed.     

A comparison of Raman scattering,resonance Raman scattering,and fluorescence from molecules adsorbed on silver island films

The enhancement of Raman scattering (RS), resonance Raman scattering (RRS), and fluorescence from molecules adsorbed on silver-island films is reported. A heirarchy of enhancements is found: 10⁵ for RS, 10³ for RRS, and 0.1–10 for fluorescence, depending on the quantum yield of the free molecule. Using the framework of the electromagnetic theory of surface-enhanced Raman scattering, generalized to treat molecular resonance phenomena, we develop a unified picture of the role of the surface plasmon resonances, and the surface-induced damping, in the light scattering processes. The observed heirarchy of enhancements is shown to have important spectroscopic consequences.     

Level crossing resonance in ice: Identification of two diamagnetic muon states

Level crossing resonance studies of ice, doped with HF and NH₃ to enhance and suppress the concentration of orientational defects, support the hypothesis that muons can trap at such defects to form complexes of the type H₂O-Mu⁺-OH₂. The doping alters the partition of the total diamagnetic fraction between this state, which is stable against charge migrationvia proton rearrangement at low temperature, and the neutral HMuO state, the isotopically substituted water molecule.     

Double scattering of fast particles. II

In the present paper we study the problem of the shape of the shadow formed in the angular distribution of particles scattered by a weakly bound atom due to subsequent scattering by a neighboring atom.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 9, pp. 29–33, September, 1971.