Dynamics of gas-surface interactions: Thermal desorption of Ar and Xe from platinum

Three-dimensional stochastic classical trajectory studies have been carried out of the thermal desorption of isolated Ar and Xe atoms from the (111) face of platinum. Realistic interaction potentials that produce quantitative agreement with experimental sticking probabilities and angular and velocity scattering distributions were employed. Energy exchange with lattice phonons was included accurately via empirically chosen friction and fluctuating forces. Application of techniques for the simulation of “infrequent events” allowed studies to be extended to experimentally accessible lifetimes. Atoms were found to desorb preferentially to wide angles, with mean energy considerably lower than 2k times the surface temperature. Significant curvature of the Arrhenius plots was exhibited, and pre-exponential factors were found to be lower than characteristic frequencies. These effects were determined to be mainly dynamical in origin; i.e., they cannot be described by conventional transition-state-theory models.     

Kinetic theory of gas-surface interaction

The kinetics of particle and energy exchange between gas and surface is treated in analogy to the transport of particles and energy in solutions using Onsager's theory of nonequilibrium thermodynamics. In addition to the coefficients of sticking and thermal accommodation a third kinetic coefficient is introduced describing the coupling of particle and energy transport. This coupling turns out to be particularly important at low temperatures reducing the accommodation coefficient from 2 to 1.The Onsager coefficients are then expressed in terms of the scattering properties of the surface. A simple model is treated to illustrate the general results and to provide estimates for the kinetic coefficients.Extract from doctoral thesis of H.M. submitted to Fachbereich Physik, Techn. Universität München, 1978     

Microscopic theory of gas-surface interaction

A kinetic theory for inelastic scattering, trapping and desorption of gas molecules by surfaces is described. The theory is valid if the time scale _l = 1/r introduced by the relaxation ratesr in the kinetic equations (which is of the order of the life time of vibrational states of adsorbates) is sufficiently large compared to the vibrational period ₀. For sufficiently large activation energies of the adsorbates another time constant _res, the residence time of adsorbed particles, can be determined from the theory. One thus may distinguish four different partly overlapping regimes defined by the time scalest _{I l} , ₀t_II, _l t_III and _rest_IV. Regime I is governed by the Schrödinger equation regime II by the kinetic equations. In the region where both regimes overlap the kinetic coefficients can be expressed in terms of microscopic quantities which have been calculated previously. The relevant quantities in the other regimes are introduced and discussed from a unified point of view thus providing a link between the regimes I and IV which have been treated in detail before.     

Direct access to subsurface sites in gas-surface O2/Ag210 interactions using supersonic molecular beams

We show with supersonic molecular beams and surface vibrational spectroscopy that, contrary to the case of Ag(100) and Ag(110), O2 undergoes total dissociation on Ag(210) at 105 K. Moreover, metastable subsurface sites can be accessed either directly or indirectly. For the direct channel, the final configuration of the oxygen atoms depends on the angle and the energy with which the gas-phase molecules collide with the surface, being largest for normal incidence on the (100) nanofacets. Access into the subsurface site is enabled only in the presence of preadsorbed oxygen adatoms.     

Molecular orientation effects in gas-surface dynamical processes

We report studies of chemical reactions on single crystalline Si surfaces induced by oriented molecular beams. We discuss the obtained steric effects on Si from the viewpoint of the strength of the gas-surface interaction, in comparison with the steric effects appearing on the graphite surface. Oriented molecular beams demonstrate the possibility for controlling surface chemical reactions by varying the orientation of the incident molecules. The steric effects found on Si surfaces hint at new ways of material fabrication on Si surfaces.     

Current trends in the theories of gas-surface interaction

Studies on gas-surface dynamics have acquired considerable importance recently not only for their intrinsic scientific interest but also for their technological potential. This article first briefly describes various experimental techniques and a number of interesting recent observations resulting from these techniques. It then discusses certain important theoretical methodologies being extensively used nowadays. There arethree broad overlapping streams of theoretical works, viz classical, semi-classical and quantum-mechanical. There are alsothree basic problems in gas-surface interaction, viz (i) the interface presents a manybody problem; (ii) the solid surface is “rough”; (iii) the number of diffractive and inelastic channels is enormously large. The semi-classical approaches appear to dominate over the others in variety and quantity. But the sources of benchmark theoretical results are still the rigorous classical-trajectory and close-coupling quantum-mechanical calculations. The coming years are likely to witness not only increased numerical accuracy through refinements in semi-classical and quantum-mechanical approaches, but also certain special approximate methods designed to yield deeper physical insights into the nature of gas-surface interaction. The authors felicitate Prof. D S Kothari on his eightieth birthday and dedicate this paper to him on this occasion. An erratum to this article is available at .     

Laser-induced gas vortices

Recently, several femtosecond-laser techniques have demonstrated molecular excitation to high rotational states with a preferred sense of rotation. We consider collisional relaxation in a dense gas of such unidirectionally rotating molecules, and suggest that due to angular momentum conservation, collisions lead to the generation of macroscopic vortex gas flows. This argument is supported using the Direct Simulation Monte?Carlo method, followed by a computational gas-dynamic analysis.     

Laser-induced nuclear excitation

An analysis is presented of the Coulomb excitation of low-lying nuclear levels by the electrons produced by strong-field ionization of atoms. It is shown that the resulting short-lived radioactivity can be as high as on the order of 10³ Ci for certain isotopes excited by using modern laser systems. Relativistic effects are demonstrated that substantially increase radioactivity as compared to that predicted by nonrelativistic theory results.     

Laser-induced etching of silicon

Conventional fabrication method of porous silicon is anodisation of single crystal silicon in hydrofluoric acid. In this report, we show that it is possible to fabricate porous silicon by laser-induced etching. An earlier report by us has demonstrated the dependence of porous silicon photoluminescence characteristic on the etching laser wavelength [1]. Here we used 780 nm line from a diode laser as the etching source, and the optimum etching conditions were obtained. A simple model was proposed to explain the etching process. Scanning Electron Microscope (SEM) images of the samples support the proposed process.     

Laser-induced doping of GaAs

Thin layers of GaAs are heavily doped locally by laser induced Se or Zn diffusion. H₂Se or diethylzinc gases are used to provide Se or Zn dopant atoms. The surface is locally heated with 3 ns light pulses from a Q-switched frequency doubled Nd-YAG laser. The doping process is described in detail. Doping profiles and sheet carrier concentrations are measured as a function of substrate temperature, laser fluence and processing time. Dopant concentrations of more than 10²¹ cm^–3, with a thickness of the doped layer of less than 20 nm can be achieved.     

Adsorbate-mediated gas-surface energy transfer: Collision geometry and surface temperature influences

We report here a classical trajectory study of how changes in the angle of incidence and the surface temperature alter the energy transfer occuring when an argon atom impinges on a partially covered tungsten surface. Our calculations suggest that while an increase in the surface temperature has only a negligible effect on the net energy exchange, the dependence on the collision geometry can be more significant.     

Laser-induced desorption of Na-dimers

We find that Na-dimers are desorbed in a thermal process if rough Na surfaces are irradiated with pulsed laser light of λ=532 nm. In contrast, for light of λ=355 nm, Na₂ can be detached in a non-thermal reaction at low laser fluences. This is concluded from the kinetic energy distributions of the dimers determined by time-of-flight measurements using a second laser at λ=248 nm for photoionization. The transition from non-thermal to thermal desorption at large fluences of the laser light can also be identified. Received: 23 July 1996 / Accepted: 26 August 1996     

Laser-induced fluorescence of estrogens

Spectra of the laser-induced fluorescence of estrogens—estradiol, estriol, and estrone—were measured under excitation by the fourth harmonic (λ=266 nm) of a Nd:YAG laser. Quantum yields of the fluorescence were evaluated by the relative technique to be 0.107 for estradiol, 0.116 for estriol, and 0.0052 for estrone. The water solution of tryptophan is used as the standard.     

Laser-induced fluorescence of GdO

A semiempirical calculation of the energy level diagram of GdO based on the ligand field theory is presented here. The main frame of the resulting scheme is very simple and should be useful in order to link together the numerous observed systems of GdO. Pulsed dye laser-induced fluorescence experiments made possible the evaluation of some of the ligand field theory predictions, particularly that a Y⁷Σ⁻ level is lying 1840 cm⁻¹ above the X⁹Σ⁻ ground state. The I and II systems are shown to arise from the B⁹Σ⁻-X⁹Σ⁻ and B₁⁷Σ⁻-Y⁷Σ⁻ transitions, respectively. Continous wave laser-excitation spectra have been recorded and rotational analysis of the A⁹Π₄ → X⁹Σ⁻ transition carried out, confirming previous results of N. N. Dmitriev, L. A. Kaledin, E. A. Shenyavkaya, and L. V. Gurvich (Acta Phys. Hung. 55, 467–479 (1984)).     

Laser-induced CVD of rhodium

Stripes of rhodium metal were deposited by focusing an Ar⁺ laser (514.5 nm) onto glass and polyimide substrates in a heated vacuum cell that contained Rh(CO)₂acac vapor. Stripes were characterized by scanning profilometry, electrical resistivity, SEM and Auger measurements. Most stripes were 100–200 m wide and 1–3 m high. Very broad stripes (>500 m) were deposited when the Rh(CO)₂acac vapor pressure was greater than 1 Torr and when the laser power was more than 200 mW. Stripe resistivities were in general around 30 times that of the bulk material. Auger spectra show the presence of carbon in the stripes.