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The dynamics of ethylene adsorption on the Si(001) surface was investigated by means of molecular beam techniques. A constant decrease of initial sticking probability s0 was observed with increasing kinetic energy indicating a non-activated adsorption channel. With increasing surface temperature, s0 decreases as well, pointing towards adsorption via a precursor state. Quantitative evaluation of the temperature dependence of s0 via the Kisliuk model was possible for surface temperatures above 250 K; below that value, the temperature dependence is dominated by the adsorption dynamics into the precursor state. Maximum surface coverage was found to be reduced with increasing surface temperature, which is discussed on the basis of a long lifetime of the precursor state at low temperatures. [ABSTRACT FROM AUTHOR]

The influence of molecular vibrations on the reaction dynamics of H2 on Si(001) as well as isotopic effects have been investigated by means of optical second-harmonic generation and molecular beam techniques. Enhanced dissociation of vibrationally excited H2 on Si(001)2×1 has been found corresponding to a reduction of the mean adsorption barrier to 390 meV and 180 meV for ν=1 and ν=2, respectively. The adsorption dynamics of the isotopes H2 and D2 show only small differences in the accessible range of beam energies between 50 meV and 350 meV. They are traced back to different degrees of vibrational excitation and do not point to an important influence of quantum tunneling in crossing the adsorption barrier. The sticking probability of H2 on the 7×7-reconstructed Si(111) surface was found to be activated both by H2 kinetic energy and surface temperature in a qualitatively similar fashion as H2/Si(001)2×1. Quantitatively, the overall sticking probabilities of H2 on the Si(111) surface are about one order of magnitude lower than on Si(001), the influence of surface temperature is generally stronger.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Thermal desorption of hydrogen from the Si(111)7×7 surface was investigated using optical second-harmonic generation to monitor the hydrogen coverage from 0.2 monolayer to below 0.01 monolayer. The results of isothermal desorption measurements are found to be compatible neither with simple first nor second-order kinetic behavior. It is suggested that different binding sites available for Si–H monohydride states on the Si(111)7×7 surface give rise to the apparent intermediate reaction order. [ABSTRACT FROM AUTHOR]

The process of desorption for the system of O2/Pd(111) under excitation by 100 fs pulses of visible light has been examined. Molecular desorption is found to occur with high efficiency and a nonlinear dependence on laser fluence. Direct time-domain measurements using a two-pulse correlation scheme reveal a dominant subpicosecond response together with a weaker, but significant correlation persisting for tens of picoseconds. These results imply a desorption process driven by the high electron temperatures produced by the femtosecond laser radiation. The slower component of the correlation response is interpreted as an enhancement of the desorption rate by adsorbate vibrational excitation. [ABSTRACT FROM AUTHOR]

Buried interface states in Ar/Cu(100) were studied by means of one- and two-photon photoemission experiments. With increasing Ar overlayer thickness, a transition from broad electron scattering resonances in the Ar conduction band into a hydrogen-like series of quasi-bound states at the Ar/Cu interface was observed. The thickness dependence of energies and lifetimes is compared to theoretical resonance positions and linewidths derived from a parameterized one-dimensional potential. [ABSTRACT FROM AUTHOR]

The article reports on the electronic excitations at surfaces. Coupling of electronic excitations at surfaces is significant to various dynamical processes. It directs diverse phenomena like charge transfer across interfaces in small scale electronic devices, desorption and dissociation processes of adsorbates.

Abstract: Site-selective adsorption of ethylene on Si(001) has been investigated by means of scanning tunneling microscopy (STM). Two different adsorption pathways are identified with ethylene adsorbed on one and two dimers, respectively. Preadsorption of atomic hydrogen is found to increase the reactivity of the two-dimer pathway, which is the minority channel on the clean surface, by a factor of 45. The results are interpreted in the framework of a precursor mediated adsorption process with a conversion barrier between precursor and final adsorption state that can be efficiently controlled by changing the local electronic structure of the surface. [Copyright &y& Elsevier]