Surface Science Perspectives surface is in fact an electronic process in which the random statistical motion of phonons occasionally con (original) (raw)
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Atomic-Scale Desorption Through Electronic and Vibrational Excitation Mechanisms
Science, 1995
The scanning tunneling microscope has been used to desorb hydrogen from hydrogenterminated silicon (1 00) surfaces. As a result of control of the dose of incident electrons, a countable number of desorption sites can be created and the yield and cross section are thereby obtained. Two distinct desorption mechanisms are observed: (i) direct electronic excitation of the Si-H bond by field-emitted electrons and (ii) an atomic resolution mechanism that involves multiple-vibrational excitation by tunneling electrons at low applied voltages. This vibrational heating effect offers significant potential for controlling surface reactions involving adsorbed individual atoms and molecules.
A microscopic theory of desorption induced by electronic transitions
Surface Science, 1996
Desorption induced by electronic transitions (DIET) in two cases is investigated from a microscopic point of view. In case A, where a single electron in a state of the localized kind (localized around adsorbates) is excited into a state of the extended kind (extended into the bosom of the substrate), the shape of the excited-state potential-energy surface (PES) may differ markedly from that of the ground-state PES for adsorbate motion. The Frauck-Condon factor then takes a finite value, giving rise to a finite desorption probability. In case B, where a single electron in a state of the extended kind is excited into another state of the extended kind, the shape of the excited-state PES is practically the same as that of the ground-state PES. The Franck-Condon factor is then zero. In such a case, one should take DIET as a single-step (coherent) process and take into account the adsorbate-position dependence of the matrix element for state transitions of the electron system in order to obtain a finite desorption probability.
Spontaneous desorption of vibrationally excited molecules physically adsorbed on surfaces
Chemical Physics, 1981
The energy within 2 vibrationally excited phy-sisorbed molecu!e often exceeds that needed to break its bond to the surface. Energy transfer from the vibrating chemical bond. to the surface bond causes the surface bond to ruptore and the vibrationdly relaxed adsorbate is released from the surface. We present a theoretical model which allows an estimation of the residence time of a vibrationally excited adsorbate on a surface. Because of uncertainties in the nature of the surface bond. the lifetimes obtained from the analytical expressions presented have only qualitative significance. The results are interpreted in terms of Franck-Condon overlaps between the wavefunctions which describe the adsorbatesu'bstrate complex and the released adsorbate. Lifetimes are calculated for hydrogen isotopes adsorbed on sapphire surfaces. Guidelines are given for estimating lifetimes of other systems in terms of a few easily calculated parameters.
Surface dissociation from first principles: Dynamics and chemistry
Physical review. B, Condensed matter, 1994
We present a detailed analysis of a fully ab initio molecular-dynamics simulation of the surface reaction for C12 molecules incident on Si(111)-2X1 at an energy of 1 eV. We 6nd that Clz adsorption on Si(111)-2X1is dissociative with short-lived molecular precursor states involved in certain dissociation geometries. Dissociation is accompanied by an enormous surface response causing large, localized rehybridization effects in the n.-bonded chains and excitation of vibrational modes in the substrate. Analysis of the charge density in the bond within the molecule during the breakup shows that the dissociation is chemically driven and proceeds by transferring charge to a more antibonding molecular orbital.
The Journal of Physical Chemistry B, 2006
The mechanism that controls bond breaking at transition metal surfaces has been studied with sum frequency generation (SFG), scanning tunneling microscopy (STM), and catalytic nanodiodes operating under the highpressure conditions. The combination of these techniques permits us to understand the role of surface defects, surface diffusion, and hot electrons in dynamics of surface catalyzed reactions. Sum frequency generation vibrational spectroscopy and kinetic measurements were performed under 1.5 Torr of cyclohexene hydrogenation/dehydrogenation in the presence and absence of H 2 and over the temperature range 300-500 K on the Pt(100) and Pt(111) surfaces. The structure specificity of the Pt(100) and Pt(111) surfaces is exhibited by the surface species present during reaction. On Pt(100), π-allyl c-C 6 H 9 , cyclohexyl (C 6 H 11 ), and 1,4cyclohexadiene are identified adsorbates, while on the Pt(111) surface, π-allyl c-C 6 H 9 , 1,4-cyclohexadiene, and 1,3-cyclohexadiene are present. A scanning tunneling microscope that can be operated at high pressures and temperatures was used to study the Pt(111) surface during the catalytic hydrogenation/dehydrogenation of cyclohexene and its poisoning with CO. It was found that catalytically active surfaces were always disordered, while ordered surface were always catalytically deactivated. Only in the case of the CO poisoning at 350 K was a surface with a mobile adsorbed monolayer not catalytically active. From these results, a CO-dominated mobile overlayer that prevents reactant adsorption was proposed. By using the catalytic nanodiode, we detected the continuous flow of hot electron currents that is induced by the exothermic catalytic reaction. During the platinum-catalyzed oxidation of carbon monoxide, we monitored the flow of hot electrons over several hours using a metal-semiconductor Schottky diode composed of Pt and TiO 2 . The thickness of the Pt film used as the catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metalsemiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. We found the chemicurrent was well-correlated with the turnover rate of CO oxidation separately measured by gas chromatography. † Part of the special issue "Charles B. Harris Festschrift".
Chemically driven molecular decomposition at semiconductor surfaces
Chemical Physics Letters, 1993
We present an analysis of the chemistry of dissociative adsorption based on an ab initio molecular dynamics simulation of Cl1 incident on Si(1 I1)-2x I at an energy of I eV. It is shown that the break-up of the molecule occurs because of charge transfer into a more antibonding molecular orbital. The process is strongly orientationally dependent and is absent if the molecule is oriented perpendicular to the surface. In this case the molecule enters a precursor state and subsequently dissociates when it escapes from the local energy minimum.
Energy transfer and chemical dynamics at solid surfaces: The special role of charge transfer
Progress in Surface Science, 2008
Molecular energy transfer processes at solid surfaces are profoundly important, influencing trapping, desorption, diffusion, and reactivity; in short, all of the elementary steps needed for surface chemistry to take place. In this paper we review recent progress in our understanding of energy transfer at surfaces with a particular emphasis on those phenomena, which are peculiar to solids with delocalized electronic structure, e.g. electronically nonadiabatic energy transfer. This area of study represents an area requiring significant extensions of our theoretical understanding, which is largely based on density functional theory. This review provides an overview of some of the experimental and theoretical tools presently being used in this field and a description of several illustrative examples of work that have helped to shape our understanding.