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The interaction of H atoms having relatively low average kinetic energy (∼0.025 eV) with both perfectly clean and D-covered HOPG surfaces is investigated using high resolution electron energy loss spectroscopy. From this study we confirm, in a controlled fashion, the presence of the theoretically predicted adsorption barrier since no adsorption is detected for such H atoms on HOPG. Moreover, we demonstrate that the exposure of a D saturated HOPG surface to these H atoms results in the complete removal of adatoms, with no further adsorption despite the prediction of the adsorption barrier to vanish for H dimers in para configuration. Therefore, the recombinative abstraction mechanism which competes with the adsorption process is more efficient.

This article deals with surface loss on quartz of atomic hydrogen (H) and its isotope deuterium (D) in a low-pressure (10 Pa) pulsed inductively coupled plasma. The atomic temporal decay in the post discharge is measured by two-photon absorption laser-induced fluorescence (TALIF). From the loss rate, the atomic surface loss probability is determined. In pure hydrogen or pure deuterium gas, no isotopic effect on surface kinetics has been observed and the surface loss probabilities of H and D were found to be almost identical and equal to ∼1.8%. However, despite the lack of difference in surface loss probability, a net isotopic effect on surface loss rate due to the mass difference between the isotopes is measured. Hydrogen atoms diffuse faster and have higher flux to the plasma chamber walls than deuterium atoms. Hydrogen atoms are therefore lost at higher rate than deuterium atoms. Based on the observed isotopic difference and on the comparison between H and D TALIF signals, the isotopic effects on H and D atom production are discussed. [ABSTRACT FROM AUTHOR]

The adsorption of H/D atoms on the graphite (0001) surface is investigated by means of both high-resolution electron-energy loss spectroscopy (HREELS) and periodic first-principle density-functional theory. The two methods converge towards two modes of adsorption: adsorption in clusters of about four hydrogen atoms and adsorption in pairs of atoms on contiguous carbon sites. The desorption energies estimated from the calculated dissociation energies range from 8 to 185 kJ mol-1 leading to an estimated surface coverage at saturations of 30–44 at. %. These results are compared with previous thermal desorption spectroscopy results. New HREEL signal assignments are proposed based on quantum calculations. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we study the abstraction of deuterium (D) by hydrogen (H) atoms (respectively H by D) on both perfectly crystalline and ion bombarded graphite surfaces using high resolution electron energy loss spectroscopy. On an initially defect free graphite surface, abstraction of D by H (reciprocally and reversibly H by D) is a quite efficient process at room temperature. Measurements demonstrate that the abstraction occurs almost essentially with the adsorbed atoms on “weakly” reactive sites. Reciprocally, atoms adsorbed on defects with sp3 configuration participate in a much lower extent to the abstraction mechanism. Obviously, this phenomenon always occurs when exposing a surface to an atomic hydrogen beam and illustrates the competition between sticking and molecules formation by abstraction. [Copyright &y& Elsevier]