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Several monolayers thick hydrogenated carbon films, C:H, were prepared by ion beam deposition from hydrocarbon process gases onto Pt and monolayer C covered Pt single crystal surfaces and investigated with Auger electron and thermal desorption spectroscopies in an UHV environment. Efficient deposition was achieved at ion energies in the 160–300 eV range. The deposited thickness and H/C ratio of the films depend on both, target temperature and H/C ratio of the process gas. It is shown that the C monolayer is crucial for efficient on-top deposition. Irrespective of the process gas used for deposition, the films grow as a C network and assume a constant H/C ratio at thicknesses greater than ∼ 3 monolayers. The H/C ratio of the films scale with the H content of the hydrocarbon process gas, a H/C ratio of 0.4 was obtained for ethane at 350 K substrate temperature. Upon thermally activated decomposition the films release molecular hydrogen as the major gaseous species and various hydrocarbons as minority species. The latter products signal chemical erosion of the film. It is shown that the rate determining step towards erosion via methane is a C–C bond breaking event which releases methyl radicals from the C network in the film. The activation energies for this step are determined as a 10 kcal/mol wide Gaussian distribution centered at 56 kcal/mol. Transport through the film is found to be so fast that it does not contribute to the observed gas release rates.

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The interaction of gas-phase H atoms with ordered and disordered adlayers of atomic oxygen, hydroxyl, and molecular oxygen on Pt͑111͒ surfaces was investigated by in situ mass spectrometry and post-reaction TPD ͑temperature programed desorption͒. Exposure of oxygen adlayers to gas-phase H atoms at 85 K leads to formation of H 2 O via two consecutive hydrogenation reactions: H(g)ϩO(a)→OH(a) followed by H(g)ϩOH(a)→H 2 O(g,a). Both reaction steps are highly exothermic, and nascent H 2 O molecules partially escape into the gas phase before being thermally accommodated on the surface. Empty surface sites and hydrogen bonding promote thermalization of H 2 O. Separate experiments performed with OH-covered Pt͑111͒ surfaces reveal that the hydrogenation of hydroxyl is a slow reaction compared to the hydrogenation of atomic oxygen; additionally, the abstraction of H from OH by gas-phase D atoms, OH(a)ϩD(g)→O(a) ϩHD(g), was detected. Abstraction of H from adsorbed H 2 O was not observed. Admission of gas-phase H atoms to O 2 -covered Pt͑111͒ surfaces at 85 K leads to the desorption of O 2 and H 2 O. The thermodynamic stability of the HO 2 radical suggests that the reaction is initiated by hydrogenation of molecular oxygen, O 2 (a)ϩH (g)→HO 2 . The intermediate HO 2 either decomposes via dissociation of the HO-O bond, HO 2 →OH(a)ϩO(a), finally leading to the formation of H 2 O ͑ϳ85%͒, or via dissociation of the H-O 2 bond thus leading to desorption of O 2 ͑ϳ15%͒. The whole reaction sequence of formation and decomposition of HO 2 is fast compared to the formation of H 2 O via hydrogenation of atomic oxygen and hydroxyl. The observed coverage dependence of the reaction kinetics indicates the dominance of hot-atom mediated reactions.

The mechanism of thermally activated chemical erosion of sputter-deposited C:H films of a few atomic layer thickness is investigated using thermal desorption spectroscopy. Methane, CH3 radicals, and various C2Hj species of molecular and radical nature desorb as gaseous products above 600 K competitively to H2. C-CiHj bond breaking is determined to be the rate limiting step of hydrocarbon production. The reaction is of first order with respect to CiHj precursors in the films with a distribution of activation energies, 56±5 kcal/mol for methane production. CH3 radical desorption occurs predominantly from the very surface of the C:H films.