Stimulated desorption of neutral molecules as a real-time probe of surface kinetics: Methanol adsorption and decomposition on Ni(110)

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Abstract
The adsorption and time-resolved decomposition of methanol on Ni(110) has been studied by stimulated desorption using a pulsed Ar+ beam at 2 keV, followed by laser ionization at 193 nm and reflecting time-of-flight (TOF) mass spectrometry of the desorbed neutral species. Total ion doses are kept well below damage thresholds. Because ion (or electron) stimulated desorption is a nonthermal process, surface chemical changes can be directly monitored even for species not subject to thermal desorption. Since a complete induced-desorption TOF mass spectrum is collected for each laser pulse, real-time measurements are possible. At 170 K methanol adsorption occurs with an initial probability of 0.95 to 1.0 which remains constant until ∼0.8 of saturation. For an initial coverage of 0.2 of saturation, decomposition of molecularly adsorbed methanol during temperature programmed reaction occurs in two steps: deprotonation, which occurs between approximately 140 and 240 K, and methoxy decomposition to adsorbed carbon monoxide and hydrogen, which occurs between 240 and 270 K.

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