Dynamical study of surface photochemistry by VUV radiation

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Photochemical reaction of the adsorbate molecules on the solid surface in a soft x-ray region is important for fundamental study of elementary chemical reactions as well as for application study of photochemical vapor deposition, etching catalysis and so on. Among these reactions, photon stimulated desorption (PSD) is a relatively simple reaction to investigate both theoretically and experimentally. From this point of view, in this project we have designed and constructed a new experimental apparatus involving an TOF ion specrtomater, Auger electron analyser and a mass spectrometer to elucidate the full mechanism of a soft x-ray photochemical reactions of the adsorbed molecules on the surfaces. Using this apparatus, we have succeeded in detecting both Auger electrons and ionic products of photon stimulated desorption in both H_2O/Si(100) and PMMA/Au/Si(100) systems. The results are summarized as follows.(1) H^+ and O^+ ions were observed in the photon stimulated ion desorption (PSID) f … More rom H_2O/Si(100) system in the range 500-700 eV. PSID yields of these ions indicate characteristic behavior near O K-edge; H^+ exhibits sharp rise at 530 eV and two broad maxima below and above O K-edge, O^+ exhibits a delayed threshold at 570 eV. These results have been discussed with the formation of multiple charged ions followed by reneutralization of the excess of charge with strong interaction with the surface.(2) Various ions involving the monomer ion were observed in PSD from PMMA/Au/Si(100) systems. Excitations of carbon 1s electron to pi^* and sigma^* resonance states at 287.9 eV show remarkable enhancement of CH^+ and CH_2^+ ions. In oxygen K-edge region, increase in COOCH_3^+ was not observed where as that in HCI^+ was observed. Some of fragment ions have been observed to occur specifically around the site of the atom where the core electron excitation takes place. Based on these observations, one might consider the use of tunable soft x-ray to stimulate chemical reactions or to selectively break large organic molecules, as one pleases. Less