PNNL

Abstract

The thermal and photochemistry of t-butyl iodide (t-buI) dosed at 100 K on rutile TiO2 (110) has been studied using isothermal (ISOMS) and temperature programmed desorption (TPD) mass spectrometry. Non-dissociative adsorption and desorption dominate the thermal behavior with dose-dependent t-buI desorption peaks at nominally 220 and 150 K. Ultraviolet photochemistry occurs readily but the behavior of submonolayers and multilayers differ qualitatively. Ejection of t-buI and i-C4H8 dominate during submonolayer photolysis at 100 K. Multilayer photolysis results are also dominated by ejection during irradiation but the t-buI component is strongly suppressed, and the maximum rates of i-C4H8 and HI ejection did not occur at the outset. A mechanistic model capturing the observations involves both direct and substrate-mediated electronic excitation t-buI. According to this model, ejection of t-buI occurs only from transient substrate-mediated formation of anionic t-buI. For either excitation path, the C-I dissociation probability is significant and the excited t-butyl product rearranges readily to form i-C4H8 that is ejected. For any local region where there is multilayer coverage, products formed at the adsorbate-vacuum interface are ejected promptly but products formed within the multilayer are trapped. Thus, ejection of t-buI is suppressed and trapped primary photodissociation products, t-butyl and I, react to either reform t-buI or rearrange to i-C4H8 and HI. The latter two products remain trapped and are subsequently induced to desorb by acquisition of momentum from collisions with subsequently formed translationally excited photodissociation products.