Tert-butyl nitrite surface photochemistry: The transition from submonolayer to multilayer behavior

The photochemistry of tert-butyl nitrite, (CH₃)₃CONO, adsorbed on Ag(111) at 355 nm has been studied using time-of-flight and reflection adsorption infrared spectroscopy (RAIRS) for coverages ranging from 0.1 to 50 ML. For all coverages a thermal and hyperthermal NO desorption distribution is observed due to photodissociation. The thermal component originates from hyperthermal NO molecules from the first and second layer, which were trapped by the silver substrate. The cross section for photodissociation is independent of coverage and matches the gas phase value. This is consistent with a direct excitation into the S₁ state. For coverages exceeding 5 ML the outermost layers still dissociate, but RAIRS shows that in the more inward layers caging is preventing dissociation. Due to caging only isomerization of tert-butyl nitrite is allowed, as is evident from the RAIRS.