Photochemistry of NH3 on Pt(111): Ejection of thermalized atomic hydrogen from ammonia multilayers

Abstract

The dissociative 193 nm photochemistry of both multilayer and submonolayer NH3 on Pt(111) has been studied. Upon irradiation with 193 nm light, the submonolayer covered Pt surface yields only a small amount of NH3 desorption. In contrast, the desorption products from the 193 nm irradiation of a multilayer NH3 covered Pt surface are both NH3 and atomic H. The NH3 photodesorption yield from the multilayer is much greater than from the submonolayer covered surface. A nearly field-free resonantly enhanced multiphoton ionization technique was used to carefully distinguish between H and NH3 photoproducts. From the multilayer, NH3 desorbs with a distinct bimodal hyperthermal velocity distribution. The atomic H velocity distribution exhibits both a hyperthermal component and, surprisingly, a ∼150 K thermalized component. Our multilayer data is consistent with adsorbate absorption, and we have proposed a model based on multiple collisions of desorbing species to explain our results. In this model, NH3 molecules at the surface photodissociate and directly eject H atoms into the gas phase with a hyperthermal kinetic energy. NH3 molecules buried within the multilayer also undergo photodissociation but their ejected H atoms suffer multiple collisions, losing kinetic energy and becoming thermalized prior to desorption into the gas phase.