The dissociative 193 nm photochemistry of both multilayer and submonolayer NH3 on Pt(lll) 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 similar to 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.