Polarized ultraviolet light from an excimer laser (193 nm) was used to photodesorb and photodissociate N₂O and NH₃ adsorbed on a cold Pt(111) surface. The photodesorbed species and their time-of-flight (TOF) were monitored by Resonantly Enhanced Multiphoton Ionization spectroscopy. For N₂O, we have observed both the ejection of ballistic O atoms and the release of slow thermalized N₂ molecules. The ballistic oxygen atoms leave the surface either in the ground state O(³P) or in the first electronically excited state O(¹D). A lobular angular distribution pointing away from the surface normal was measured for the ballistic O(³P) in agreement with a binding geometry where the linear N₂O is tilted N-end down on the Pt(111) surface. Evidence for the production of N₂ photofragments thermalized by the surface includes both low (approximately 90 K) rotational and translational temperatures of the N₂ as well as a lack of correlation between rotational and translational energy. For NH₃ the irradiation of a submonolayer coverage largely favors the desorption rather than the dissociation of NH₃. For multilayer coverages however, a strong dissociation channel is activated and atomic H is seen to desorb from the surface. A bimodal distribution was found for the TOF of the H photofragments. The fastest channel (0.7 eV) corresponds to the ejection of undeflected ballistic H fragments produced near the surface. For bulk NH₃ photodissociated deep within the multilayer, the TOF of the H radicals shows an unexpected thermalized distribution.

Date Published: 25 September 1995
PDF: 12 pages
Proc. SPIE 2547, Laser Techniques for Surface Science II, (25 September 1995); doi: 10.1117/12.221477

Published in SPIE Proceedings Vol. 2547:
Laser Techniques for Surface Science II
Janice M. Hicks; Wilson Ho; Hai-Lung Dai, Editor(s)