Velocity and internal state distributions of photodesorbed species from N2O/Pt(111) by 193 nm light

Abstract

Polarized ultraviolet light from an excimer laser (193 nm) was used to photodesorb and photodissociate N2O adsorbed on a cold (80 K) Pt(111) surface. The photodesorbed species and their time of flight (TOF) were monitored by resonantly enhanced multiphoton ionization (REMPI) spectroscopy. We have identified three major channels. The photodesorption of molecular N2 is observed only in the slowest channel where N2 produced by fragmenting the N2O is thermalized on the surface before desorbing. Evidence for this behavior includes both low (∼90 K) rotational and translational temperatures of the N2 fragments as well as a lack of correlation between rotational and translational energy. In the next fastest channel, hyperthermal N2O with a kinetic energy of 0.4±0.1 eV is seen to photodesorb. The photodesorbed hyperthermal N2O also has a substantial degree of internal vibrational excitation. The angular distribution of the N2O channel is peaked toward the surface normal. In the fastest channel, the release of ballistic oxygen atoms, a prompt axial recoil with no collisions with neighboring adsorbates, is seen along the tilted N2O molecular bond axis. The ballistic oxygen atoms leave the surface either in the ground state O(3P) or in the first electronically excited state O(1D). The kinetic energy of the O(3P) and of the O(1D) photoproducts is similar (0.5 eV) suggesting a common dissociative intermediate. In all of the channels observed, the dependence of the photoproducts yield on the polarization of the photodesorption laser indicates a hot carrier mediated mechanism at the surface. We propose a dissociative electron attachment model to explain the photochemistry of N2O/Pt(111) with 193 nm light.