Photodissociation dynamics of (NO)2 on LiF(001): Characterization of vibrationally excited NO fragments

Abstract

The photodissociation of adsorbed NO dimers on LiF(001) was studied in the monolayer regime at 248 nm using resonantly enhanced multiphoton ionization (REMPI) and Fourier transform infrared (FTIR) absorption spectroscopy. Vibrationally excited NO photofragments were observed in v′=0–9. The vibrational energy distribution was found to have a maximum at v′=0 and a second region of substantial population between v′=2 and 9. The rotational and translational energy distributions of the photofragments showed no major change with vibrational excitation. By contrast, the translational energy displayed a systematic increase with increasing rotational excitation. Photodissociation at 1 ML (NO)2 coverage yielded average vibrational, rotational, and translational energies of 0.48, 0.05, and 0.13 eV, respectively, in the NO fragments. The vibrational and rotational energy distributions of the fragments were unchanged for 0.06 ML, while the translational energy increased by approximately 30% in going to this lower coverage. The angular distribution was peaked in the normal direction at both coverages. The results are interpreted in terms of an excitation/deexcitation mechanism, for which the observed energy distributions can be rationalized by assuming differing equilibrium geometries between the ground and excited states of the adsorbed dimer.