PHOTOLYSIS OF CLNO ADSORBED ON MGO(100)

The 365 nm pulsed laser photolysis of nitrosyl chloride adsorbed on a rough MgO(100) surface at 90 K has been studied. Mass spectrometric de tection was used to record time-of-night (TOF) spectra by monitoring C l+ and NO+. These ions can derive from parent ClNO, which fragments co mpletely in the mass spectrometer, as well as from Cl and NO photofrag ments. The TOF distributions are considerably slower than for the corr esponding gas phase photodissociation process. NO was also detected st ate selectively by using resonance enhanced multiphoton ionization (RE MPI), and a channel corresponding to direct adsorbate photolysis was i dentified. The state selective detection of NO molecules that emerge f rom the surface following photolysis shows unambiguously that their ro tational degrees of freedom reflect the surface temperature (T-rot 100 -140 K), even at low coverages. At similar photolysis wavelengths, gas phase ClNO photodissociation is known to produce highly rotationally excited NO with a distinctive non-statistical distribution peaked at J '' =46.5. Our studies suggest that, contrary to the gas-phase photolys is results, Cl and NO are not ejected rapidly following photolysis of surface-bound species on a repulsive potential energy surface. We post ulate that ClNO grows in islands, with MgO defect sites Serving as nuc leation centers. Photofragment rotational and translational excitation s are quenched efficiently due to strong attractive interactions that equilibrate NO-to the surface temperature. Desorption of intact ClNO m ay also take place, but following (i.e., not during) the photolysis pu lse. Such desorbed species can contribute to the TOF spectra, but not the REMPI spectra.