365-NM PHOTON-INDUCED DYNAMICS OF CINO ADSORBED ON MGO(100)

Temperature programmed desorption (TPD) and 365 nm photolysis of ClNO adsorbed on MgO(100) at 90 K were investigated under ultrahigh vacuum conditions. The crystal was treated in a way that largely eliminated o xygen vacancies and yielded a relatively smooth surface. Angularly res olved time-of-flight (TOF) mass spectra and state-selective resonance- enhanced multiphoton ionization (REMPI) spectra of NO photoproducts we re obtained. The TPD data indicate that ClNO desorbs at surface temper atures above 160 K for exposures (Theta) below 0.2 Langmuirs (L), whil e for higher values of Theta the main desorption peak is near 120 K. T he higher temperature feature, which saturates at Theta similar to 0.3 L, is probably associated with binding to defect sites. Thermal desor ption is believed to be molecular at ail coverages. irradiation at 365 nm for 0.1 less than or equal to Theta less than or equal to 5.0 L yi elds products having low average translational energies and broad tran slational energy distributions. NO fragment REMPI spectre were recorde d at Theta greater than or equal to 0.7 L. The rotational distribution s could be fit with a temperature of 110+/-10 K, i.e., comparable to t hat of the substrate. These results differ from those obtained in the photodissociation of gas-phase ClNO, where the NO fragment has high tr anslational and rotational energies. However, the present results are similar to those obtained on rougher MgO(100) surfaces. Possible mecha nisms are discussed.