Ultraviolet laser photodesorption of NO from condensed films: Translational and internal energy distributions
In this paper we report that ultraviolet laser induced desorption from the surface of a thin nitric oxide film proceeds via two mechanisms which are present simultaneously. One mechanism is attributed to laser induced thermal desorption while the other is due to a nonthermal, single photon process. A film of 1-2000 ML (layers) of NO condensed on a Ag(111) substrate under UHV conditions at 25-50 K was irradiated by 5 ns pulses of 220-270 nm laser light (4.6-5.5 eV) with 0.5-5 mJ/cm2 (0.1-1 MW/cm2 ) power density at the surface. Translational energies of desorbed molecules were measured from time-of-flight (TOF) spectra taken with a mass spectrometer, while the internal energy distribution of molecules desorbed in the nonthermal channel was determined by a (1+1) resonance enhanced multiphoton ionization (REMPI) probe. NO monomer in the 2Π3/2,1/2 electronic ground states was the only significant product. There were two distinct characteristic TOF components, which we associate with different desorption mechanisms. Each component had a different velocity and angular distribution, and their relative yields varied with laser pulse energy and NO layer thickness. Under conditions where both mechanisms gave comparable desorption yields, we obtained TOF distributions which were bimodal. A ``slow'' peak with an average translational energy up to 0.06 eV was Maxwellian with temperatures between 160 and 280 K and a broad angular distribution. Yield in this peak increased strongly with layer thickness and exponentially with laser pulse energy. A ``fast'' TOF peak with average energy of 0.22 eV was non-Maxwellian, with an angular distribution peaked toward normal, and yield increasing linearly with laser pulse energy. REMPI of the fast peak showed a vibrational population ratio v=3:v=2 of 0.85. A Boltzmann plot of the rotational population distribution of v=2 molecules, if fit with a single line, gave a temperature of 2500 K. We use these angular, velocity, rotational, and vibrational distributions to suggest mechanisms for the nonthermal desorption. We also discuss factors determining the relative extent of thermal and photochemical effects, which control the morphology of ablated surfaces.