Dissociative desorption from CH3I by ultraviolet-laser radiation

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Abstract
CH3 I molecules condensed in thick layers are exposed to pulsed UV-laser radiation at a wavelength of 266 nm. Atoms and molecules emitted from the surface are detected after a flight path by species and state selective resonant multiphoton ionization combined with a time-of-light mass spectrometer. This detection method is tested by photodissociation experiments of CH3 I in the gas phase. The two dissociation channels known from gas phase dissociation, yielding I(2 P3/2 ) and I*(2 P1/2 ) are identified in the velocity distribution of CH3 fragments desorbed from the surface. At low UV fluences this ‘‘direct’’ dissociative photodesorption dominates. Its similarity with gas phase photodissociation is studied in some detail. At UV fluences above 1.5 mJ/cm2 a strong increase of the overall desorption yield is observed and the composition of desorbed species changes. The process becomes similar to UV-laser photoablation of polymers.