Experimental energy spectra and energy-resolved angular distributions of photoelectrons produced by multiphoton ionization of atomic hydrogen are presented for several wavelengths between 596 and 630 nm, peak intensities of about 10 W/cm, and laser pulse durations of about 0.5 ps.Resonant ionization processes via ac Stark-shift-induced resonances with excited states dominate the spectra. The results of Sturmian-Floquet calculations of these spectra and angular distributions are presented; they are in fair agreement with the experimental data.PACS number(s): 32.80.Rm, 42.50.Hz
Multiphoton ionization of atomic hydrogen is investigated experimentally in the short-pulse regime (T< 0.5 ps) at wavelengths around 600 nm and intensities up to 10 14 W/cm 2 . Sharp peaks in the energy spectrum of the photoelectrons can be attributed to resonant ionization processes via excited states which are ac-Stark shifted into resonance. Energies and angular distributions of the electrons allow one to identify some participating excited states with quantum numbers n >: 4 and / > 3. PACS numbers: 32.80.Fb, 32.80.Rm 2 4 57 1 1 II 1 1 II J
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.
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