Nonthermal desorption of molecular ions of polyatomic molecules induced by uv laser radiation

The paper presents the results of studies into ion desorption from the surface of adenine and anthracene molecular crystals and anthracene molecules adsorbed on a metal as the surface is irradiated by an uv laser at λ=249, 308, and 337 nm. A sharp increase in molecular ion yield has been revealed with a decrease in radiation wavelength down to 249 nm. At this wavelength the effect of molecular ion desorption is observed at the radiation intensity of 20∶50 kW/cm² and pulse duration of 20 ns, it cannot be explained by thermal mechanisms. The initial velocity distribution of photoions is nonthermal by character, with maximum velocity of 2·10⁵ cm/s in case of adenine. The mechanisms responsible for the effect observed are discussed.