Mechanism of ion formation by irradiation of molecular crystal surfaces with laser pulses

An investigation was made of ion formation as a result of irradiation of molecular crystals by ultraviolet laser pulses of moderate intensity. Formation of ions from molecules of the crystals themselves and from molecules adsorbed on the surface was studied. The mechanism of ion formation was as follows: a laser radiation pulse heated slightly the surface and this increased the rate of evaporation (desorption) of molecules during a laser pulse; the free molecules formed as a result of such detachment from the surface became ionized just above the surface via an intermediate electronic state by the same laser radiation pulse. Calculations were found to agree well with the experimental data. A method was developed for increasing significantly the sensitivity of photoionization detection of molecules by adsorption on the surface and pulsed desorption.