Ionization and fragmentation of small molecules under intense short pulse laser radiation

Abstract

The interaction of OCS, CO/sub 2/, CH/sub 3/I and CCl/sub 4/ with intense short pulse duration laser radiation is studied experimentally using time-of-flight mass spectroscopy. Laser pulses of 248 nm at 9 and 0.5 psec, 496 nm at 5 and 0.5 psec, 800 and 400 nm at 50 fsec are used in order to investigate the effect of the wavelength and the pulse duration on the molecular ionization and fragmentation. The fragmentation channels were investigated in detail and as was found, for low incident intensities the ionization is more important since the parent molecule peak is the most pronounced mass peak for all molecules under investigation. Increasing the intensity, the ionization is still more pronounced for the long wavelengths (497 nm and 800 nm) whereas, at short wavelengths (248 nm and 400 nm) the fragmentation of the molecule is preferable, the effect becoming more important for longer laser pulses. At very high intensities, extending fragmentation was observed in all cases. Multiply charged fragments appeared to be of great importance, their corresponding mass peaks being split due to Coulombic explosion. In addition, multiply charged parent ion peaks were observed for the 800 nm radiation. Finally, the emitted fragments were found to be preferentially aligned along or perpendicular to time-of-flight axis, the alignment being more pronounced with increasing charge of the fragment.