MULTIPHOTON IONIZATION OF MOLECULES - A COMPARISON BETWEEN FEMTOSECOND AND NANOSECOND LASER-PULSE IONIZATION EFFICIENCY

Multiphoton ionization mass spectra of nonvolatile molecules laser des orbed into a supersonic beam are recorded. It is shown by indirect mea surements that the laser desorption of neutrals is not mass limited, b ut lead to the formation of neutrals with intensities large enough for intense signals. To investigate the efficiency of the multiphoton ion ization process with varying laser pulse durations, simultaneous laser pulses of 500 fs and 5 ns or 100 fs and 5 ns have been applied to the neutral beam. The energies of both femtosecond and nanosecond laser p ulses are held in a comparable magnitude, and thus produce, in the res ulting ion intensity, very large differences up to 4 orders of magnitu de. For larger evaporated molecules (> 500 u) the ionization efficienc y from nanosecond laser pulses drops significantly in comparison to fe mtosecond laser pulse excitation. A variety of possible reasons for th e different ionization and dissociation behavior in femtosecond and na nosecond laser pulse excitations are discussed in this paper. It is ra tionalized that even with very short laser pulses and large molecules the ''ladder switching model'' for ionization and fragmentation is val id.