FEMTOSECOND VERSUS NANOSECOND MULTIPHOTON IONIZATION AND DISSOCIATIONOF LARGE MOLECULES

Under nearly identical conditions, the difference between 500-fs and 5 -ns laser pulse ionization and dissociation of small volatile molecule s and large laser-desorbed polypeptides in the gas phase is demonstrat ed. By applying in each experimental cycle almost simultaneously but i ndependently both femtosecond and nanosecond laser pulses for photoion ization, reproducible intensity relations in the mass spectra have bee n achieved even for laser-desorbed molecules. For femtosecond and nano second laser pulse excitation, a large laser intensity difference resu lts for equal laser pulse energy. Nevertheless, we observe the ion yie ld to be nearly the same for small molecules. For large molecules femt osecond excitation is found to be much more efficient than nanosecond excitation. We explain this by fast relaxation rates which can compete either with photon absorption or with ionization. The fragmentation o f large ions demonstrates a most important difference between the two modes of excitation. Femtosecond excitation shows large fragmentation near the photoexcited chromophores, while nanosecond excitation produc es statistical energy redistribution and thus a lot of different fragm ents. This lack or presence of internal energy randomization in the sa me molecular system has consequences for future applications in site-s pecific bond-breaking processes and for the unimolecular rate theory o f very large molecular systems.