Multiphoton ionization mass spectrometry of small biomolecules with nanosecond and femtosecond laser pulses

This paper reports on the multiphoton ionization and dissociation (MPID) processes that occur in gas phase dipeptides excited with femtosecond and nanosecond laser pulses. The MPID products of the compounds tyrosyl-tyrosine, tyrosyl-leucine, and valyl-valine resulting from excitation with 5 ns and 250 fs laser pulses at 266 nm have been analysed by time-of-flight mass spectrometry. There are two important conclusions from this study. First, ion fragmentation patterns depend on the pulse length of laser radiation used to excite the neutral molecule. Photofragmentation is generally less extensive and more tuneable with femtosecond pulses compared with nanosecond pulses. Second, the efficiency of the ionization with femtosecond pulses seems relatively insensitive to the presence or absence of a chromophore in the systems we have studied; this is not true for nanosecond pulses. This suggests that although coherent processes clearly play little part in ionization with nanosecond pulses, these effects become significantly more important as the time scale of the ionization reaches the subpicosecond regime. (Int J Mass Spectrom 176 (1998) 77-86) (C) 1998 Elsevier Science B.V.