MULTIPHOTON DISSOCIATION OF ORGANIC-MOLECULES - STEP BY STEP INVESTIGATION WITH LASER TANDEM MASS-SPECTROMETRY

Multiphoton ionization of molecules is connected with strong multiphoton dissociation of the molecular ions for high enough laser intensities. By varying the laser intensity, the degree of dissociation can be tuned from very soft (no fragmentation) to extremely hard. In earlier work we proposed and proved the "ladder-switching" model to explain this process for excitation with nanosecond laser pulses. Recently, we developed a new laser tandem mass spectrometric technique which allows to investigate the fragmentation paths due to the "ladder-switching" mechanism step by step. We applied this method to benzene cations and could verify the proposed tree of fragmentation pathways. Also a wavelength dependence of the fragmentation pattern has been detected and explained. Our new technique includes (i) a two stage ion source with laser ionization and second order space focus, (ii) secondary laser excitation and dissociation in this space focus and (iii) a reflectron time-of-flight analyzer.