ON THE FRAGMENTATION OF NITROBENZENE AND NITROTOLUENES INDUCED BY A FEMTOSECOND LASER AT 375 NM

The photodissociation of nitrobenzene and the nitrotoluene isomers at 375 nm, induced by a 90 femtosecond laser, is analyzed and compared wi th the fragmentation by a 10 nanosecond laser at the same wavelength, The molecular ionization is attributed to a nonresonant multiphoton pr ocess, and the observed fragmentation can be explained predominantly b y an above ionization mechanism (ladder climbing). The mass spectra of the three nitrotoluene isomers show differences which can be used for analytical purposes. The molecular rearrangement taking place prior t o the dissociation is also discussed. For nitrobenzene, it is suggeste d that most of the dissociation occurs from the nitrobenzene structure rather than that of phenyl nitrite. In the case of o-nitrotoluene, it seems that the hydrogen transfer from the -CH3 to the NO2 group (orth o effect) is favored in ionic states, while the rearrangement to a nit rite structure is possible in the excited electronic states.