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 with the fragmentation by a 10 nanosecond laser at the same wavelength. The molecular ionization is attributed to a nonresonant multiphoton process, and the observed fragmentation can be explained predominantly by 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 to the dissociation is also discussed. For nitrobenzene, it is suggested 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 (ortho effect) is favored in ionic states, while the rearrangement to a nitrite structure is possible in the excited electronic states.