A high mass resolution study of the interaction of aromatic and nitro-aromatic molecules with intense laser fields

The technique of femtosecond laser mass spectrometry has been applied to benzene, nitrobenzene, toluene and nitrotoluene using pulses of 80 fs and λ = 800 nm (1014 − 1016 W cm-2). The ultrafast laser pulses used were able to largely defeat the dissociation pathways associated with nanosecond ionization and produce a molecular ion for both the aromatics and the two photounstable nitro-aromatics. The high mass resolution (m/Δm = 800) permitted, for the first time, the observation of various doubly charged species and allowed a study of the effect of the substituent NO2 group on the multiple ionization process. It was found that the femtosecond laser irradiation of benzene and toluene enabled the production of a doubly charged cation envelope in each case along with an additional doubly ionized contribution from certain lower mass fragments. Doubly ionized species were also observed for the nitro-aromatics including, most notably the loss of NO2 doubly charged ion ([M−NO2]2+) although a doubly charged parent was not observed. In addition, an NO2+ ion was detected for both nitro-aromatics which was thought to be evidence of a 'charge-separation' process involving a transient doubly charged molecular ion.