A High Mass Resolution Study of the Interaction of Aromatic and Nitro-Aromatic Molecules with Intense Laser Fields

Abstract

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 (10 14 -10 16 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/∆m ) 800) permitted, for the first time, the observation of various doubly charged species and allowed a study of the effect of the substituent NO 2 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 NO 2 doubly charged ion ([M-NO 2 ] 2+ ) although a doubly charged parent was not observed. In addition, an NO 2 + 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.

Femtosecond laser photoionization time-of-flight mass spectrometry of nitro-aromatic explosives and explosives related compounds

“…These results agree well with femtosecond measurements of Kosmidis et al [25] at 375 and 750 nm, and those of Ledingham et al [24] at 375 nm. The results of this study, and that of Tasker et al [28], differ considerably in the relative amounts of…”

“…Tasker et al [28] have also conducted a high mass resolution (m/⌬m ϭ 800) ultrafast laser ionization study on NB at 800 nm with 80 fs pulses at power densities between 10 14 to 10 16 W/cm 2 . For NB, they observe the presence of species are generated through the dissociation of the doubly charged transient molecular ion and the channels for their production should be similar to those observed for the singly charged species.…”

“…The difference in the mass spectra is most likely attributed to the difference in the power density of the experiments. Tasker et al [28] conducted their experiments at 3.8 ϫ 10 15 and 1.2 ϫ 10 16 W/cm 2 , while the current experiments were conducted at a calculated power density of 3.5 ϫ 10 14 W/cm 2 . The higher power density is thought to be responsible for the suppression of these two high-mass signals.…”

“…A singlet structure of this peak would indicate that the C 6 H 3 2ϩ species came from the dissociation of a multiply charged molecule into one ionic and one or more neutral fragments. Tasker et al [28] have previously identified this ion peak in their study of NB using 800 nm, 80 fs pulses at 1.1 ϫ 10 15 W/cm 2 and 3.0 ϫ 10 15 W/cm 2 and attributed its presence to the dissociation of the transient doubly charged molecular ion, i.e., NB 2ϩ ¡ C 6 H n 2ϩ ϩ NO 2 . The doublet structure observed in our experiment is inconsistent with this assignment, and it is unclear whether the difference in power density between the two experiments, ϳ10 15 W/cm 2 in the Tasker et al study versus ϳ10 14 W/cm 2 in the current study could account for the difference.…”

“…The first reports of ultrafast laser ionization on explosives and ERCs found that the fast rate of up-pumping provided by the femtosecond laser pulse led to an enhancement in molecular ion formation for these species as well as a suppression of the dissociative states that lead to the highly fragmented mass spectra found when nanosecond pulses were employed. These studies, conducted at 800 nm [28], 412 nm [27], 375 nm [24 -26], and 206 nm [29] found that the visible and UV wavelengths were generally better at producing the parent molecular ion. Further, the mass spectra taken at high power density in the 800 nm studies suggested the activity of Coulomb explosion (CE) channels.…”