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.

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Femtosecond laser time-of-flight mass spectra of solid samples of trinitrobenzene (TNB), trinitrotoluene (TNT) and trinitrophenol (TNP) have been recorded. Desorption of the solid samples was enacted by the fourth harmonic output (266 nm) of a 5 ns Nd:YAG laser. Subsequent femtosecond post-ionisation of the plume of neutral molecules was achieved using 800 nm laser pulses of 80 fs duration. Mass spectra have been recorded for desorption laser intensities from 2-6 x 10(9) W cm(-2) with ionisation laser intensities between 2 x 10(14) and 6 x 10(15) W cm(-2). Femtosecond laser ionisation has been shown to be capable of generating precursor and characteristic high-mass fragment ions for labile nitro-aromatic molecules commonly used in high-explosive materials. This feature is critical in the future development of femtosecond laser-based analytical instruments that can be used for complex molecular identification and quantitative analysis of environmentally important labile molecules. Furthermore, a comparison of femtosecond post-ionisation mass spectra with standard 70 eV electron impact data has revealed similarities in the spectra and hence the fragmentation processes.