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Femtosecond ionization and dissociation of laser desorbed nitro-PAHs

Tasker, A.D. and Robson, L. and Ledingham, K.W.D. and McCanny, T. and McKenna, P. and Kosmidis, C. and Jaroszynski, D.A. (2003) Femtosecond ionization and dissociation of laser desorbed nitro-PAHs. International Journal of Mass Spectrometry, 225 (1). pp. 53-70. ISSN 1387-3806

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Abstract

Mass spectra of six nitro-PAHs, 5-nitroacenaphthene, 9-nitroanthracene, I -nitropyrene, 3-nitrofluoranthene, 6-nitrochrysene, and 3-nitrobenzanthrone, have been investigated using laser desorption/femtosecond laser mass spectrometry (LD/FLMS). A prominent parent ion was observed for each molecule along with the structurally-characteristic [M - NO](+) and [M - NO2](+) fragments. The consistent observation of the [M - NO](+) and [M - NO - CO](+) ions, in the mass spectra of all the nitro-PAHs, along with the presence of certain doubly charged fragments, is thought to be indicative of a molecular rearrangement. Although this photorearrangement may be occurring within the pulse duration (<80 fs), it is thought that it is more likely to be taking place within the low intensity regions of the laser pulse. In addition to this, an abundance of doubly charged polyatomic ions were observed for the first time in the mass spectra of these molecules under laser irradiation. It was found that only the two smallest molecules (5-nitroacenaphthene and 9-nitroanthracene) were able to generate an observable dication, with the highest-mass doubly charged species corresponding to the [M - NO2](2+) fragment in the remaining molecules. An investigation has also been conducted into the effect of varying the molecular structure of the analyte and the position of the focussed beam. It was found that variation of molecular structure had little effect on the observed fragmentation pathways. However, movement of the focussed beam was found to exert a considerable influence over the observed mass spectra.