Photodissociation of the Fluorene Cation: A Fourier Transform Ion Cyclotron Resonance Mass Spectrometric Study

The photodecomposition of the vapor phase fluorene cation has been studied using Fourier transform ion cyclotron resonance (ICR) mass spectrometry. Broad-band visible and ultraviolet irradiation for relatively short times (∼0.5 s) is shown to lead predominantly to sequential hydrogen atom loss, with one to five hydrogens lost. A relatively minor decomposition route for several precursor ions is the loss of H 2 (or 2H atoms). Longer irradiation times lead to various products resulting from multiple C−C bond cleavages. The “action” spectrum for the m / z 166 → 165 process reveals two major electronic absorption bands which correlate well with a previous matrix absorption spectrum of the parent ion. The C 13 H + 10 - n ( n = 0−5) species have been isolated in the ICR mass spectrometer, irradiated further, and their photodecomposition followed kinetically. Solution of the appropriate coupled differential equations that govern the kinetics has enabled fits to the experimental data. Rate constants and branching ratios for the photodecomposition of fluorene and its dehydrogenated partner ions have been determined. The photoproduct ions observed suggest rapid isomerization in some excited states with ring openings playing a possible role in certain decomposition pathways.