Coincidence studies of the slow dissociation reactions of benzene monocations in the 21-35 eV photon energy range

Metastable ion transitions are investigated, with benzene as an example, using reflectron mass spectrometry and synchrotron radiation. Photoelectron-photoion coincidence (PEPICO) spectra contained two types of metastable peaks, m^* and m^**. These are shown to be due to dissociation in the dirft tube and the electrostatic mirror section respectively through the use of double and triple coincidence measurements between photoelectrons, photoneutrals and photoions. The combined results enabled precursor ions to be assigned in both single-step and sequential fragmentations. The results confirmed previous analyses of benzene metastable transitions induced by electron excitation. The evolution of product ion yields, and the ratio of metastable to fast dissociation reactions, as a function of photon excitation energy are rationalized in terms of the internal energy deposited in the parent cation.