VUV peaks in absorption spectra and photoion yield curves of polycyclic aromatic hydrocarbons and related compounds

Abstract

Photoion mass spectrometry (PIMS) was used to obtain the parent and fragment photoion yield curves in the 8–35 eV range of 25 aromatic hydrocarbons, mainly polycyclic aromatic hydrocarbons (PAHs) and related compounds. An intense peak, also observed in the absorption spectrum, occurs in the 16–18 eV region in the parent monocation yield curves and at similar or higher energies for fragment ions. Its origin, shape and width are discussed in terms of the electronic structure and photophysics of these carbonaceous species. The general behaviour of the ion yield curves as a function of photon energy is modelled for the prototype PAH benzene using data on the partial cross-sections for photoionization of this species. The appearance energies of fragment ions and the corresponding VUV peak profiles provide information about fragmentation mechanisms. In particular, the double peaks observed in the (M - 3) + ion yield curves for several species are interpreted as resulting from sequential mechanisms in the production of these ions. This constitutes a new diagnostic use of ion yield curves.