VUV PEAKS IN ABSORPTION-SPECTRA AND PHOTOION YIELD CURVES OF POLYCYCLIC AROMATIC-HYDROCARBONS AND RELATED-COMPOUNDS

Photoion mass spectrometry (PIMS) was used to obtain the parent and fr agment photoion yield curves in the 8-35 eV range of 25 aromatic hydro carbons, mainly polycyclic aromatic hydrocarbons (PAHs) and related co mpounds. An intense peak, also observed in the absorption spectrum, oc curs 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 photophy sics of these carbonaceous species. The general behaviour of the ion y ield curves as a function of photon energy is modelled for the prototy pe PAH benzene using data on the partial cross-sections for photoioniz ation of this species. The appearance energies of fragment ions and th e corresponding VUV peak profiles provide information about fragmentat ion 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 cons titutes a new diagnostic use of ion yield curves. Published by Elsevie r Science B.V.