PHOTOIONIZATION AND ENSUING ION MOLECULE REACTIONS OF POLYCYCLIC AROMATIC-HYDROCARBONS IN ALKANE AND ALCOHOL-SOLUTIONS

A common reaction pathway involving ion-molecule reactions of excited ions is indicated for a series of polycyclic aromatic hydrocarbons photoionized using intense pulsed excimer lasers (248 and 308 nm). The overall yields of free electrons and free aromatic radical cations created by two-photon ionization are measured by laser flash photolysis in cyclohexane and in 2-propanol solutions. The electron yields increase with increasing photon energies and photon flux, but the yield of radical cation is found to decrease with increasing photon flux. Photoproducts derived from aryl and solvent radicals are observed and quantified by GC and GC-MS, and their yields increase with increasing photon energy and photon flux. The diminished yield of radical ions at higher photon flux suggests an alternate higher energy reaction pathway for the radical cation. While aryl radical cations are relatively unreactive, an excited aryl radical cation could protonate the surrounding solvent, leading to aryl radical formation (AH.+* + RH --> A. + RH2+). Solvent radical formation would then result from hydrogen atom abstraction by aryl radicals. Product studies support this ion-molecule reaction pathway and exclude alternate mechanisms involving neutral excited state homolysis.