Photoionization of polycyclic aromatic hydrocarbons in alkane solutions: “High energy” chemical pathways

Unusual “high energy” chemical pathways can result when ionizing radiation is deposited into condensed-phase systems. In photoionization it has often been surmised that all excess energy above that needed for ionization appears as kinetic energy of the ejected electron. Alternatively, some of the excess energy could remain in the radical cation, resulting in new modes of radical cation reactivity.

The multiphoton ionization of aromatic hydrocarbons (AH, e.g. anthracene) in alkane (RH, e.g. cyclohexane) solvents was carried out using intense pulsed 248 or 308 nm irradiation. Photoproducts derived from solvent radicals (e.g. bicyclohexyl) are observed, and the yields increase with increasing photon energy. Flash photolysis and transient conductivity studies were also carried out, and the relative yield of aromatic radical cations was found to decrease with increasing photon energy. Our observations suggest an alternate “high energy” pathway for aromatic radical cations. One possible pathway is proton transfer to the solvent which would lead to aryl radical formation (AH^+∗_· + RH → A + RH⁺₂). Subsequent hydrogen atom abstraction by intermediate aryl radicals (A + RH → AH + R) would then result in the observed increase in solvent-derived radicals.