The fluorescence and photofragmentation of liquid saturated hydrocarbons at energies above the photoionization threshold

Fluroescence quantum yields (φF) are reported for saturated hydrocarbons excited by photon absorption in the liquid phase at energies below and above the photoionization threshold. The quantum yields of fluorescence obtained in the subionization region are in excellent agreement with those reported by Lipsky and colleagues. In the phtotionization region, emission occurs as a result of both charge recombination processes and deactivation of the superexcited molecule to the vibrationally relaxed first excited singlet state. The presence of these two populations of fluorescing species above the ionization threshold is manifested in differences in the slopes of plots of φF as a function of energy above and below the ionization onset. Since, for saturated hydrocarbons, photofragmentation is the only nonradiative channel, the variations in the relative importances of the major modes of fragmentation have been examined as a function of energy for selected molecules. It is observed that the two main dissociative processes, H and H2 elimination, show a continuous variation with photon energy. While at the absorption threshold H2 elimination predominates, at the highest energy used in this study (11.6 eV) H atom detachment from the excited molecules (both those formed by direct photon absorption and those formed by charge recombination) is of comparable importance. Liquid phase radiolysis experiments show a ratio of H atom elimination to H2 formation which is substantially higher than that measured at the highest photon energy. The role of equilibrated excited singlet state (S1)0 in saturated hydrocarbons is examined in the context of the wavelength-dependent fluorescence and photochemical data. The results indicate that, at energies above the ionizatin thresholds for the various molecules, ion pair recombination has a probability below unity of leading to the formation of vibrationally relaxed (S1)0 emitting states. In the radiolysis of liquid C6–C10 alkanes, it is suggested that charge recombination does not always lead to the formation of vibrationally relaxed (S1)0 molecules. This is explained in terms of LET effects and a high mean energy associated with the ionization process.