Liquid n-alkanes (from CS to Cia), as well as selected cyclic and branched alkanes, were irradiated with 0.4-ps pulses of 10 GW/cm2 intensity at 248.5 nm. Transient absorption signals at 497 nm were recorded in pumpand-probe experiments. The observed time profiles are attributed to the result of UV two-photon excitation.The properties of these profiles appear incompatible with the assumption of ultrafast (subpicosecond) recombination of geminate electron-ion pairs. Instead, slower time scales for geminate electron-hole recombination are determined, such as those derived from pulse radiolysis scavenger studies. An ultrafast component of the absorption decay is attributed to fragmentation from higher excited singlet states, which competes with internal conversion into the SI state of the parent molecules.

Chemical Kinetics J Ionization / Liquids J Multiphoton Processes J Solvated ElectronsDecomposition and ionization reactions of neat liquid alkanes, alcohols, water, and halomethanes after UV-two-photon absorption were investigated by pump-and-probe measurements with subpicosecond time resolution. Previous suggestions are revised and new mechanisms proposed which consistently explain the transient absorption signals obtained. In particular, it is shown that the importance of free or "quasi-free'' electrons as intermediates in the photodecomposition of liquids has commonly been largely overestimated.

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Fast processes occurring in liquid alkanes upon radiolytical or optical excitation have been under investigation for many years. However, pulse radiolytical experiments have so far been limited to time scales larger than 10 ps and the only two-photon laser-experiments with subpicosecond resolution [1] have been performed at excitation energies below the ionisation threshold of alkanes. The attempted interpretation of the transient absorption signals in [1] as due to geminate electron-hole recombination therefore has to be rejected.