A subpicosecond pump-probe laser study of ionization and geminate charge recombination kinetics in alkane liquids

We report on time resolved optical pump-probe investigations of the electron-cation recombination in alkane liquids. The alkanes were excited via two-photon excitation, using intense sub-picosecond pulses at 266 nm. Adding an electron scavenger to the liquid and probing the transient absorption at three different wavelengths (800 nm, 1500 nm, and 2250 nm) made it possible to distinguish the transient absorption due to excess electrons from that due to other absorbing species. The experimentally observed charge-recombination kinetics in n-hexane and n-octane could be fairly well reproduced by computer simulations in which the initial electron thermalization distance distribution was taken to be f(r)r²dr=(1/b)exp(-r/b)dr. Other distributions, such as a Gaussian, gave unsatisfactory results. The average electron thermalization distance in n-hexane was found to be 35+ 5 A and in n-octane it was found to be 70±10 Å. The results for isooctane could be described either by the distribution (1/b)exp(-r/b)dr with an average thermalization distance of 25±5 Å or by a Gaussian distribution with an average thermalization distance 50±10 Å. The transient absorption in n-hexane and isooctane was observed to exhibit a red shift during the first 2 ps after the onset of the absorption. This spectral relaxation could be due to the slowing down of the ejected electron to thermal energies.