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 el ectron-cation recombination in alkane liquids, The alkanes were excite d via two-photon excitation, using intense sub-picosecond pulses at 26 6 nm. Adding an electron scavenger to the liquid and probing the trans ient absorption at three different wavelengths (800 nm, 1500 nm, and 2 250 nm) made it possible to distinguish the transient absorption due t o excess electrons from that due to other absorbing species. The exper imentally observed charge-recombination kinetics in n-hexane and n-oct ane could be fairly well reproduced by computer simulations in which t he initial electron thermalization distance distribution was taken to be f(r)r(2)dr = (1/b)exp(-r/b)dr. Other distributions, such as a Gauss ian, gave unsatisfactory results. The average electron thermalization distance in n-hexane was found to be 35+/-5 Angstrom, and in n-octane it was found to be 70+/-10 Angstrom. The results for isooctane could b e described either by the distribution (1/b)exp(-r/b)dr with an averag e thermalization distance of 25+/-5 Angstrom or by a Gaussian distribu tion with an average thermalization distance 50+/-10 Angstrom. The tra nsient absorption in n-hexane and isooctane was observed to exhibit a red shift during the first 2 ps after the onset of the absorption. Thi s spectral relaxation could be due to the slowing down of the ejected electron to thermal energies. (C) 1997 American Institute of Physics.