Ultrafast photoionization dynamics of indole in water

indole in aqueous solution is photoionized near threshold following single photon absorption from a femtosecond laser pulse at 260 nm. Transient absorption measurements are performed using a white-light continuum probe pulse. Excited state absorption of neutral indole molecules is characterized accurately in I-propanol when photoexcitation at 260 nm does not lend to photoionization. The presence of 0.75 M carbon tetrachloride in a solution of indole/l-propanol leads to the formation of indole radical cations on a picosecond time scale. While solvated electrons are formed in aqueous indole within our time resolution of 200 fs, measurements of the transient absorbance out to 100 ps are flat and indicate that geminate recombination is insignificant on this time scale. This result contrasts sharply with the geminate recombination dynamics observed following the photoionization of neat water. This indicates that the bimolecular reaction between indole radical cations and solvated electrons is considerably slower than the diffusion limit. We suggest that geminate recombination arising from solute photoionization in polar solvents may be slower than previously thought.