The presolvated electron in water: Can it be scavenged at long range?

Femtosecond transient absorption measurements were carried out to investigate the scavenging effect, at low concentration, of the radiosensitizing molecule 5-bromo-2'-deoxyuridine (BrdU) on presolvation states of the hydrated electron. Hydrated electrons were generated by two photon absorption of bulk water at 267 nm, and transient absorption at 1200 nm was monitored. Results from experiments performed with the sample in a 5-mm cell showed a dramatic BrdU concentration dependence strongly suggestive of dynamic scavenging of the presolvated electron observable at concentrations as low as 0.2 mM, but no scavenging was detected in experiments on a thin sample jet with BrdU concentrations of up to 10 mM. Further analysis reconciled these results, confirming the absence of scavenging and attributing the concentration effect in the cell measurements to a group-velocity-dispersion-induced concentration dependence of the temporal response function. However, low concentrations of BrdU were seen to enhance the generation of hydrated electrons for excitation at both 267 and 3 10 nm.