Ultrafast Study of the Photodissociation and Recombination of Aqueous O3-

The photodissociation of the strongly solvated radical O 3 - (generating O 2 and O - ) has been studied in aqueous solution by femtosecond pump−probe spectroscopy. The near-UV absorption band of O 3 - exhibits a prompt bleach due to photolysis (390 nm excitation), which is followed by a partial recovery (3.5 ps) that is assigned to delayed geminate recombination. The transient spectrum of O 3 - shows no evidence of excess vibrational energy content, indicating that vibrational relaxation of O 3 - occurs on a faster time scale than the recombination process (3.5 ps). This overall picture is in strong contrast to the standard prototype for solution recombination reactions, i.e. the widely investigated photodissociation of I 2 in nonpolar solvents. For I 2, photodissociation and prompt recombination (∼0.3 ps) lead to hot I 2 molecules that relax on the tens of picoseconds time scale. The extremely fast vibrational relaxation and the slow recombination of O 3 - in water apparently stem from strong solute−solvent interactions for both O 3 - and O - . The O 3 - behavior may in fact be a common situation for polar reactions in polar solvents. In these studies, O 3 - was generated via a known reaction by photolysis of an oxygenated basic solution of hydrogen peroxide. The quantum yield of cage escape for O 3 - photodissociation has been measured to be 0.50 ± 0.02, assuming that the long-lived bleach is due entirely to cage escape. The long-lived bleach may, however, also be due to other photophysical pathways that involve long-lived, low-lying O 3 - excited states. Both the apparent cage escape yield and recombination dynamics are identical within experimental error in H 2 O and D 2 O. The rotational reorientation time of O 3 - has been measured to be 2.3 ps.