ULTRAFAST STUDY OF THE PHOTODISSOCIATION AND RECOMBINATION OF AQUEOUSO-3-

The photodissociation of the strongly solvated radical O-3(-) (generat ing O-2 and O-) has been studied in aqueous solution by femtosecond pu mp-probe spectroscopy. The near-UV absorption band of O-3(-) exhibits a prompt bleach due to photolysis (390 nm excitation), which is follow ed 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 relaxat ion of O-3(-) occurs on a faster time scale than the recombination pro cess (3.5 ps). This overall picture is in strong contrast to the stand ard prototype for solution recombination reactions, i.e. the widely in vestigated photodissociation of I-2 in nonpolar solvents, For I-2, pho todissociation and prompt recombination (similar to 0.3 ps) lead to ho t 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 si tuation for polar reactions in polar solvents. In these studies, O-3(- ) was generated via a known reaction by photolysis of an oxygenated ba sic solution of hydrogen peroxide. The quantum yield of cage escape fo r O-3(-) photodissociation has been measured to be 0.50 +/- 0.02, assu ming that the long-lived bleach is due entirely to cage escape. The lo ng-lived bleach may, however, also be due to other photophysical pathw ays that involve long-lived, low-lying O-3(-) excited states. Both the apparent cage escape yield and recombination dynamics are identical w ithin experimental error in H2O and D2O. The rotational reorientation time of O-3(-) has been measured to be 2.3 ps.