FEMTOSECOND PHOTOLYSIS OF CLO2 IN AQUEOUS-SOLUTION

The photolysis of aqueous ClO2 has been studied with a new femtosecond transient absorption spectrometer, allowing absorbance changes as sma ll as Delta A approximate to 1 x 10(-4) to be recorded with a time res olution of 150 fs. ClO2 was photolyzed at 390 nm and the ultrafast for mation and decay of photoproducts were monitored at 260, 390, and 780 nm. As expected from earlier studies, Cl atoms are formed with a quant um yield of Phi(Cl) = 0.1. However, the rate of formation is nearly 2 orders of magnitude higher than that reported. Moreover, Cl is the onl y photoproduct that survives 25 ps after the photolysis pulse. The mai n photolytic products, ClO + O, formed with a quantum yield of 0.9, di sappear through fast germinate recombination, producing vibrational ex cited ClO2 in the electronic ground state. The vibrational relaxation of this species occurs with a time constant of 10 ps. The vanishing yi eld of cage escape for ClO + O, which contrasts with the reported resu lt of the photolysis at 355 nm, indicates that the amount of excess en ergy imparted to these products at 390 nm is insufficient to enable th em to separate. The decay of a photoinduced dichroism observed at 390 nm is interpreted as an orientational relaxation of ground-state ClO2, the time constant (0.6 ps) agreeing with that calculated from the hyd rodynamical slip model.