Titanium dioxide layers, composed of 5 nm diameter closely packed nanocrystallites prepared by spin coating of concentrated TiO 2 sols (titanium isopropoxide hydrolysis), were exposed to pulsed laser photolysis, in the presence as well as in the absence of added reactants. Time profiles in the range 390-700 nm have been studied in the nanosecond time range. TiO 2 layers immersed in liquids (acidic or alkaline water, CCl 4 , CCl 4 / CBr 4 mixture, cyclohexane) show the same absorption vs time profiles as the dry layers. Iodide ions (0.5-7.6 M in water) convert the holes to I 2 -within less than 10 ns (quantum yield approaching unity is observed at the highest concentration). The absorption of I 2 -(peaking at 390 nm) is relatively stable during the first 4 µs, in contrast to the decay of the electron absorption which is only slightly different than in iodide-free solutions. This result is unexpected if the decay of the electron absorption is because of electron-hole recombination. Alcohols (methanol and 2-propanol) at high concentrations unexpectedly reduce the initially observed electron absorption (time resolution 10 ns) by up to 4-fold, without affecting the shape of the nanosecond time profile. The alcohol effect is assigned to formation of an alcoholic positive ion radical which is more reactive in recombination with conduction band electrons than the original hole. The electron scavenger H 2 O 2 reduces the initial electron absorption without affecting the shape of the nanosecond time profile. It is concluded that (a) the decay of the visible absorption in the nanosecond time range is largely because of gradual electron trapping, with only a partial contribution of electron-hole recombination; (b) reactions with scavengers are important in the femtosecond-picosecond time range (reactions of h vb + and e cb -) and in the microseconds or longer time (reactions of the respective trapped species), but the absorbance changes in the nanosecond time range are not affected by scavengers; (c) even in the absence of hole scavengers, trapping of the electron competes successfully with recombination when no more than one electron-hole pair is produced in a nanocrystallite. Most electrons still exist after several microseconds.

The existence of methyl radicals in the photo-Kolbe reaction was confirmed by means of in situ measurements with flow electron spin resonance for deuterated acetic acid. From the dependence of the flow rate, two reaction paths were deduced for methyl radical formation. These were assigned to those caused by trapped holes and •OH radicals. It is suggested that the former path is dominant when platinum has not been loaded onto TiO2 powder.

Radical cations of pyrene were investigated by the pulse radiolysis technique in solutions of benzonitrile, acetone, and dichloroethane. A 450-nm absorption band of a pyrene monomer cation was replaced by new bands with maxima at 500, 580, and 750 nm as the pyrene concentration was increased. The latter absorption bands were exclusively assigned to a pyrene dimer cation in equilibrium with the monomer cation at room temperature. Equilibrium constants evaluated were (2.7 ±0.2) X 10 2 and (5.1±0.5) X 102M~1 at 20°C for benzonitrile and acetone solutions, respectively. The decay processes of both the monomer and dimer cations were discussed. In addition, rate constants of the reactions of both the monomer and dimer cations with triethylamine were found to be 2.7XI09 and 5.4X108JI~1·sec-l, respectively.