Light intensity dependence of a two-photon catalytic cycle: photoionization via absorption–electron transfer–absorption Original Research Article

The kinetics of the title mechanism, in which the substrate is excited by a light pulse and the radical anion resulting from quenching by a suitable electron donor is then photoionized within the duration of the same pulse, is analyzed theoretically and experimentally. A characteristic of this mechanism as compared to other two-photon ionizations with an intervening chemical step is the regeneration of the substrate. As a consequence, the yield of hydrated electrons e•−aq can grossly exceed the substrate concentration; for continuous illumination, it is only limited by depletion of the donor, which must be present in high excess anyway to ensure high quenching rates and efficiencies. When the absorption steps are much faster than the quenching, the amount of e•−aq produced within a given time depends on the shape of the excitation pulse and saturates at high light intensities, whereas in the opposite case the yield of e•−aq is independent of the pulse envelope and becomes a linear function of the integrated intensity I in the limit of high I. Tests of the theoretical predictions by laser flash photolysis (pulse duration, 20 ns) of 1,5-anthraquinonedisulfonate or 4-carboxybenzophenone with triethylamine as the quencher in water gave excellent agreement. Substantial quantum yields (0.16 and 0.23) were found for the photoionizations of the two radical anions at 308 nm.