Superposition of different photoionization mechanisms upon laser irradiation of pyrene in tetrahydrofuran solutions at room temperature

The photoconductivity signal consecutive to a light pulse from a 337.1 nm N₂ laser can be understood as the superposition of photocurrents stremming from two distinct photoionization mechanisms. Low light intensity (5 × 10²¹ quanta/cm²s) favors the triplet—triplet annihilation path where light absorption produces an initial population of excited triplets which subsequently undergo diffusion controlled collisional encounters resulting partly in the generation of charge carriers; high light intensity (10²³ quanta/cm²s) favors a biphotonic photoionization mechanism in which ionization occurs during the light pulse by consecutive absorption of two photons. Oxygen quenching does not affect the ion quantum yield in the biophotonic process. This fact and evidence derived from a kinetic simulation involving only the triplet and singlet states are discussed in terms of the possible intermediate state(s) necessary for consecutive biphotonic ionization.