The photoionization process of pyrene in non-polar solvent has been investigated using a two-color two-photon ionization technique. The ionization spectrum near threshold of pyrene in n-hexane was measured by a delayed irradiation of a dye laser 20 ns after an N laser at total excitation energies of 5.9-6.22 eV. The two-photon ionization threshold of pyrene was in the vicinity of 5.9 eV which is larger by 0.7 eV than that expected from the onset of one-photon ionization. The ionization spectrum shows maxima at 5.92 and 5.99 eV which may be assigned to an autoionization process.

Pyrene has two photoconductivity thresholds in the wavelength region of 360–435 nm (two-photon energy: 5.70–6.89 eV) in nonpolar hydrocarbon and tetramethylsilane. The first one is located at 6.04–6.44 eV and the second one at 5.18–6.11 eV. The former was assigned to direct ionization and the latter to an ion-pair formation process. Direct ionization is the major process.