Two-photon-induced ionization

This process can be accounted for in terms of a consecutive two-photon-induced ionization. Absorption of the first photon results in the formation of the 2AP singlet excited state, and absorption of the second photon causes photoionization according to the following scheme ... 

Most other studies on multiphoton ionization were carried out on solutions. Early work is due to Pilloff and Albrecht (1968), who studied the biphotonic ionization of TMPD (N,N,N N" tetramethyl-p-phenylene diamine) in 3-methylpentane. They found a quadratic dependence on the light intensity indicating that only virtual states were involved. Siomos et al. (Siomos and Christophorou, 1980 1982 Siomos et al., 1981) investigated the two-photon-induced ionization threshold of several aromatic molecules in n-pentane. The values obtained were lower than the corresponding thresholds measured in single photon ionization experiments. In a later paper Faidas and Christophorou (1987) attributed these lower values to three-photonic processes near the threshold which were not identified in the earlier work. For fluoranthene in tetramethylsilane they report a threshold of 5.7 eV. In another more complete study two-photon ionization onsets were reported for several aromatic molecules in various hydrocarbons (Faidas and Christophorou, 1988). Some of their results are compiled in Table 7. Within the variation of the relative dielectric constant and the limits of error, the polarization P+ energy is constant. 

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