Photo-Ionization of Crystalline Anthracene

The photo-ionization spectra of the ground, first singlet, and first triplet states of crystalline anthracene are calculated assuming transitions to a simple continuum. The calculations neglect vibrational-overlap factors which will reduce the cross sections perhaps by as much as a factor of 10. Recombination is also neglected, and is blamed for discrepancies between the calculation and experiments which measure photo-currents. Kepler has reported a cross section of 2×{}10^-19 cm² from the first singlet, Courtens et al., one of 0.6×{}10^-19; the calculation gives 6×{}10^-18. Holzman et al. report 10^-20 cm² from the first triplet; the calculation gives 6×{}10^-18. If one accounted for vibrational overlap and recombination (which will be larger in the second experiment, because of the smaller kinetic energy), the agreement would be good. The agreement is not so good with the magnitude of transitions from the ground state, though the energy dependence gives a good fit. The discrepancy may be due to the (neglected) influence of auto-ionizing states on the final-state wave functions. Based on a theory by Choi, the singlet exciton-exciton ionization rate constant is calculated to be 2×{}10^-9 cm³ sec^-1 (vibrations and recombination neglected) compared with Silver's experimental value of 0.8×{}10^-10; again the agreement is good. Finally, the time needed for a photo-ionized electron to lose kinetic energy by exciting a triplet is calculated to be 10^-9 sec or longer, so this mechanism is ruled out as an energy-loss process in comparison with the emission of optical vibrations, which has been estimated to take about 10^-13 sec.