Energy dependence of two-photon-absorption cross sections in anthracene

Two-photon-absorption cross sections were determined in the energy range 3.10-3.56 eV for crystalline anthracene and for the anthracene molecule in solution. The experimental method was based on the measurement of the excitation spectrum for fluorescence induced by two-photon absorption from a Raman-shifted ruby laser. From these results we conclude that: (a) a symmetry-forbidden two-photon transition corresponding to the ¹A_1g→ ¹B_2u excitation was observed both in the crystal and in the solution; (b) this transition is vibronically induced by a nontotally symmetric vibration of the frequency 1200±400 cm^-1; (c) a crystal state is observed at 3.5-3.6 eV which has no counterpart in the solution spectrum; (d) this crystal state is tentatively assigned to a g-symmetry-type charge-transfer state; (e) the possible observation of a quadrupole two-photon ¹A_1g→¹B_2u to the 0-0 band seems likely since the dipole two-photon transition is forbidden and vibronic mixing within the true 0-0 band does not occur.