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 A11g→B12u 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 A11g→B12u 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.