The polarised two-photon excitation spectrum of benzene monocrystals at 6 eV

Abstract

The two-photon excitation spectrum of a benzene single crystal at 4.2 K has been recorded in the region of the second absorption system. The onset of two-photon absorption occurs near 46 500 cm −1 (quoted as a two-photon frequency). The spectrum has the appearance of a forbidden transition in that the system begins with weak lines which become dominated by an intense continuum at higher energies. The two-photon cross section at 55 000 cm −1 (the limit reached in this study) is about 200 times greater than at 47 490 cm −1 although the peak of this strongly allowed system has not yet been reached. The fwhm of the bands near 47 000 cm −1 is 280 cm −1, the same as in the one-photon spectrum at these energies. The polarisation ratio is much the same over the entire energy range, and is consistent with the two-photon operators ( xz, yz) or ( zz). An analysis of all the data available from the one- and two-photon spectra suggests that the transition is 1B 1u ← 1A 1g in which the vibronic intensity is derived from the 1E 1u state in the one-photon and 1E 1g in the two-photon spectrum.