Electron–phonon coupling in two-photon spectral gratings: role of molecular symmetry

We study spectral gratings obtained as a result of two-photon excitation of the lowest electronic transition of several tetrapyrrole molecules with phase-locked pairs of femtosecond pulses. A particular dependence of these gratings on temperature indicates that mirror symmetry is present between two-photon absorption (TPA) and one-photon fluorescence spectra for non-centrosymmetrical chlorin, but fails for centrosymmetrical naphthalocyanine. The latter case is best described if the homogeneous TPA spectrum contains phonon sideband, but not the zero-phonon line. These results are discussed in terms of alternative selection rules for one- and two-photon transitions in centrosymmetrical molecules.