C–H vibrational states of benzene, naphthalene, and anthracene in the visible region by thermal lensing spectroscopy and the local mode model

Abstract

Visible (red) absorption spectra of benzene (h6 and d6), methyl substituted benzenes, naphthalene, and anthracene are presented as seen by thermal lensing spectroscopy. It is shown how this highly sensitive laser-based technique can easily detect transitions having absorption coefficients as small as 10−6 cm−1. The observed transitions are principally due to C–H stretching vibrations at the sixth quantum of excitation. The local mode (LM) model for describing such molecular states is explored in some detail with reference to the C–H excitations in benzene. Based on a two parameter empirical fit, the LM model can give a true C–H stretch ’’mechanical’’ frequency; it successfully predicts isotope effects; and it fully determines the energy level scheme at any quantum of excitation. The LM energy level scheme for the 462 C–H stretching states at the sixth quantum of excitation is presented. A majority of these states are only mildly anharmonic; one state in fact is a perfect harmonic of the fundamental. However, only the most anharmonic state of the 462 is the spectroscopically allowed one. The LM model, already introduced in the literature, is seen to be an excellent zeroth order description of those very same highly excited vibrational states most readily accessible to a sensitive absorption technique such as that offered by the thermal lensing effect.