Investigation of non-linear absorption effects in pulsed-laser thermal lens spectrometry of dye solutions

Abstract

Non-linear optical absorption effects arising in pulsed-laser thermal lens spectrometry of dye solutions have been investigated. Experiments have been carried out with rhodamine 6G, quenched rhodamine 6G and two non-fluorescent compounds, pseudoisocyanine and erythrosine, which deactivate mainly either through internal conversion or after intersystem crossing to a triplet state, respectively. It is shown that experimental signals can be greater or smaller than those expected from the molecular parameters of the dyes, depending on the solvent, the excitation wavelength and the presence of quenching agents, such as dissolved oxygen or added iodides. For every dye solution, it is necessary to perform irradiance-dependent measurements and to calibrate the experimental data with respect to a reference absorber in order to elucidate the origin of the discrepancy between experimental and expected signals. Various kinds of non-linear effects, including two-photon absorption, singlet state bleaching, triplet state bleaching and absorption of the probe beam by transient triplet states, have been demonstrated.