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

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-fluoresce nt compounds, pseudoisocyanine and erythrosine, which deactivate mainly eit her through internal conversion or after intersystem crossing to a triplet state, respectively. It is shown that experimental signals can be greater o r smaller than those expected from the molecular parameters of the dyes, de pending on the solvent, the excitation wavelength and the presence of quenc hing agents, such as dissolved oxygen or added iodides. For every dye solut ion, it is necessary to perform irradiance-dependent measurements and to ca librate the experimental data with respect to a reference absorber in order to elucidate the origin of the discrepancy between experimental and expect ed signals. Various kinds of non-linear effects, including two-photon absor ption, singlet state bleaching, triplet state bleaching and absorption of t he probe beam by transient triplet states, have been demonstrated. (C) 1999 Elsevier Science B.V. All rights reserved.