Two-photon absorption photothermal spectroscopy using a synchronously pumped picosecond dye laser. Thermal lensing spectra of naphthalene and diphenylbutadiene

We demonstrate the use of a synchronously pumped picosecond dye laser as the excitation source for two-photon photothermal (thermal lensing) spectroscopy. The two-photon absorption spectrum of naphthalene in CCl4 is recorded in the range 31 450–35 200 cm−1, and the measurement is in agreement with the previously reported two-photon excitation spectrum. Similar agreement is found for the spectrum of diphenylbutadiene in the range 30 000–35 200 cm−1. It is demonstrated how the technique can provide accurate values for two-photon absorption cross sections by comparison of the thermal lens signals from one- and two-photon absorption. The two-photon absorption measurement in naphthalene indicates a symmetry-forbidden, vibronically induced transition, while the measurement in diphenylbutadiene indicates a fully symmetry-allowed two-photon transition. Both cross section measurements are in agreement with previously reported results.