Abstract

THE recently developed high-sensitivity opto–acoustic (OA) absorption measurement technique¹ has proved ideal for the measurement of a variety of weak absorption phenomena in liquids^2–4. We report here the results of an investigation of two-photon absorption in liquid benzene using the OA technique. Weak two-photon spectra (cross-section 10^-51 cm⁴ molecule^-1 photon^-1 s) in nonfluorescing liquids have been difficult to measure. The only previous published study for such a measurement used thermal blooming⁵, which is difficult to interpret theoretically and apparently produces a poor signal-to-noise ratio. We demonstrate here that far superior spectra can be obtained by the simple and sensitive two-photon OA spectroscopy (TPOAS) technique. Polarisation dependence, vibrational intervals and electronic origin are obtained, and two-photon absorption cross-sections are derived for the first time in this study for the case of a weak two-photon absorption band in liquid benzene. Note that Bonch-Bruevich et al.⁶ have recently attempted similar two-photon OA measurements, but they can only measure strong absorption (cross-section 10^-48 cm⁴ molecule^-1 s) with a poor signal-to-noise ratio.