Analysis of the thermal lensing effect for an optically thick sample—A revised model

A revised model of the thermal lensing effect is presented which treats the sample as an extended optical medium having a transverse quadratic index of refraction gradient. The revised model relaxes the restriction on sample cell length inherent in earlier models which treated the sample as an optically thin lens. It is shown how the increased sample length allows higher-sensitivity measurement of optical absorption. Possible sources of error in the thermal lens experiment are discussed. The present model specifically addresses the problem of accounting for the elliptical beam shape of the cw dye laser used in the thermal lens experiment. The absorptivity of benzene at 607 nm (the 6←0 transition of the C-H stretching vibration) is measured to be (2.3±0.2) ×10−3 cm−1.