Pulsed optoacoustic spectroscopy of condensed matter

Abstract
The authors discuss the theory and experiments dealing with the pulsed optoacoustic effect (i.e., generation of a transient acoustic wave by absorption of an optical pulse) in condensed matter. Their primary interest lies in the measurement of small absorption coefficients (≪101 cm1). At present an experimental capability of measuring absorption coefficients as small as 106 cm1 has been demonstrated, and further improvement is foreseen. The pulsed optoacoustic absorption measurement technique has been applied to the following linear spectroscopic studies: (1) precise measurements of the optical absorption spectra of H2O and D2O; (b) accurate determination of absorption strengths and profiles of high harmonics (n=6, 7, and 8) of vibrational modes in transparent organic liquids (e.g., benzene); (c) quantitative absorption spectra of thin (∼ 1-10 μm) liquid films; and (d) quantitative absorption spectra of solids and finely powdered crystals. The usefulness of the pulsed optoacoustic technique to nonlinear spectroscopy has been demonstrated in the following studies: (a) quantitative two-photon absorption spectroscopy of the weak two-photon (B2μ1A1g1) transition in benzene; and (b) optoacoustic Raman-gain spectra for a variety of liquids where an ability to measure Raman gains as small as 105 cm1 has been demonstrated. In addition to reviewing the above studies the authors discuss future possible applications and compare the pulsed optoacoustic spectroscopy technique with other optoacoustic absorption measurement techniques.