The technique of pulsed indirect photoacoustic spectroscopy is applied to the examination of free liquid surfaces, and the prospects are assessed for remote detection and identification of chemical species in a field environment. A CO<sub>2</sub> laser (tunable within the 9–11-μm region) provides pulsed excitation for a variety of sample types; the resulting photoacoustic pulses are detected at ranges of the order of a few centimeters. The phenomenon is investigated as a function of parameters such as temperature, sample depth, laser-pulse energy, pulse length, and beam diameter. The results are in good agreement with a theoretical model that assumes the mechanism to be expansion of air resulting from heat conduction from the laser-heated surface of the sample under investigation. Signal and noise processing issues are discussed briefly, and the possible extension of the technique to ranges of the order of 10 m is assessed.
© 2000 Optical Society of America
(140.3470) Lasers and laser optics : Lasers, carbon dioxide
(280.1910) Remote sensing and sensors : DIAL, differential absorption lidar
(300.6430) Spectroscopy : Spectroscopy, photothermal
(350.5340) Other areas of optics : Photothermal effects
Michael Harris, Guy N. Pearson, David V. Willetts, Kevin Ridley, Paul R. Tapster, and Brian Perrett, "Pulsed Indirect Photoacoustic Spectroscopy: Application to Remote Detection of Condensed Phases," Appl. Opt. 39, 1032-1041 (2000)