Remote chemical sensing by pulsed photoacoustic spectroscopy

Summary form only given, as follows. The technique of pulsed photoacoustic (PA) spectroscopy has recently been applied to the examination of free liquid surfaces at short range (a few cm). The mechanism involves expansion of air resulting from heat conduction from the laser heated surface of the sample under investigation. This drives an acoustic pulse that is detected by a conventional microphone. The technique has been successfully extended to ranges across the laboratory (/spl sim/ 8m) using: (i) an increased pulse energy to exploit the non-linear dependence of PA generation efficiency, (ii) a directional microphone to discriminate more effectively between signal and background clutter and (iii) improved processing that exploits prior knowledge of the PA signal. Identification of chemicals can be achieved by looking at only one or two of the spectral lines and doing a "fingerprint" analysis of the substance. An initial examination of the PA spectrum of some of the expected substrate materials has been performed to determine how easy discrimination between chemicals and background materials would be.