Photoacoustic spectroscopy: a measurement technique for low absorption coefficients

The case in which a laser beam modulated at angular frequency omega passes through the weakly absorbing windows of a gas cell which contains a nonabsorbing gas is investigated in this paper. In particular, the frequency dependences of the acoustic stresses in the gas which arise from the surface and bulk absorption are derived. An intermediate range of frequencies exists for which the acoustic stress due to surface absorption varies approximately as omega(-1) and has an approximate 90 degrees phase shift relative to the modulated laser beam and for which the acoustic stress due to bulk absorption varies approximately as omega(-3/2) and has an approximate 45 degrees phase shift. Numerical examples for a representative laser glass and air (nitrogen) are given.