Frequency dependence of photoacoustic spectroscopy: Surface‐ and bulk‐absorption coefficients

摘要

Researchers seek improved ways to measure separately the surface‐ and bulk‐absorption coefficients of highly transparent materials. The case in which a laser beam modulated at angular frequency ω 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 as ω-1 and has a 90 ° phase shift relative to the modulated laser beam and for which the acoustic stress due to bulk absorption varies as ω-3/2 and has a 45 ° phase shift. In addition, expressions for the acoustic stress and phase shift which are valid for all frequencies are given. These expressions enable one to develop numerical procedures by which the surface‐ and bulk‐absorption coefficients may be determined separately. Numerical examples for a representative laser glass and air (nitrogen) are given.