Phase measurements in the frequency domain photoacoustic spectroscopy of solids

Photoacoustic spectroscopy provides information about both the amplitude and phase of the response of a system to an optical excitation process. This paper presents a theoretical model of photoacoustic processes in the frequency domain which includes the relaxation time of the radiationless deexcitations and a two-layer absorbing system. Emphasis is placed on the effect of these conditions on the phase of the photoacoustic signal and the utility of this measurement in evaluating material parameters. Circumstances under which the phase may be used to measure the optical absorption coefficient of the solid and the nonradiative relaxation times are defined. The value of the phase measurement in the study of surface films is discussed.