Photoacoustic phase shift

The use of phase shifts for PA/FT-IR depth profiling was first discussed by Dittmar et al. [8], who correlated the depth from which a spectral feature originated with its phase spectrum, 9, as calculated by Eq. 20.3. The photoacoustic phase shift, 0 can be calculated for thermal wave propagation across a layer of thickness, t, from the velocity of the thermal wave [9,10] ... 

Photoacoustic spectroscopy is based on measurement of the heat generated by the radiationless processes for the deactivation of excited molecules (Rosencwaig, 1980). The population of the excited species and the heat emission will be modulated with the same frequency as the exciting light source. At the appropriate frequencies, the excited species will emit their excess heat phase-shifted with respect to the heat emitted by the fast relaxation processes. A gas-coupled microphone and a phase-sensitive detector in combination with an intensity-modulated light source can be... 

Phase angle Phase angle shift of photoacoustic signal relative to the modulation frequency. 

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