Profiling by Varying the Photoacoustic Phase

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]  

The distance that a thermal wave travels in one cycle is V /f. Since L = the phase shift of the PA signal emanating from a distance t below the surface, 0, is tIL radians, or 3( Qtl2nL degrees. McClelland et al. [10] have listed the thermal wave propagation parameters with different modulation frequencies for a material with D = 0.001 cm s these parameters are hsted in Table 20.3. 

The phase shifts associated with different absorption bands are best observed by plotting the amplitudes of absorption peaks against the phase angle. If we denote the in-phase and quadrature interferograms as /o(5) and /9o(5), respectively, the interferogram at any phase angle, 9, can be calculated as 

Phase adjustment is usually necessary when a PA spectrum is measured using an LIA because the microphone and preamplifier add their own phase delay to 

In summary, photoacoustic spectrometry is a versatile technique for studying solid samples, especially when a step-scan interferometer is available for the measurement. 

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