Comparison of phase rotation, phase spectrum, and two dimensional correlation methods in step-scan Fourier transform infrared photoacoustic spectral depth profiling

This paper compares three photoacoustic phase-related analytical approaches for spectral depth profiling of heterogeneous materials, particularly in conjunction with step-scan Fourier transform infrared spectroscopy, The three approaches compared are (1) the phase rotation method, (2) the direct analysis of the phase spectrum, and (3) the calculation of two-dimensional (2D) frequency correlation maps. The experimental data for a representative two-layer sample consisting of a 12-mu m-thick layer of ethylene-vinyl acetate on a 60-mu m-thick polypropylene substrate are compared for the three approaches. Similar/identical phase difference information can be obtained from either the continuous phase rotation plot or the phase spectrum. However, the phase spectrum method provides more accurate, direct, and efficient results with a higher depth resolution than does the phase rotation method. Qualitative agreement between the 2D correlation analysis and the photoacoustic phase analysis is observed, although the 2D approach does not clearly distinguish between spectral bands with similar response phase, limiting it to qualitative use.