Use of step-scan FT-IR to obtain the photoacoustic/photothermal response phase

This paper describes the extraction of photoacoustic (PA)/photothermal (PT) or other experimental response phase from data measured by step-scan FT-IR. The signal processing used with step-scan FT-IR is discussed and contrasted to the signal processing used to extract the experimental phase from data acquired by conventional continuous-scan FT-IR methods. Step-scan operation decouples the FT-IR spectral multiplexing from time, simplifying a variety of time-dependent spectroscopic measurements both conceptually and practically. A consequence of the decoupling is the separation of the temporal phase from the FT-IR instrument (beamsplitter) phase. Data are presented which show clearly that the temporal phase is completely separated from the FT-IR phase by step-scan. This is in contrast to the case of continuous-scan FT-IR, in which the phases are added together. In continuous-scan operation, a single interferogram, which is the average of a number of scans, is recorded. Typically, in step-scan operation, two interferograms, the in-phase and quadrature responses, are recorded simultaneously point-by-point. The beam splitter phase is the same for both of the step-scan interferograms, while the temporal response (sample) phase appears as their relative intensities, specifically, the arctangent of their ratio. Photoacoustic and photothermal beam deflection data are presented here as illustrations of the points discussed.