Phase correction and two-dimensional correlation analysis for depth-profiling photoacoustic step-scan FTIR spectroscopy

Abstract

A method is described which corrects for the instrument response function in step-scan FT-IR photoacoustic spectroscopy (PAS) without distorting the thermal response information necessary for depth profiling layered samples. The instrumental phase correction functions are determined from the phase-sensitive photoacoustic signals of a strong surface absorber, such as carbon black. Phase correction functions generated from the monosignate spectrum of carbon black are used to compute the in-phase and quadrature interferograms of the sample. This treatment allows unratioed PAS spectra to properly change sign as a function of lock-in amplifier phase angle, thereby making a 2D correlation analysis of the results meaningful. This approach is used to clearly separate individual signals originating from a three-layer sample film of polydimethylsiloxane (PDMS), polyethylene (PE), and polystyrene (PS).