Depth profiles in coated paper: Experimental and simulated FT-IR photoacoustic difference magnitude spectra

Experimental photoacoustic (PA) magnitude spectra of a coated paper and the uncoated basepaper are presented. The normalized and scaled PA magnitude s pectra are used to calculate difference magnitude spectra. It was decided t o scale all PA magnitude spectra to (low) equal intensity at the approximat ely optically thin spectral range before subtraction. Then no infrared (IR) bands of identical band shape and height (as needed for common difference spectroscopy) in either PA magnitude spectrum are needed. Contributions of the two individual layers to the IR-PA magnitude spectrum of the coated pap er are separated in the difference spectrum hy their sign. An increasing re lative contribution of the coating layer with an increasing phase modulatio n frequency is found. On decreasing the thermal length to a value near the coating thickness, the difference spectra increasingly show positive coatin g bands and negative bulk signals. The extension of the Rosencwaig-Gersho t heory to a double-layered system introduced by N. C. Fernelius [J. Opt. Sec . Am. 70, 480 (1980) and J. Appl. Phys. 50, 650 (1980)] applied to syntheti c spectra confirms the experimental observation. It is found that photoacou stic difference spectroscopy may provide quantitative depth-resolved spectr al information due to the presented scaling procedure, and photoacoustic di fference magnitude spectra of any polymeric laminate may therefore be calcu lated.