Photoacoustic Depth Profiling of Polymer Laminates by Step-Scan Fourier Transform Infrared Spectroscopy

Abstract

The use of step-scan Fourier transform infrared (FT-IR) spectroscopy with photoacoustic (PA) detection for depth profiling studies of polymer laminates is demonstrated. Step-scan FT-IR simplifies the extraction of depth profile information due to the single modulation frequency that can be applied over the entire spectral range. Because a single modulation frequency is generally used in step-scan FT-IR, the thermal diffusion length, μ, is constant for all wavelengths in a single scan. In addition, lock-in detection allows for easy extraction of the signal phase. Two methods of depth profiling are discussed and illustrated. The first is the conventional method of varying the probe depth by changing the modulation frequency. The other method depends on the direct use of the signal phase. The phase analysis technique is particularly useful for cleanly separating the signal due to a thin (<5 μm) surface layer from that of the bulk or substrate.