The step-scan experiment on the same PMMA/ABS sample was conducted with a ΦM frequency of 1,000Hz. A uniform probing depth of about 5.6μm across the spectrum is expected from this measurement. Since ΦM calibration phase was set to maximize the surface absorption in the in-phase (I) channel, as shown in Figure 3.8, the I spectrum is dominated by spectral features of the surface layer PMMA, and subsequently minimal contribution of PMMA and maximum contribution of ABS can be found from the Q spectrum.
Kapton Film. The three-layer Kapton film consists of a 50μm polyimide layer sandwiched between two 12μm Teflon layers. Figure 3.9 shows frequency-resolved PA magnitude spectra of this sample collected in S2ΦM mode at frequencies from 50-800Hz. At 50Hz, the probing depth is about 25μm, thus polyimide bands over the spectral region of 1800-1350cm-1 are clearly detected. As the modulation frequency increases to 800Hz, the probing depth decreases to 6μm and the spectrum only shows abroad Teflon band around 1225cm-1 (C-F stretch).
The I and Q spectra of the Kapton sample obtained with a ΦM frequency of 400Hz are shown in Figure 3.10. The surface reference phase calibration was performed using the glassy carbon as described earlier. It can be seen that the I spectrum contains contributions mainly from the top Teflon layer and Q spectrum contains contributions mainly from the intermediate polyimide layer.
The PA spectra of the Kapton film over a 180° range of phase angles are calculated and displayed using the OMNIC SST phase rotation and 3D display routine and the results are shown in Figure 3.11. As the phase rotation angle increases from 0 to 90°, the spectral features associated with the deeper polyimide layer replace those from the surface Teflon layer.
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