In‐depth analyses by confocal Raman microspectrometry: experimental features and modeling of the refraction effects

Abstract

In-depth confocal Raman microspectrometry (CRM) studies through a planar interface between materials of mismatched refraction indices are known to be affected by a decrease in both collected Raman intensity and axial resolution as a function of focal depth. A complete treatment of these phenomena would require diffraction and refraction effects to be taken into account. Baldwin and Batchelder have recently modeled the refraction effects by considering the influence of the dimensions of the confocal pinhole aperture on the collection efficiency. Their theoretical predictions are compared here with experimental results obtained for a standard 200 µm thick polyethylene (PE) sample. It is shown that the decrease in Raman intensity as a function of focal depth is weaker than predicted, suggesting that off-axis refraction effects cannot be neglected. We therefore propose a simple two-parameter relation which reproduces the observed Raman intensities down to 150-200 µm focal depths. Other in-depth experiments on various test samples, a silicon wafer buried in Nujol oil, PE films of different thicknesses and a polycarbonate slab, were then performed in order to show how the Raman intensity decrease, the radial resolution and the axial interfacial broadening can be directly estimated as a function of the focal depth position. Also, CRM experiments were carried out on a four-layer polymer laminate in order to find the best optical conditions. 'Edge' analyses remain the most efficient way to investigate polymer interfaces but, when in-depth analyses are needed, we tried to evaluate to what extent interfacial broadenings impede the investigation of interpenetration effects at buried interfaces. It is demonstrated that the refraction effects sharpen the interfacial broadenings and the apparent axial resolution does not drastically deteriorate with increasing depth. Indeed, when focused 120 µm deep into a non-absorbing sample, the interface is found to be broadened by a factor of only 7.5 or 3.7 using a 100× or a 50× objective lens; the axial resolution then takes a reasonable value of about 13 µm.

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“…Also if depth resolution is important, immersion objectives (oil, water) give better results [109]. Generally the axial resolution is around twice the lateral resolution [110].…”

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“…The focus depth can be obtained from the full-width-at-half-maximum (FWHM) of the intensity profile by depth-scanning [15,29,30]. It can also be predicted by several theoretical or empirical models [16,31,32].…”