We have developed a model to describe the effect of refraction through a planar interface on the collection efficiency and depth of focus when performing confocal Raman microspectroscopy. The planar interface introduces spherical aberration, which can substantially degrade the performance of the microscope, especially for large-numerical-aperture microscope objectives. This spherical aberration will increase the range of focal depths spanned by the paraxial and marginal rays of the illuminating laser beam within the sample. In the collection path, it will also distort the scattering volume defined by the confocal aperture; this results in a dramatic fall in the collected light intensity with increasing depth. We demonstrate that there is an optimum numerical aperture for collected light intensity at a given depth. The prediction of this theoretical model is compared to empirical results obtained by mapping the stress distribution within the diamond anvil of a high-pressure cell. Both the collected Raman intensity and the effective depth of focus are compared to the predictions from the theory.

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The laser excitation wavelength is an important parameter in obtaining definitive Raman spectra from pharmaceutical drug substances. This paper investigates the effect of changing wavelength on the Raman spectra of four different drug substances: paracetamol, paroxetine and ranitidine (polymorphic forms I and II). Excitation wavelengths in the range 244 nm to 785 nm have been used. The fluorescence intensities in the Raman spectra were found to be negligible at the two extreme laser wavelengths but tended to interfere in the visible range. Resonance Raman enhancement was observed with 244 nm excitation but there was evidence for sample degradation. Furthermore, the site selective nature of the resonance enhancement reduced the differences between the Raman spectra of the two ranitidine polymorphs.

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We studied the anchoring of 6CB on a series of self-assembled monolayers (SAMs) with a functional group that mimics that of the nematic liquid crystal (LC). The SAMs were first characterized by wetting, Fourier-transform infrared spectroscopy, and surface potential measurements. We found that, in two of these SAMs, the end group dipoles were oriented close to the normal of the surface and that these promoted homeotropic anchoring. In the case of the other SAM, the dipole was oriented parallel to the surface, and planar anchoring was obtained. Raman scattering by adsorbates on thin metal films is enhanced by the electromagnetic fields of surface plasmon polaritons (SPPs). Despite the inherent polarization of SPPs, there have been few reports in which SPP Raman scattering has been used to study molecular orientation. We have developed optical instrumentation to provide efficient excitation and collection of SPP Raman scattered light using attenuated total reflection geometry. The Kretschmann prism coupling configuration was used to excite SPPs on thin (500 A) gold films with adsorbed SAMs of alkanethiols in contact with thin films (50 microm) of the nematic liquid crystal 4'-hexylbiphenyl-4-carbonitrile (6CB, Merck). The anchoring and orientational wetting of the LC 6CB at the interface with omega-functionalized SAMs was studied using this arrangement. In agreement with the results of previous studies, a high-energy surface (-COOH) was found to promote planar anchoring, whereas a low-energy surface (-CF(3)) was found to induce homeotropic anchoring.