Raman sample arrangements optimization

A sample arrangement should make it possible to record Raman spectra within a short time period with a high SNR by using a small amount of sample. In order to optimize the... 

This is a useful prerequisite for the optimization of sample arrangements the low intensity of the Raman radiation can be considerably enhanced by utilizing multiple reflections of the exciting and the emerging Raman radiation at the sample and an external spherical mirror. 

Figure 3.5-5 Sample arrangement for Raman spectroscopy. Optimal illumination of a a grating spectrometer in the 90° arrangement b of an interferometer in the 180° arrangement.

Conventional Raman spectroscopy utilizes rectangular or cylindrical cuvettes. A given spectrometer collects maximum intensity of Raman radiation of a sample, if the sample is placed in the focal region of a laser beam and if a maximum amount of the Raman radiation emerging from this sample is collected by a sample optics of the spectrometer within a maximum solid angle (Schrader, 1980). As mentioned in Sec. 3.1, the optical conductance of the entrance optics should have the same value as that of the interferometer or monochromator. Inspection of conventional sample arrangements shows that these conditions were often not fulfilled optimally ... 

A circular spot is more aberration-tolerant than a line of the same area and thus can be demagnified further (Hirschfeld, 1977). In other words, the Raman radiation of a smaller spot on the sample can be collected within a larger solid angle. This means a further optimization of the arrangement. 

Another optical arrangement for Raman mapping has proved to be convenient for a variety of cumbersome surface samples and holders, such as variable temperature or pressure cells. The focused laser spot is scanned over the stationary sample and the spectra recorded in sequence. This method is achieved by a new kind of transfer optics placed between the microscope and the spectrometer, which enables an optimized coupling. The coupling optics consist of a pair of lenses. One lens, optically coupled to the back aperture of the objective, can be moved in two orthogonal directions perpendicular to the laser beam. Thus, this lens can focus the light beam on any point... 

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