Raman Microprobe and Microscopy

Principles and Characteristics Raman microscopy takes advantage of the fact that the intensity of Raman scattered light is independent of sample volume [503]. Thus, the light intensity remains essentially constant with decreasing sample size down to the dimension determined by the diffraction limit, and hence the wavelength, of the laser excitation. Raman intensity always comes from 

A Raman microscope consists of five basic components excitation source (laser), focusing component (microscope), signal analyser (spectrometer or interferometer), photon detector (either monochannel or 2D array) and mapping unit such as a computer-controlled micromanipulator. Raman microscopes are usually equipped with low-power UVA IS or NIR lasers, with laser spot sizes (focused laser beam) below 10 pm. Raman microscopy 

Object line scanned in x-direction by laser beam focused ) the difraclion limit 

Both dispersive (D) and Fourier transform (FT) micro-Raman are commercially available [507-509]. The choice between D- and FT-Raman has been discussed cfr. Chp. 1.2.3). However, as the intrinsic superiority of interferometers in terms of high resolution and geometrical extent cannot be exploited in micro-Raman spectroscopy, dispersive spectral analysers are preferred in combination with the microscope. FT-Raman instruments equipped with a microscope have thus far been limited to a sensitivity far lower than that of modern Raman microspectrometers equipped with multichannel detectors, which employ visible excitation. Consequently, FT-Raman microscopes often yield deceiving results with poor sensitivity and spatial resolution (from 15 

The NIR region is a compromise between the trade-off that must be made in choosing between Raman and IR spectral imaging. Dispersive Raman microprobes using near-IR excitation beyond 1000 nm and linear array detectors with good sensitivity are useful for the investigation at the microscopic level or for remote analysis by means of optical fibres of samples which fluoresce under visible illumination. This allows manufacturing quality control. 

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