Microscope laser Raman spectrometer

This method is based on measuring the shift of the optical energy levels of fluorescent elements such as Cr3+ in response to a change of the stress state. This results in undergo as the result of altering the distance of ions within the strained crystal structure of the host lattice (Yu and Clarke, 2002). Equipment used to record photoluminescence spectra include confocal laser-Raman spectrometers equipped with a liquid nitrogen cooled CCD detector and a motorised X-Y microscope table to allow point-by-point mapping. 

There are three types of Raman spectrometers in general use today for the characterization of protein or nucleic acid structure (a) the classic laser Raman spectrometer,(b) the laser Raman microscope, " and (c) the ultraviolet resonance Raman spectrometer. In the classical system... 

The test equipment of crystal type of gas hydrates consists of a laser Raman spectrometer, gas supply system, jacketed cooling type high-pressure visual cell, temperature control system, data acquisition and other parts. The experiment using a laser Raman spectrometer for the JY Co. in French produced Lab RAM HR-800 type visible confocal Raman microscope spectrometer. Laboratory independently designed a cooled jacket visible in situ high-pressure reactor, reactor with sapphire window to ensure full transparency of laser, and high pressure performance, visual reactor effective volume 3 ml, compression 20 MPa effective volume, to achieve characteristics of gas hydrate non-destructive and accurate measurement. The schematic representation of equipment is shown in Eigure 1. 

Figure 3-36 Raman spectrometer built around an AOTF. The sample is mounted on a microscope slide positioned 45 degrees relative to the incident laser. Raman scattering is collected and spectrally filtered with the AOTF. Holographic Raman filters are placed after the AOTF to Eliminate intense Rayleigh scatter befre the image is focused onto a liquid-nitrogen-colled CCD. (Reproduced with permission of Ref. 101.)...

Modern Raman spectrometers allow scanning of a surface up to A4 size, with XY translation stage and fine spatial adjustment to allow the operator to align the laser probe onto samples as small as 5 pm in diameter. Sample selection is assisted by the use of an integral video microscope with a rotating turret that can be fitted with up to four objective lenses to provide on-screen magnification of up to 500 X. 

Raman spectroscopy requires highly monochromatic light, which can be provided only by a laser source. The laser source is commonly a continuous-wave laser, not a pulsed laser. The laser source generates laser beams with the wavelengths in the visible light range or close to the range. In a Raman microscope, sample illumination and collection are accomplished in the microscope. The microscope s optical system enables us to obtain a Raman spectrum from a microscopic area this is the main difference between the micro-Raman and conventional Raman spectrometers. 

Brooker (1997) measured the Raman spectra using a Laser Raman Microprobe Renishaw and a conventional spectrometer Coderg PHO. A super-notch filter served as a monochromator in front of the entrance slit of a single grating, which in turn disperses the Raman beam onto a 400 x 600 CCD detector. The Laser Raman Microprobe was equipped with a 632.8 nm helium-neon laser of 10 mW power and a 514.5nm argon ion laser of 50 mW power with the appropriate super-notch filters. The laser beam was focused into the sample by a lens with an Olympus microscope and the back-scattered Raman light was collected by the same lens. Samples of molten salts were sealed in capillary tubes under dry nitrogen or vacuum. 

Raman microspectroscopy was not a completely new concept. In 1966, Delhaye and Migeon [35] showed that a laser beam could be hghtly focused at a sample, and that Raman-scattered light could be collected and transferred to a spectrometer, with minimal loss. Their calculahons showed that the increased irradiance more than compensated for the decrease in the size of the irradiated volume. The first Raman microscope was reported by Delhaye and Dhamelincourt in 1975 [36], and an instrument based on these principles (the MOLE) was introduced by Jobin Yvon at about the same time. However, the optical scheme used for imaging, which employed global illumination, was inefHcient and it was not until the advent of CCD-Raman spectrometers that the advantages of Raman microscopy became apparent. 

FIGURE 18-9 Raman spectrometer with fiber-optic probe. In (a) a microscope objective focuses the laser radiation onto excitation fibers that transport the beam to the sample. The Raman scattering is collected by emission fibers and carried to the entrance slit of a monochromator or to the entrance of an interferometer. A radiation transducer, such as a photomultiplier tube, converts the scattered light Intensity to a proportional current or pulse rate (b) end view of the probe (c) end view of collection libers at entrance slit of monochromator. The colored circles represent the input fiber and the uncoiored circles the collection fibers. (Adapted from R. L. McCreery. 

Dispersive Raman spectrometers are used with excitation in the visible range (typically He—Ne or Ar+ lasers are used), Fourier transform Raman spectrometers are used with excitation in the near infrared range (Nd YAG laser). For both ranges, microscopic techniques working with a laser beam diameter of micrometer size, are commercially available. 

In all the examples the measurements were performed with a Raman spectrometer RXN-1 from Kaiser Optical Systems. A laser line of 532nm with an output power of lOOmW was used for the excitation. Spectra were recorded on a range from 150 to 4300 cm with a spectral resolution estimated at 1 cm Data were acquired with a CCD camera (Andor) of 1024 pixels and cooled at — 40°C and treated with the ICRaman software. A microscope objective X50 Olympus) was generally used to realize the measurements through a liquid sample cell. 

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... 

A Renishaw inVia Raman Microscope was utilized for scratch induced residual mechanical stress measurements. Using a Si-laser (532 nm), Raman spectra were collected from several SiC particles present within the scratch grooves. The laser spot size used was around I pm. For comparison purposes, Raman spectra were also collected from several SiC particles residing outside the scratch grooves. All the Raman measurements were performed at room temperature. The Raman spectrometer was calibrated with a Si standard using a Si band position at 520.3 cm". ... 

---