The choice of laser wavelength has an important impact on experimental capabilities:. Spatial resolution. For example, with a nm laser, and a 0. Thus, achievable spatial resolution is partially dependent on choice of laser. Ultra-violet UV lasers for Raman spectroscopy typically include laser wavelengths ranging from nm through to nm.
Theoretically UV Raman spectroscopy is not different from standard analysis using visible laser wavelengths.
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However, in practice there are a number of practical difficulties and disadvantages which must be considered. Near infra-red NIR lasers for Raman typically include a range of wavelengths greater than nm, such as nm, nm, nm and nm. Whilst at times NIR Raman is invaluable, it should certainly not be considered the best solution for every sample.
Each laser wavelength requires an individual filter. There are two main types of filters used, both of which can be used without user intervention or optimization:. They are used for Raman spectroscopy because they are extremely sensitive to light and thus suitable for analysis of the inherently weak Raman signal , and allow multichannel operation which means that the entire Raman spectrum can be detected in a single acquisition.
CCDs are widely used, not least as the sensors in digital cameras, but versions for scientific spectroscopy are of a considerably higher grade to give the best possible sensitivity, uniformity and noise characteristics. CCD detectors are typically one dimensional linear or two dimensional area arrays of thousands or millions of individual detector elements also known as pixels. At the end of the measurement read out, electronics pull the charge from the elements, at which point each individual charge reading is measured.
In a typical Raman spectrometer, the Raman scattered light is dispersed using the diffraction grating, and this dispersed light is then projected onto the long axis of the CCD array. CCDs require some degree of cooling to make them suitable for high grade spectroscopy. Most Raman systems use peltier cooled detectors, but for certain specialized applications, liquid nitrogen cooled detectors still have advantages.
This enhancement is particularly valuable when the Raman signal is very weak, since the electron multiplication process can result in good spectrum quality, unlike the conventional CCD where only a few of the stronger features can just be observed above the noise. The benefits of EM gain are clearly obvious in fast Raman spectral imaging,where the necessary short integration times can often result in signals which are barely visible above the noise when measured with a conventional CCD.
In the EM register, the clocking voltages used are higher than for conventional clocking, causing the electrons to acquire sufficient energy that impact ionization can occur. The key benefit of an EMCCD is that the amplification occurs before readout of the signal, which means that the signal is not readout noise limited.
Spectral resolution is the ability to resolve spectral features and bands into their separate components. The spectral resolution required by the analyst or researcher depends upon the application involved.
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In contrast, characterization of polymorphs and crystallinity often requires high resolution, since these phenomena exhibit only very subtle changes in the Raman spectrum, which would not be visible in a low resolution experiment. Spectral resolution is an important experimental parameter. If the resolution is too low, spectral information will be lost, preventing correct identification and characterization of the sample.
If the resolution is too high, total measurement time can be longer than necessary. Higher resolution becomes necessary to characterize more subtle spectral features — for example, minor changes in the shape or position of a peak. There are a number of chemical phenomena which cause such subtle spectral changes:. Spectral resolution in a dispersive Raman spectrometer is determined by four main factors. In the discussions below, the effect of each factor is considered under the assumption that all other factors remain unchanged.
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The longer the focal length e. It is sometimes forgotten that a long focal length spectrometer is not limited to high resolution work only — With a suitable choice of gratings see below , a high resolution spectrometer can be run in a low reso-lution mode. The higher the groove density of the grating typically measured as number of grooves per millimeter , the higher the spectral resolution. The use of higher groove density gratings cannot be applied ad infinitum to increase spectral resolution, since they will have fixed practical and physical limits linked with the spectrometer itself.
Thus, gratings provide an initial way to improve resolution, but once their limit is reached, it is necessary to move to a longer focal spectrometer. However, Raman spectra use an energy related unit Raman shift, or wavenumber, cm -1 which means that the spectral resolution decreases as the laser excitation is changed from infrared to visible to ultra-violet wavelengths. Most systems have a single detector, so practically the user does not have control of this factor.
However, it should be noted that different detectors can be configured with different pixel sizes. The smaller the pixel the higher the achievable spectral resolution.
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A Raman microscope combines a Raman spectrometer with a standard optical microscope. The Raman signal from the sample is collected from a similar area, passes back through the microscope into the spectrometer and is there analyzed for spectral information. The Raman microscope allows Raman spectroscopy to be performed with microscopic spatial resolution.
Thus it opens up a new dimension in chemical analysis:.
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Simply adding a microscope assists in giving lateral XY spatial resolution, but does not give depth Z spatial resolution. For this confocal optics are required. There are several methods in use today, some truly confocal, others pseudo confocal, which work with varying success. Some Raman microscopes do not have confocal optics. For this, confocal optics are required. With a true confocal design which incorporates a fully adjustable confocal pinhole aperture micron depth resolution is possible, allowing individual layers of a sample to be discretely analyzed.
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Measurement and Control Techniques. What is Raman Spectroscopy? Instrument Presentation. Typical laser wavelengths used for Raman spectroscopy Laser wavelengths ranging from ultra-violet through visible to near infra-red can be used for Raman spectroscopy. Optimization of resulting based on sample behavior For example: Blue or green lasers can be good for inorganic materials and resonance Raman experiments e. Red or near infra-red nm are good for fluorescence suppression. Advantages and disadvantages of ultra-violet UV lasers for Raman Ultra-violet UV lasers for Raman spectroscopy typically include laser wavelengths ranging from nm through to nm.
Such an approach is of interest because XAS experiments can be relatively easily performed at the required temperature and pressure. In this case, XAS acts as a bridge between experiment and theory. Most of the advanced tools are installed in the clean room facilities. This objective is already being achieved, with the Institute now world-renowned for the development of knowledge and skills in synchrotron-based experiments and XAS.
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