Dispersive spectrometer systems

Traditional Raman spectrometers used a monochromator with two or even three gratings to eliminate the intense Rayleigh scattering. The optical layout is similar to that for the UV/VIS single grating monochromators discussed in Chapters 2 and 5. Holographic interference filters, called 

As described in Chapter 2, spectral resolution determines the amount of detail that can be seen in the spectrum. If the resolution is too low, it will be impossible to distinguish between spectra of closely related compounds if the resolution is too high, noise increases without any increase in useful information. Spectral resolution is determined by the diffraction grating and by the optical design of the spectrometer. With a fixed detector size, there is a resolution beyond which not all of the Raman wavelengths fall on the detector in one exposure. Ideally, gratings should be matched specifically to each laser used. A dispersive Raman echelle spectrometer from PerkinElmer Instruments covers the spectral range 3500-230 cm with a resolution better than 4 cm .  

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In Chapter 2, Jansson describes the determination of the system response function for a dispersive spectrometer system. We have made a number of such determinations using very-low-pressure samples of, for example, CO in the 5-fim region. As discussed by Jansson, one records the data and then removes the Doppler profile using deconvolution, yielding the system response function. 

In the early work of Bewick and Robinson (1975), a simple monochromator system was used. This is called a dispersive spectrometer. In the experiment the electrode potential was modulated between two potentials, one where the adsorbed species was present and the other where it was absent. Because of the thin electrolyte layer, the modulation frequency is limited to a few hertz. This technique is referred to as electrochemically modulated infrared reflectance spectroscopy (EMIRS). The main problem with this technique is that data acquisition time is long. So it is possible for changes to occur on the electrode surface. 

Preparation of data for deconvolution must begin prior to the acquisition of the first data point. Once the resolution of the system is set (in a dispersive spectrometer this is equivalent to setting the slit width), the density of data points per resolution element must be chosen as discussed in Sections III.D and III.E. There are some subtle factors that must be taken into account. For example, for continuous scanning, approximately 10 data points per resolution element are recommended (Blass, 1976a) to capture all of the information required by the data and the noise. On the other hand, the data-point density must be great enough to characterize the spectral lines after... 

The imaging detectors, whether for point mapping, line scanning, or array detection, can be coupled with different types of spectrometers. Instrument types are classified by wavelength selection modality into imaging Fourier transform (FT) and tunable filter (TF) spectrometers, both of which are presented below, and dispersive spectrometers. FT imaging systems are classical laboratory instruments while TF spectrometers are compact and robust systems for chemical imaging. 

A modification of an interferometrically-based system, which was first described by Dohi and Suzuki (24), is known as a selectively-modulated interferometric dispersive spectrometer, this system is a hybrid in that a rotating grating (a dispersive element) is used to limit the number of wavelengths which can interfere at any one time in a modified Michelson interferometer. 

Non-dispersive systems are very compact, and potentially less expensive than dispersive spectrometers. For this reason they may be used for multi-channel instruments, measuring upto six elements almost simultaneously.39,40 Although... 

Time resolved hole burning spectra were measured by means of a femtosecond transient absorption spectrometer system. A second harmonics of a mode locked cw Nd + YAG laser (Quantronix, 82MHz) was used for a pumping source. A synchronously pumped rhodamine 6G dye laser with a saturable absorber dye jet (DODCl/DQOCI) and dispersion compensating prisms in the cavity was used. The output of the dye laser (lOOfs fwhm, 600pJ/pulse) was... 

The most simple dispersive spectrometer (Fig. 12.2) comprises a source, a monochromator and a detector. The monochromator, made up of an entrance slit, an output slit and prisms or gratings, is u,sed to separate the light into its basic components. The role of the slit system is to enhance the spectral resolution and compensate for intensity variations. The transmission infrared spectrum of the sample is the recording of the light intensity transmitted as a function of the wave-numbers w hich are scanned in front of the monochromator output slit by rotating the dispersive element. In the infrared domain, the wave-numbers are always recorded sequentially, due to the single-channel nature of the detectors. This recording is compared to that of the reference or the source in order to deduce the absorption due to the sample. 

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