Double monochromators

Stray light can be reduced and higher resolution obtained by using double monochromators. An echelle spectrometer equipped with a predisperser has been used. As the two instruments are operated in tandem, the exit slit of the predisperser is the entrance slit of the monochromator. 

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Fig. 10. Scattered light including ghosts appearing in a single, double, and triple monochromator. Although scattered light plunges to the square root in a double monochromator, interference with Raman spectra still occurs occasionally. In a triple monochromator, instrumental scatter has never been known to interfere with Raman scatter...

A detachable monochromator (19) developed by Spex Industries, was another approach in minimizing stray light. It is a modified Czerny-Turner spectrograph which can be coupled to the exit slit of a double monochromator and function as a variable bandpass, variable frequency filter. This accessory, while providing the versatility of a triple monochromator, does not add much mechanical and optical complexity and can be removed when not wanted. 

In Raman measurements [57], the 514-nm line of an Ar+ laser, the 325-nm line of a He-Cd laser, and the 244-nm line of an intracavity frequency-doubled Ar+ laser were employed. The incident laser beam was directed onto the sample surface under the back-scattering geometry, and the samples were kept at room temperature. In the 514-nm excitation, the scattered light was collected and dispersed in a SPEX 1403 double monochromator and detected with a photomultiplier. The laser output power was 300 mW. In the 325- and 244-nm excitations, the scattered light was collected with fused silica optics and was analyzed with a UV-enhanced CCD camera, using a Renishaw micro-Raman system 1000 spectrometer modified for use at 325 and 244 nm, respectively. A laser output of 10 mW was used, which resulted in an incident power at the sample of approximately 1.5 mW. The spectral resolution was approximately 2 cm k That no photoalteration of the samples occurred during the UV laser irradiation was ensured by confirming that the visible Raman spectra were unaltered after the UV Raman measurements. 

The first laser Raman spectra were inherently time-resolved (although no dynamical processes were actually studied) by virtue of the pulsed excitation source (ruby laser) and the simultaneous detection of all Raman frequencies by photographic spectroscopy. The advent of the scanning double monochromator, while a great advance for c.w. spectroscopy, spelled the temporary end of time resolution in Raman spectroscopy. The time-resolved techniques began to be revitalized in 1968 when Bridoux and Delhaye (16) adapted television detectors (analogous to, but faster, more convenient, and more sensitive than, photographic film) to Raman spectroscopy. The advent of the resonance Raman effect provided the sensitivity required to detect the Raman spectra of intrinsically dilute, short-lived chemical species. The development of time-resolved resonance Raman (TR ) techniques (17) in our laboratories and by others (18) has led to the routine TR observation of nanosecond-lived transients (19) and isolated observations of picosecond-timescale events by TR (20-22). A specific example of a TR study will be discussed in a later section. 

The infrared spectrophotometers are based on either single monochromation or double monochromation ... 

Figure 22.3 Optical Diagram of a Double-Monochromator Infrared Spectrophotometer (Beckman Model IR-9)...

The various components of a double-monochromator infrared spectrophotometer shown in Figure 22.3 are as follows below ... 

Raman spectra were recorded with a computer-controlled Spex 1401 double monochromator equipped with a cooled photomultiplier and photon counting electronics. Excitation was provided by Spectra Physics Kr+ and Ar+ cw lasers. 

Instruments with double monochromator configurations, or equivalent multi-pass configurations, can greatly reduce stray light (which is any radiation of wavelength other than that of the columnated light beam). Absorbance in the presence of stray light can be expressed as ... 

In the case of the sulphur triimide S(NBu-f)3, the dispersive Raman technique applying a double monochromator and a CCD camera was employed to obtain the information from polarized measurements (solution studies) and also to obtain high-resolution spectra by low-temperature measurements. In the case of the main group metal complex, only FT-Raman studies with long-wavenumber excitation were successful, since visible-light excitation caused strong fluorescence. The FT-Raman spectra of the tetraimidosulphate residue were similar to those obtained from excitation with visible laser lines. 

Laser Raman spectra were obtained with the 647.1 nm line of a krypton ion laser (Lexel, Model 95). The radiation intensity at the sample was 300 mW. The scattered radiation was passed through a double monochromator... 

High-quality spectrometers could have two monochromators in series (called a double monochromator) to reduce stray light. Unwanted radiation that passes through the first monochromator is rejected by the second monochromator. 

Additional help is to be gained by eliminating spectrometer "stray light . This is normally achieved by using double monochromators" in the more sophisticated spectrometers (by Spbx, Cary, Coderg and Jarbia-Ash). 

Most available infrared instruments use the Littrow mount for the prism, the beam being reflected from a plane mirror behind the pnsm and thus returning it through the prism a second time. This doubles the dispersion produced. Actually, a double-pass system is also used so that the beam goes through the pnsm four times. Other design modifications include those with single beam and double monochromator, double beam and double monochromator, and related combinations, See also Infrared Radiation. 

For the determination of the spectral sensitivity complicated and expensive laboratory equipment are required in conjunction with experienced personnel. Such a facility should be capable in providing calibrated monochromatic radiation of sufficient power to be sensed by the broadband detector in question. A typical system should be comprised by a double monochromator coupled with a stable, high-power source (e.g. a 1000-Watt Xenon lamp). A spectrally calibrated detector should also be available to measure the quasi-monochromatic radiation emitted by this system. Such facilities are rarely available and only a few laboratories worldwide are able to cany out such characterizations. 

Luminescence. Both excitation and emission spectra were recorded using a Spex Fluorolog 202 B double monochromator fluorescence spectrometer. The spectrometer was operated in the front face mode with bandpass slits of 1.0 nm. 

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