Monochromator triple

A dispersive element for spectral analysis of PL. This may be as simple as a filter, but it is usually a scanning grating monochromator. For excitation spectroscopy or in the presence of much scattered light, a double or triple monochromator (as used in Raman scattering) may be required. 

A modern laser Raman spectrometer consists of four fundamental components a laser source, an optical system for focusing the laser beam on to the sample and for directing the Raman scattered light to the monochromator entrance slit, a double or triple monochromator to disperse the scattered light, and a photoelectric detection system to measure the intensity of the light passing through the monochromator exit slit (Fig. 7). 

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. 

The probability of Raman scattering is quite small. This normally requires the use of intense laser sources and concentrated samples. A high-resolution double or triple monochromator is used to separate the Raman lines from the intense Raleigh line. 

Development of laser sources was followed by the use of special monochromators that can resolve the more-intense, elastically scattered light (Rayleigh line) from the weak, inelastically scattered, Raman signal. The requirement of frequency matching in the double or triple monochromators presents a challenging, coupling problem for frequency-scanning systems. 

Raman spectroscopy Raman spectra from small SWNT pieces with typical dimensions of 100 pm were recorded in the back-scattering geometry using two different micro-Raman setups comprised of a triple monochromator DILOR XY and a CCD detector system, cooled either to liquid nitrogen temperature or -100°C. The 488 or 514.5 nm line of an Ar+ laser, as well as the 647.1 nm line of a Kr+ laser, were used for excitation, while the beam intensity on the sample was =0.5 mW. The laser line was focused on the sample by means of a lOOx objective with a spatial resolution of 1 pm. 

In aqueous solution, one is faced with the scattering of pure water (9), which is a strong scatterer below 200 cm-l with some broad structure around 150 cm-- -. Since water scattering is not an instrumental artifact, one does not evade this problem in going from a double to a triple monochromator or by using the iodine filter technique. The rotating divided-cell technique (10)... 

A typical Raman system consists of the following basic components (1) an excitation source, usually a laser (2) optics for sample illumination (3) a double or triple monochromator and (4) a signal processing system consisting of a detector, an amplifier, and an output device. A diagram showing various components of the Raman spectrometer is shown in Fig. 4.6.2. 

Dispersive spectrometers usually achieve this end by employing double or triple monochromators. Each monochromator distributes stray radiation of about 10 of the power of the exciting radiation over the entire spectrum. Consequently, sequential spectrometers for the scanning of Raman spectra employ multiple monochromators, i.e., double or even triple monochromators with additive dispersion (Fig. 3.5-1 a). 

Inefficient double and triple monochromators were required to eUminate stray light from the Rayleigh line. 

For the in situ Raman measurements, the UHV system is optically aligned with a triple monochromator Raman spectrometer (Dilor XY) equipped with a CCD camera for multichannel detection [1]. The samples were excited with the 488 nm (2.54 eV) Ar laser line that lies in the first absorption maximum of both organic molecules and thus ensures resonance conditions for the Raman process. 

Due to the rather stringent requirements placed on the monochromator, a double or triple monochromator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm, and the linewidths are only tens of cm j, it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monochromator to collect the spectrum. In fact, this is the approach taken with Fourier transform Raman spectrometers. 

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