Monochromators focusing

Figure 2.15. The three different monochromator/sample geometries used in powder diffraction a) flat diffracted beam monochromator, parallel arrangement b) curved diffracted beam monochromator, angular arrangement, and c) flat primary beam monochromator, parallel arrangement. F - focus of the x-ray source, S - sample, M - crystal monochromator, D - detector, Rm - radius of the monochromator focusing circle, Rq - radius of the goniometer focusing circle.

The X-ray beam from the source is monochromated, focused, and collimated to deliver a parallel beam of defined size and wavelength to the crystal. Because of the intrinsically superior optical qualities of synchrotron beams, the radiation delivered to the crystal is also superior to that from conventional sources. The crystal is mounted on a goniostat, which allows the crystal to be rotated. The crystal is usually flash-cooled to a temperature of 100 K by a cold stream of nitrogen gas to reduce radiation damage. X-rays are ionizing radiation and the free radicals produced as they pass through the protein destroy the crystal. Without flash cooling, protein crystals last only seconds on a synchrotron beamline. 

Figure 5 Small spot XPS system using a monochromated focused x-ray source, demagnification retardation lens and a parallel data acquisition system, (after reference 13).

The sizes of microfluidic devices are becoming smaller and smaller for automatic, inexpensive, and accurate characterizations of infinitesimal amounts of samples. However, to characterize chemical and biomedical samples, expensive and bulky instruments have been always attached to microfluidic chips. Various optical units, such as detectors, fight sources, cameras, monochromators, focusing lenses, add-drop filters, amplifiers, optical fibers, and microscopes, should be integrated and aligned in a single chip in order to obtain a proper signal. 

Fluorometry and Phosphorimetry. Modem spectrofluorometers can record both fluorescence and excitation spectra. Excitation is furnished by a broad-band xenon arc lamp foUowed by a grating monochromator. The selected excitation frequency, is focused on the sample the emission is coUected at usuaUy 90° from the probe beam and passed through a second monochromator to a photomultiplier detector. Scan control of both monochromators yields either the fluorescence spectmm, ie, emission intensity as a function of wavelength X for a fixed X, or the excitation spectmm, ie, emission intensity at a fixed X as a function of X. Fluorescence and phosphorescence can be distinguished from the temporal decay of the emission. 

The point F is either the focai point on an X-ray tube or the focai point of a focusing monochromator. 

Light from an appropriate light source (a xenon arc or a halogen or tun ten lamp) passes through a monochromator (probe monochromator). The exit intensity at wavelength "k, IqCK), is focused onto the sample by means of a lens (or mirror). Tbe reflected light is collected by a second lens (mirror) and focused onto an appropriate detector (photomultiplier, photodiode, etc.). For simplicity, the two lenses (mirrors) are not shown in Figure 2. For modulated transmission the detector is placed behind the sample. 

Because the laser beam is focused on the sample surface the laser power is dissipated in a very smaU area which may cause sample heating if the sample is absorbing and may cause break-down if the sample is susceptible to photodecomposition. This problem sometimes may be avoided simply by using the minimum laser power needed to observe the spectrum. If that fails, the sample can be mounted on a motor shaft and spun so that the power is dissipated over a larger area. Spinners must be adjusted carefully to avoid defocusing the laser or shifting the focal spot off the optic axis of the monochromator system. 

A simple spectrometer that we have used successfully is shown in Figure 2. Electrons from an electron microscope hairpin tungsten filament are focused with an Einzel lens onto the monochromator entrance slit, pass through the monochromator and exit slit, and are focused on the sample s surface by additional electrostatic... 

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). 

Scattering on the Triple-Axis-Diffractometer [1,2] at the HASYLAB high-energy beamline BW5 is performed in the horizontal plane using an Eulerian cradle as sample stage and a germanium solid-state detector. The beam is monochromatized by a singlecrystal monochromator (e.g. Si 111, FWHM 5.8 ), focused by various slit systems (Huber, Riso) and iron collimators and monitorized by a scintillation counter. The instrument is controlled by a p-VAX computer via CAMAC. 

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