Harmonics monochromator

As area detectors (other than multiwire systems) are not energy discriminating devices, apotential source of error lies in the contamination of the data with harmonics of the assumed wavelength of the primary beam. The importance of this effect has been estimated for molybdenum Ka radiation using a graphite monochromator [1],... 

X-ray Instrumentation. All experiments were performed at the Cornell High Energy Synchrotron Source (CHESS) operated at 5.8 GeV (Stations A-3, B and C-2). Monochromatic radiation was obtained with a Si (220) double crystal monochromator. In order to eliminate higher harmonics, 50% detunning was typically employed. ... 

Figure 12.1 SGX-CAT beamline schematic. The components of the beamline include (1 (not shown), 8) photon shutters (2,4) beam transport tubes (3, 5) collimators and vacuum pumps (6) beam-defining slits (7) monochromator (9,10) focusing and harmonic rejection mirrors and (12) CCD detector, supporting base, and sample robot.

Figure 8.4 Principle of autocorrelation. S, second harmonic generator crystal or sample cell M, movable mirror F, filter or monochromator D photomultiplier...

Quick EXAFS measurements were performed at the HASYLAB synchrotron (station XI. 1) in Hamburg, Germany. The measurements were done in transmission mode using ion chambers filled with a mixture of Ar and N2 to have an absorption of 20% in the first and of 80% in the second ion chamber. The monochromator (a double Si-111 crystal) was detuned to 50% at maximum intensity to avoid the presence of higher harmonics in the X-ray beam. In QEXAFS mode, the monochromator is in continuous motion. 

X-ray absorbance of 20% in the first and 80% in the second ion chamber. The monochromator was detuned to 50% maximum intensity to avoid higher harmonics present in the beam. 

Evidently the design of present cameras is determined by the size of the source point and the large value of F, which is due to the existing shielding. A simplified optics with a flat or segmented monochromator plus a flat mirror, which is used for the supression of the higher harmonics, could be used in case the optical elements could be placed closer to a smaller source point. This is the case for a camera at VEPP-3 where the distance source point to monochromator is only 3 m. A more detailed discussion of this point can be found in Ref. 60. 

Fig. 11. Experimental setup for the in situ detection of chemisorbed CO during catalytic combustion of CO on Pt using optical infrared-visible sum frequency generation (SFG) and mass spectrometry. A mode-locked Nd YAG laser system is used to provide the visible laser beam (second harmonic 532 nm) and to pump an optical parametric system to generate infrared radiation (wir) tunable with a pulse duration of 25 ps. MC monochromator, PMT Photomultiplier, AES Auger Electron Spectrometer, LEED Low Energy Electron Diffraction Spectrometer, QMS Quadrupole Mass Spectrometers for CO Thermal Desorption (TD) and CO2 production rate measurements.

Figure 2. Schematic layout of a typical suprananosccond kinetic spectrometer Q Q-switched laser with harmonic generators L lens X cw monitoring lamp S shutter A aperture (see Figure 3 for detail) C sample cuvette M monochromator PM, PD photodetectors O digital oscilloscope PC personal computer.

The channel-cut monochromator is the simplest type employed experimentally. A channel is cut in a perfect crystal (e.g.. Si) to provide two parallel reflecting surfaces that have a particular crystal plane [e.g., the Si (220)] parallel to the surface. The Bragg condition is used to select a particular wavelength and the reflected beam emerges parallel to the incident beam but is vertically displaced by 2D cos d, where D is the distance between the two faces and 0 is the angle between the beam and the Bragg planes. The accuracy of data collected using channel-cut crystal monochromators may be limited due to harmonic con-... 

Various optical elements can be placed in the beam path to tailor the characteristics of the X-ray beam. These can work by diffraction e.g. a monochromator crystal), reflection e.g. a mirror), or absorption e.g. a filter or slits). A monochromator is used to select a particular wavelength, a mirror can focus the beam or suppress higher harmonics, and filters can be used to remove unwanted radiation. 

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