Focussing mirrors

As shown in the table, the smallest focal size potentially achievable with XI3 is considerably smaller than with X33 in the vertical direction, although not in the horizontal direction. Nevertheless, the smaller vertical size means that a much larger fraction of the total flux could be concentrated on a thin specimen using XI3 with focussing mirrors than with X33. 

Double focussing, mirror-monochromator cameras are optimized for maximum flux at the sample. This type of camera is hence mainly used for real time diffraction studies on biological samples and polymers (see Sect. 4). Such a camera is shown in Fig. 23. The first optical element could only be placed at 20 m... 

Fig. 23. Schematic design of a double focussing mirror-monochromator camera at DORIS The middle of the mirror is at 20 m from the source point. A bent, triangular monochromator crystal is used for horizontal focussing and a segmented mirror (quartz) for vertical focussing. The ionization chamber is designated by-I-...

Fig. 1. Scheme of a Fourier transform infrared spectrometer with a Michelson interferometer. BS, beam splitter FM, focussing mirror PM, parallelizing mirror. (Adapted from [16]). 

Table 5.2. Details of calculations of intensities on various possible sources cases 1-9 assume 1 1 vertical focussing mirror 4- 10 1 demagnifying horizontally focussing germanium monochromator combination case 9A utilises the natural line-width of the third harmonic without monochromatisation but with point focussing 1 1 mirror. (The symbols are defined in section 5.2.3.) Based on Helliwell and Fourme (1983). 

This instrument (station 9.5) is currently under development (figures 5.17 and 5.18) at Daresbury on the wiggler line for macromolecular crystallography (Thompson and Helliwell 1986 Brammer 1987 Brammer et al 1988 Habash et al 1990). It is primarily intended for very rapid Laue and rapidly tunable monochromatic experiments. The sample position is at 32m from the tangent point. A point focussing mirror is situated at 18m and a rapidly tunable monochromator at 30m (in the hutch). 

The polarisation state of the synchrotron beam has been discussed in section 4.4. The polarisation state of the beam at the sample is determined by the fraction of the parallel and perpendicular components reflected by the monochromator (there may be several reflections, i.e. two for a double crystal monochromator), and the relative fraction of the parallel and perpendicular components incident on the monochromator from the source. This depends on the vertical angular aperture of the source that the sample sees and can be calculated from the source characteristics the size of the vertical aperture is dependent on whether a focussing mirror is used as this increases the aperture subtended by the sample at the source. 

Radiation from a xenon or deuterium source is focussed on the flow cell. An interchangeable filter allows different excitation wavelengths to be used. The fluorescent radiation is emitted by the sample in all directions, but is usually measured at 90° to the incident beam. In some types, to increase sensitivity, the fluorescent radiation is reflected and focussed by a parabolic mirror. The second filter isolates a suitable wavelength from the fluorescence spectrum and prevents any scattered light from the source from reaching the photomultiplier detector. The 90° optics allow monitoring of the incident beam as well, so that dual uv absorption and fluorescence... 

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

Where a is the aperture made by a single mirror. On the basis of this expression, it is of interest to calculate the effect of focussing on the size of the vertical focus on the currently used bench X33. Taking approximate values for s (3 mm), u (22 m) and v (7 m) from Table 1, and with a = 0.7 mm, the contributions of the first term... 

Fig. 4. Experimental arrangement on the XAS instrument 8.1 at the SRS, where a slitless monochromator is used and the crystals are mechanically bent to account for the vertical divergence (0.2 mrad) of the x-rays from the source. A toroidal mirror is placed after the monochromator, which provides the focussing...

The necessary pump powers can be achieved either by other lasers (e.g. nitrogen lasers, solid-state lasers or even focussed He-Ne- or Ar+-gas lasers) or by flash-lamps. The simplest practical arrangement is a square spectrophotometer cell, polished on all sides, containing the dye solution which is pumped by a nitrogen laser whose beam is focussed into a line parallel to and directly behind one of the cell windows. Then the Fresnel reflection from the two adjacent windows gives enough feedback in most cases, so that no additional resonator mirrors are needed and the dye laser oscillation starts. 

The advantage of the arc lamp is that the actual light source is very small. It is concentrated between the electrodes in a volume of a few mm3, and the outgoing beam can be focussed accurately by optical devices such as lenses and curved mirrors. However, the power supplies needed for arc lamps are much more complex and expensive than those for incandescent lamps. Arc lamps are not electrically conducting when cold, so that a high voltage pulse is required to start them. The current must be controlled very accurately to ensure a steady light intensity. 

Fig. 1.10. The fully electrostatic high-brightness positron beam developed by the Brandeis group. The positron Soa gun is located near B. The beam is deflected at C using a cylindrical mirror analyser and focussed onto a remoderator in chamber D. The extracted beam is then focussed and remoderated at the lower left of D. The double brightness-enhanced beam is then transported into the target chamber, E. Reprinted from Nucl. Instrum. Methods B143, Charlton, Review of Positron Physics, 11-20, copyright 1998, with permission from Elsevier Science.

In the "one laser experiment (Figure 1), the ions were desorbed by a single laser pulse using a focussing lens. A mirror... 

---