Quartz monochromator

Fig. 12. Eosin in glycerol (7 X 10 Af) and eosin in ethanol (1.5 X 10 W). (a) Fluorescence emission spectrum at +30°C. (6) delayed emission spectrum (DES) at +69°C. (c) DES at +48°C. (d) DES at + 18°C. (e) DES at -40°C. Delayed emission spectra at a sensitivity 600 times greater than that for the fluorescence emission spectrum. (/) Fluorescence emission spectrum at -J-22°C. (g) delayed emission spectrum (DES) at +71 °C. (h) DES at +43°C. (j) DES at +22°C. (Z) DES at — 7°C. (m) DES at —58°C. (s) Sensitivity of 1)558 photomultiplier with quartz monochromator (unite of quanta and frequency). Delayed emission spectra at a sensitivity 3000 times greater than that for the fluorescence emission spectrum.

For measurements of ultraviolet light from a quartz mercury arc passes through a quartz monochromator, silica lens, and window, and is focused onto the crystal at an angle of incidence of about 45°. The saturated photocurrent is collected by the surrounding metal drum to determine ip by the Fowler method (4). A vacuum thermopile measures the light intensity. Photoelectric and diffraction currents are measured by a vibrating reed electrometer. Ultra-high-vacuum techniques are employed, which result in residual pressures of less than 10-9 mm. of Hg. 

X-ray Diffraction Studies. They are performed under vacuum with a Guinier type focussing camera equipped with a bent quartz monochromator giving a linear collimation of the CuK (X = 1.54 A) radiation (8). 

Fig. 44. A crystal quartz monochromator. See Calvert and Pitts p. 725. References pp. 104-JIl...

Figure 2.52. Diffraction pattern of a sample of cerium oxide powder obtained by using a Bragg-Brentano diffractometer equipped with a copper anode andfront quartz monochromator [BAL 04]...

Bragg-Brentano diffractometer, front curved quartz monochromator, front Sober slits... 

The excitation wavelength, ex> or fluorescence wavelength, Xem. of light is often selected through a monochromator. The dispersion element in a monochromator may be either a grating or a quartz prism. A grating monochromator is preferred since the reciprocal dispersion, D" is constant over all X, while for a prism this parameter varies with X and is cumbersome to account for. Hence a quartz monochromator is rarely used in most standard laboratory instruments. 

Monochromators employing prisms for dispersion use a Littrow 60° prism plane mirror mount. Midinfrared instruments employ a sodium ehloride prism for the region from 4000-650 cm (2.5-15.4 pm), a potassium bromide or cesium iodide prism and optics extend the useful speetrum to 400 em (25 pm) or 270 em (37 pm), respectively. Quartz monochromators, designed for the ultraviolet visible region, extend their eoverage into the near infrared to 2500 cm (4 pm). 

Figure 8.3 An X-ray monochromator using a bent quartz crystal Q T is the target chamber...

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

Quartz also has modest but important uses in optical appHcations, primarily as prisms. Its dispersion makes it useful in monochromators for spectrophotometers in the region of 0.16—3.5 m. Specially prepared optical-quality synthetic quartz is requited because ordinary synthetic quartz is usually not of good enough quality for such uses, mainly owing to scattering and absorption at 2.6 p.m associated with hydroxide in the lattice. 

Monochromators. Replica gratings and narrow band-pass filters are used commonly, more so than quartz prisms. Computer control of the monochromator is available in some instruments, so that optimum intensity at the desired wavelength or maximum absorption by the examined substance can be obtained. 

Filter-photometer or spectrophotometer incorporating prism or grating monochromator, phototube photomultiplier or diode array, glass, quartz or plastic cells. 

The line width of the X-ray source is on the order of 1 eV for A1 or Mg Ka sources but can be reduced to better than about 0.3 eV with the use of a monochromator. A monochromator contains a quartz crystal which is positioned at the correct Bragg angle for A1 Ka radiation. The monochromator narrows this line significantly and focuses it onto the sample. It also cuts out all unwanted X-ray satellites and background radiation. An important advantage of using a monochromator is that heat and secondary electrons generated by the X-ray source cannot reach the sample. 

The absorption sample is mounted in the standard filter holder located between the monochromator crystal and the geiger-counter receiving slit. Pertinent distances are as follows target to quartz ciystal, 17 cm. quartz crystal to receiving slit, 17 cm. absorption sample to receiving slit, 6 cm. 

---