Question

What is the difference between a Soller slit and a divergence limiting slit?

What is the function of a monochromator? How is it constructed? When should it be used?

Answer 1

Difference between a Soller slit and a divergence limiting slit:

Soller slits used to limit the axial (vertical / out-of-plane) divergence of the incident & diffracted X-ray beams. Using soller slits improves peak shape and the resolution in -type scans, especially at low scattering angles.

Divergence slits are used to limit the divergence of the incident X-ray beam. The slits block X-rays that have too great a divergence. The size of the divergence slit influences peak intensity and peak shapes. Narrow divergence slits reduce the intensity of the X-ray beam, the length of the X-ray beam hitting the sample and produce sharper peaks. The instrumental resolution used, is improved so that closely spaced peaks can be resolved. Divergence slits are fitted in the incident beam path to control the equatorial divergence of the incident beam, and thus, the amount (length) of the sample that is irradiated by the incident x-ray beam.

Monochromator

A monochromator is an optical dispersing device that is used to select a narrow band of light (i.e., optical radiation) from a wider range of wavelengths available at the input. Ideally, a monochromator should produce a single wavelength of optical radiation at its output.

Function:

The monochromator is included as an important device of the optical system of an atomic absorption spectrophotometer. The function of this device in atomic absorption is to separate the spectral line of interest from others spectral lines with different wavelengths emitted by the hollow-cathode lamp or to isolate a single atomic resonance line from the spectrum of lines emitted by the hollow cathode lamp. The desired spectral line is chosen with the preferred wavelength and bandwidth by an appropriate monochromator's setting named grating. A grating is a reflective surface, scored either mechanically or holographically with parallel grooves that can be designed for different wavelength regions. Generally, most of the instruments are equipped with two gratings with the goal to cover a wavelength range from 189 to 851 nm which is used in atomic absorption.

Construction:

A typical monochromator design is shown below. It consists of the diffraction grating (dispersing element), slits, and spherical mirrors.

The monochromator comprises a dispersive element, an entrance slit and mirrors to create a parallel beam similar to sunlight, and an exit slit and mirrors to extract the monochromatic light.

Monochromators are used in many optical measuring instruments and in other applications where tunable monochromatic light is wanted. Sometimes the monochromatic light is directed at a sample and the reflected or transmitted light is measured. Sometimes white light is directed at a sample and the monochromator is used to analyze the reflected or transmitted light. Two monochromators are used in many fluorometers; one monochromator is used to select the excitation wavelength and a second monochromator is used to analyze the emitted light. In hard X-ray and neutron optics, crystal monochromators are used to define wave conditions on the instruments.