A monochromator ( Greek : mono = a + chroma = color) is an optical device for the spectral isolation of a certain wavelength from an incident beam (mostly electromagnetic radiation such as light, X-rays, synchrotron radiation, but also neutron radiation ). Electromagnetic radiation without treatment - due to its origin - is polychromatic , i.e. composed of different wavelengths (poly = a lot). When using a monochromator, the unwanted portion of the radiation is absorbed or deflected. The functionality of monochromators differs for different wavelength ranges of electromagnetic radiation.
The following principles can be used for light, i.e. electromagnetic radiation in the visible wavelength range or nearby secondary ranges.
The incident light is imaged in front of the monochromator on its entrance slit, which serves as a secondary light source for the monochromator . The light is then fanned out continuously within the monochromator depending on its wavelength (see also: electromagnetic wave ). This is done using a dispersing element (e.g. a prism ) or an optical grating (on which the light is not dispersed, but rather diffracted ).
By means of a further slit diaphragm, the exit slit, the smallest possible wavelength range (= spectral color ) is allowed to pass through from this fanned out light with the desired wavelength. This gap serves as a secondary light source for the rest of the measurement setup. The unwanted part of the radiation is absorbed by the screen . So that this selection is as pure as possible, the entrance slit is mapped onto the exit slit using optical means (mostly concave mirrors ).
The width of the entry and exit slit can usually be adjusted manually; they are usually set to the same width (typically 0.5 to 2 mm). The optimal width results from the compromise between the required intensity of the light (i.e. not too narrow) and the required spectral resolution (i.e. not too wide). The spectral slit width indicates which wavelength range is covered from the left to the right edge of the exit slit, i.e. the wavelength resolution or the spectral resolving power . In addition to the gap width, the gap heights can sometimes also be adjusted using another pair of diaphragms (typically 10 to 20 mm). Depending on the area of application and the intensity of the incident light, the screen must be cooled, as the absorption of the light causes the absorber to heat up .
A prism is used when a large wavelength range is to be covered. Depending on the wavelength range, prisms made of glass (visible VIS, near IR, near UV) or made of rock salt (NaCl, for far IR) are used. The latter require intensive care because of their hygroscopicity .
In contrast to the prism, the deflection angle of the grating depends on the wavelength via a sine function . The spectral resolving power depends not only on the wavelength-dependent deflection angle but also on the slit width. (In addition, higher-order light is produced.) So-called holographic gratings avoid the disadvantages mentioned last more and more. There are now also versions of the holographic gratings that are themselves additionally shaped like a concave mirror ( concave blazed holographic gratings ); This means that all other imaging elements (mirrors) between the entry and exit slits are superfluous, which further reduces losses and imaging errors.
To set the desired wavelength, the dispersing element (sometimes also one of the other imaging elements such as a mirror) is usually mounted on a turntable, which is controlled externally via a shaft as a mechanical drive. In automatic operation (manual operation is still possible), an electric motor drive and a rotary encoder (e.g. multi-turn analog potentiometer) for recording the current position are flanged onto this shaft.
To reduce the scattered light, double monochromators are also built, which practically consist of two single monochromators mounted one behind the other in a common housing. They have a third optical gap in the middle and have the two rotary drives coupled without play for adjusting the wavelength. The series connection of identical monochromators does not increase the spectral resolution. Scattered light is light of a different wavelength that does not fall within the wavelength interval of the monochromator. With a non-ideal dispersive element (e.g. optical grating ), this scattered light is falsely mapped onto the exit slit and detected. By applying the frequency selection twice with the double monochromator, the disturbing stray light is reduced. The double monochromator is used, for example, in the spectroscopy of Raman scattering .
Before a measurement, the association between the mechanical position of the dispersing element and the wavelength selected must be determined, i.e. the calibration . For this purpose, a light source with known, narrow-band spectral lines is usually used and the intensity profile behind the monochromator is measured as a function of the monochromator position. The mercury vapor lamp is a suitable light source here , as it has well-known lines in the entire visible and UV range.
Between the interpolation points found by these lines, the calibration curve is later interpolated in the form of a curve that is as smooth as possible in order to obtain the assignment at any intermediate points. One can even carefully extrapolate something beyond the range of the observed spectral lines .
A monochromator for X-rays, i.e. electromagnetic radiation in the wavelength range of 10–0.002 nm , basically fulfills the same function, but in a different way: the radiation is reflected on a suitable crystal at a suitable angle according to the Bragg condition . Since the X-rays penetrate into the crystal, the radiation will not only be reflected on the crystal surface, but also on many lattice planes of the crystal lattice . A ray reflected from the outermost lattice plane travels a shorter distance than a ray reflected from a plane within the crystal. This distance difference is called the path difference . This path difference leads to interference of the rays. Due to the large number of different path differences and the high number of reflective grating levels, almost all wavelengths experience destructive interference. Only the wavelength that meets the Bragg condition at the given angle interferes constructively. For radiographic measurements, curved crystal monochromators are usually used, from which a curve has been milled. Such a monochromator can be used for focusing or precollimation of a divergent X-ray beam.
Use crystal monochromators z. B. for:
- X-ray diffractometry
- X-ray fluorescence analysis
- Photoelectron Spectroscopy
- Absorption processes such as EXAFS , XANES , AAS .