Figure 7.7
Operating principle of a diffraction
grating, φ is the angle of incidence;
d is the groove spacing; θ is the
angle of reflection.cos θ and hence the angular dispersion can be considered constant. This offers a decided advantage over
prism dispersion when interpolations are to be made from standard line spectra.
Monochromators for dispersing X-radiation utilize single crystals which behave like a diffraction
grating. The spacing of the crystal lattice determines the angles at which radiation is reflected and
generally two or more different crystals are required to cover the X-ray region of the spectrum.
In the microwave and radiowave regions, virtually monochromatic incident radiation is generated and
the need for a monochromator is thus obviated.
Interferometers
Infrared spectrometers employing an interferometer and having no monochromator are now
predominant. These non-dispersive instruments, known as Fourier transform (FT) spectrometers, have
increased sensitivity and can record spectra much more rapidly than the dispersive type. This is because
instead of scanning a spectrum over a given wavenumber range, a process that takes a dispersive
instrument from 1 to 4 minutes, the interferometer enables all the data to be collected virtually
simultaneously in the form of an interferogram then mathematically transformed (using Fourier
integrals) by computer into a conventional spectrum.
A diagram of a typical interferometer (Michelson type) is shown in Figure 7.8. It consists of fixed and
moving front-surface plane mirrors (A and B) and a beamsplitter. Collimated infrared radiation from the
source incident on the beamsplitter is divided into two beams of equal intensity that pass to the fixed
and moving mirrors respectively. Each is reflected back on itself, recombining at the beamsplitter from
where they are directed through the sample compartment and onto the detector. Small