IR Spectroscopy
In IR spectroscopy, absorptions of the sample are observed starting from the excitation radiation in the infrared spectral range. The energy of the radiation stimulates oscillations between the atoms in the molecule or a solid state lattice. Theoretically, the oscillation frequencies of the molecules can be described as harmonic oscillators using the spring force model. If one continues the considerations on the basis of wave mechanics and considers only the accessible, discrete energy levels, one obtains the equation for two-atom molecules:
E=h/2π * √[f/µ*(ν+½)]
(E energy, h Planck's effective quantum, f force constant, μ reduced mass, ν oscillation quantum number)
In reality, molecules do not behave completely harmonic (at too large oscillation amplitudes, molecules would dissociate resp. would break open grid), so overtone-vibrations contrary to previous equation do not occur completely at double energy of spectrum. With polyatomic molecules the oscillations of the individual atoms are coupled and the theoretical description of the oscillations becomes clearly more complicated. However, the number of oscillations in a molecule can be determined very precisely according to the following equation[1]:
Z=3n-6
(Z number of oscillations, n number of atoms in the molecule; for linear molecules: Z = 3n - 5)
The vibration types are described with Greek letters and basically divided into valence vibrations (also called stretching vibrations, νs symmetrical, νas antisymmetrical) and deformation vibrations (δ and γ bending vibrations, τ twisting-v., ρ rocking-v., ω wagging-v.). In IR spectroscopy, only those oscillations can be observed that lead to a change in the dipole moment of a molecule. Consequently, the oscillations are divided into IR-active and IR-inactive.
The instrumentation of IR spectroscopy differs significantly between near, middle and far infrared. While lenses, optics and detectors from UV-VIS spectroscopy can still be used up to a wavelength of about 1000 nm, observation in the near, middle and far infrared requires the special detectors mentioned above based on InGaAs or MCT. Some of the latter have to be cooled with liquid nitrogen. From the mid-infrared range from 2500 nm, the use of special optical materials such as potassium bromide (KBr), zinc selenide (ZnSe), germanium (Ge) or silicon (Si) is mandatory, as only selected materials are transparent in this spectral range. Up to 2500 nm the excitation can also be carried out by halogen lamps as they emit a considerable part of the emission in the near infrared as thermal radiation. From the mid infrared onwards, red hot SiC ceramics ('Globar'), for example, are used as emission sources. In general, the heat input of the IR sources must also be taken into account for sensitive samples.
[1] A molecule with n atoms has 3n degrees of freedom of movement corresponding to the three spatial directions. Of these 3 are the translational motion and 3 the rotational motion (with linear molecules the rotation is identical for two spatial directions and only 2 degrees of freedom are subtracted), each along the spatial axes. The remaining degrees of freedom are accounted for by the oscillation movements.