Ultraviolet, Visible, and Infrared Spectrometry UV, Vis, IR

Another complication arises in the interpretation of absorption spectra. If a molecule vibrates with pure harmonic motion and the dipole moment is a linear function of the displacement, then the absorption spectrum will consist of fundamental transitions only. If either of these conditions is not met, as is usually the case, the spectrum will contain overtones (multiples of the fundamental) and combination bands (sums and differences). Most of these overtones and combination bands occur in the near-infrared (0.8-2.0/un). 

Not all vibrations and rotations are infrared-active. If there is no change in dipole moment, then there is no oscillating electric field in the motion, and there is no mechanism by which absorption of electromagnetic radiation can take place. An oscillation, or vibration, about a center of symmetry, therefore, will not be observed in the infrared spectrum (absorption) but can be observed in the Raman spectrum (scattering). 

In summary, therefore, there are five regions of the electromagnetic spectrum of interest  

For gases, we generally need about 250 cm3 at 1 atm to obtain a spectrum. 

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