IR spectrum:

Because each electronic energy level is related along with several vibrational levels and each of the vibrational level is associated with several of the rotational energy levels, the transitions caused through UV-VIS radiation have associated vibrational and rotational transitions. Accordingly a molecular spectra are quite complex and consist of a huge number of transitions.  For this purpose, electronic absorption bands for molecules are commonly broad and the spectrum is known as band spectrum. Figure provides a simplified energy level diagram for a molecule showing the purpose for observing band spectrum. Same the transitions caused through radiations in the IR region have associated rotational elements. Sample IR and UV spectra are given in Figure.

(a)                                                                                         (b)

Figure: Sample a) IR spectrum and b) UV spectrum

Sometimes the sample does not absorb radiation of a given kind under ordinary conditions but on placing the sample within a magnetic field it does. This phenomenon is of immense analytical applications and is the origin of possibly the most exploited molecular structure determination method that is, nuclear magnetic resonance (NMR).

The other important technique of ESR also need magnetic field. Now let us learn about the magnetic field induced absorption.