Spectroscopy is the branch of science which deals with the study of interaction of electromagnetic radiation with matter.
Because of such interaction the matter absorbs or emits the energy in discrete amounts called “quanta”.
The molecule has always characteristic electronic, vibrational and rotational energy levels.
The difference between the electronic energy level is very large and hence the spacing between \(E1\) and \(E2\) is large. The difference between two electronic energy levels goes on decreasing. \(i.e. \ (E2 – E1) > (E3 – E2),\) \((E3 – E2) > (E4 – E3)\) and so on.
Each electronic energy level is further sub-divided into group of vibrational energy levels i.e., between \(E1\) and \(E2\) there are \(V0, V2, V3…. Vn\) vibrational energy levels. The difference between two vibrational energy levels goes on decreasing \(i.e. \ (V1 – V0) > (V2 – V1),\) \((V2 – V1) > (V3 – V2)\) and so on.
Each vibrational energy levels are further sub – divided into number of rotational energy levels. i.e. \(J0, J1, J2, …… Jn\). The difference between two rotational energy levels goes on increasing. \(i.e. \ (J1 – J0) < (J2 - J1),\) \((J2 - J1) < (J3 - J2)\) and so on.
When there is a change in the electronic energy levels, there will be simultaneous change in vibrational and rotational energy levels. The total energy absorbed causes simultaneous changes in electronic transition, vibrational changes and rotational changes.