Interaction of Em Radiation with Matter:

In sequence to understand the nature and mechanism of interaction of radiation with matter we would first like you to recall what you have learnt about nature of matter in previous classes. You would recall from your understanding of the quantum mechanical description of the electronic structures of atoms and molecules constituting matter, in which these have unique set of quantised electronic energy states available to them.

Those arise from the motion of electrons around the nucleus and the electrons in atoms and molecules are present in the permitted or permissible energy states only.

Therefore in case of molecules, the rotation and vibration motions are also quantised further to these electronic energy states. The permitted energy level with the least energy is termed as the ground state while the rest are called excited states. The energy levels for a molecule could be schematically display as given in Figure.

Figure: Schematic diagram showing energy levels for a molecule