Fate of the Absorbed Radiation:

As a consequence of the absorption, energy of the photon is transferred to the absorbing species and it obtains an excited state.

M   +   hν       →        M*       (Excitation)

The concentration of the excited species so acquires is quite small and its life time is extremely short.  The excited species commonly loses the extra energy to the other atoms or molecules, present within the medium and reverts back to the ground state. This process is known as non-radiative relaxation and occurs in a short period of time (10^-6 to 10^-9 s). The excess energy is transferred in a series of small steps to the neighboring species as their kinetic energy and is manifested in the form of heat. As a consequence of this, the temperature of the system increases slightly.

M* → M         +          heat     (Relaxation)

In a few cases the relaxation procedures is accompanied through an emission of radiation and is referred to as radiative relaxation. Let us learn about different categories of radiative relaxation mechanisms and the spectra that arise because of these.