The intensity distribution of a particular vibrational structure progression in the electronic absorption spectrum or progression in the electronic emission spectrum displays different kind of pattern.
Let us first discuss the electronic absorption intensity distribution. There are mainly three types of intensity distribution seen in the experiments. In the following we will discuss the nature of these three intensity distributions.

Case-I :
As discussed in the previous lectures that for a progression with , the
 transition, which is known as the vibration origin transition, is the lowest in energy. This first band is seen to be very intense as shown in the figure-33.1.

Further bands in progression with  progression appear withy very rapidly decreasing intensities.

This kind of intensity distribution is observed for the atmospheric oxygen in the red part of the solar spectrum.