What happens when higher energy ultraviolet radiations act on the oxygen at the higher level of the atmosphere ?

When higher energy ultraviolet (UV) radiation acts on molecular oxygen ($O_2$) at higher levels of the atmosphere, it splits the diatomic oxygen molecules into free, highly reactive oxygen ($O$) atoms. These free oxygen atoms then combine with intact molecular oxygen ($O_2$) to form ozone ($O_3$).

The chemical reactions representing this process are:

$$O_2 \xrightarrow{\text{UV}} O + O$$

$$O + O_2 \rightarrow O_3 \text{ (Ozone)}$$