Question

We have stimulated emission and spontaneous emission. Do we also have stimulated absorption and spontaneous absorption?

Answer 1

Yes, we have Stimulated Absorption. But Spontaneous Absorption does not exist.

Explanation:

When an energy photon $\left(E_{2}-E_{1}=h v\right)$ occurs on an atom in ground condition, the ground-state atom E1 will absorb the photon and jump to a higher energy ($E_2$).  This process is called accelerated absorption or absorption by induction.

We have no mechanism, like spontaneous absorption but stimulated absorption is possible(Stimulated absorption is nothing but the mechanism by which an atom unexpectedly leaps to a higher energy state in its ground state, which results in photon absorption). It is because we need to incident a photon of enough energy into the atom for absorption to activate the atom.