In a monomolecular reaction, how can a reactant experience collision to react? Monomolecularity means no molecules which experience collision with each other taking part in reaction. So how it is possible that in monomolecular reaction only one reactant experiences collision?
One reactant, e.g. O3, a pure chemical species, has many molecules that are in continual motion and can collide with each other (as well as with the walls of a container or with a catalyst), so you'll get some O2 generated. Or the converse, if you have UV photons colliding with pure O2 gas.