Molecular Physics seminar

Surprises in the UV photolysis of ozone

by Prof. Gus Hancock (Physical and Theoretical Chemistry Laboratory, Oxford University)

FA31 ()


Ozone is best known for its important role in the absorption of otherwise harmful uv radiation in the atmosphere. It has a weak bond, and when it absorbs it dissociates, and this talk will review some of the surprises that we have found by looking at the dynamics of the process. For instance, the dominant dissociation products appear from the spin allowed decomposition to give both electronically excited molecular and atomic oxygen: O3 + hv → O2 (a1Δg) + O(1D) The energetic threshold for this process when ground state ozone is photolysed is 310 nm: at longer wavelengths however we still see both products because of the high probability of absorption by internally excited molecules, and this has a profound effect on the chemistry of the troposphere. The molecular product is found to have marked alternations in the populations of levels associated with even and odd rotational quantum numbers J, but that is not all – the vector properties of the dissociation which involve J also show alternations, the magnitudes of which depend upon the speed of separation of the fragments. This seemingly simple dissociation process involving just three oxygen atoms is continually springing surprises upon us.