Excited-State Roaming Dynamics in Photolysis of a Nitrate Radical

Roaming dynamics has been recognized as one of the key mechanisms in atmospheric and combustion processes. In this Letter, we report the first example of roaming dynamics that occurs on an excited-state potential energy surface. In the photodissociation of a nitrate radical, systematic CASPT2 reaction path search and DFT dynamics calculations show that the roaming dynamics occurs on the first excited doublet state (D1). The direct dissociation on D1 gives the minor vibrationally cold O2, while the indirect dissociation after the nonadiabatic transition to the ground doublet state (D0) produces the major vibrationally hot O2; this proposal explains the recent experimental results.