Conical Intersection Induced Valley-Ridge Inflection Drives Chemiluminescence in the SO₂ Afterglow Reaction

The prevailing mechanism of chemiluminescence is largely based on cyclic peroxide studies, while insights into other systems are limited, and a general mechanistic framework remains elusive. Here, we investigate the chemiluminescence mechanism in the SO₂ afterglow (SO + O) reaction, which produces SO₂ in both ground and excited electronic states, employing XMS-CASPT2 and nonadiabatic dynamics calculations. The T₁ surface features a valley-ridge inflection (VRI) originating from a T₁/T₂ conical intersection (CI), hence termed a CI-induced VRI. Unlike conventional VRI bifurcations that produce products of the same electronic character, the pathway here branches to generate SO₂ in distinct electronic states. Furthermore, in contrast to the previously reported CI-induced VRI on the ground-state surface of the air afterglow, nonadiabatic effects significantly influence the dynamics around this topology in the present reaction. Collectively, this CI-induced VRI mechanism is fundamentally distinct from peroxide-based chemiluminescence and may represent a recurring mechanistic motif in gas-phase chemiluminescence.