Multistate electronic quenching: Nonadiabatic pathways in NO A ²Σ⁺ + O₂X ³Σ_g ^- scattering

The quenching of NO A ²Σ⁺ with O₂ as a collisional partner is important for combustion and atmospheric processes. There is still a lack of theoretical understanding of this event, especially concerning the nature of the different quenching pathways. In this work, we provide potential energy surfaces (PESs) of 20 electronic states of this system. We computed the spin-doublet and spin-quartet PESs using SA-CASSCF and XMS-CASPT2. We find two potential quenching pathways. The first one (Q₁) is a two-step orientation-specific process. The system first undergoes an electron transfer (NO⁺ X ¹Σ⁺ + O₂ ^-X ²Π_g) at short distances, before crossing to lower neutral states, such as NO X ²Π + O₂a ¹Δ_g, O₂b ¹Σ_g ⁺, O₂X ³Σ_g ^-, or even 2 O(³P). The second quenching pathway (Q₂) is less orientation-dependent and should be sudden without requiring the proximity conditioning Q₁. The Q₂ cross section will be enhanced with increasing initial vibrational level in both O₂ and NO. It is responsible for the production of NO X ²Π with higher O₂ excited states, such as O₂c ¹Σ_u ^-, A'³Δ_u, or A ³Σ_u ⁺. Overall, this work provides a first detailed theoretical investigation of the quenching of NO A ²Σ⁺ by O₂X ³Σ_g ^- as well as introduces a weighting scheme generally applicable to multireference, open-shell bimolecular systems. The effect of spin-multiplicity on the different quenching pathways is also discussed.