High resolution vibronic state-specific dissociation of NO2+ in the 10.0–15.5 eV energy range by synchrotron double imaging photoelectron photoion coincidence

Abstract

Vacuum ultraviolet (VUV) photoionization and dissociative photoionization of NO₂ in the 10.0–15.5 eV energy range have been investigated in detail by using high-resolution double imaging photoelectron photoion coincidence (i²PEPICO) at synchrotron SOLEIL. Five low-lying electronic states of the NO₂⁺ cation, X¹Σ_g⁺, a³B₂, b³A₂, A¹A₂ and B¹B₂, are prepared with well-resolved vibronic structures and their state-specific dissociation mechanisms are unraveled and discussed. The present experimental results clarify that except the X¹Σ_g⁺ ground electronic state and the first three vibrational levels of the a³B₂ electronic state, the other cationic states of NO₂⁺ within the present energy range are totally dissociative towards the NO⁺(X¹Σ⁺) + O(³P) and/or NO⁺(X¹Σ⁺) + O(¹D) dissociation limits. An energy barrier exists along the direct dissociation route of the a³B₂ state, and the b³A₂ electronic state is a quasi-bound state with a very shallow well, both of which adiabatically correlate to the NO⁺(X¹Σ⁺) + O(³P) dissociation limit. The a³B₂ state mainly with bending vibration excitations undergoes a non-adiabatic transition to the 2³A′′(³B₁) repulsive state along its bending potential energy curve and then predissociates into the NO⁺(X¹Σ⁺) + O(³P) products. Our experimental results firstly demonstrate that the NO⁺(X¹Σ⁺, v) fragment ions produced from individual vibronic levels of the dissociative NO₂⁺(a³B₂, b³A₂) states are produced at the v = 0 ground vibrational level with a high rotational population due to the excitation of the vibrational bending mode of NO₂⁺ and the associated imparted torque upon dissociation. The slower predissociations of the A¹A₂ and B¹B₂ electronic states via their spin–orbit couplings with the repulsive 2³A′′(³B₁) state produce the NO⁺(X¹Σ⁺) and O(³P) fragments with a long vibrational progression. In addition, the B¹B₂ state can also undergo a radiationless transition such as internal conversion into the hot X¹Σ_g⁺ state and then dissociate into the second dissociation channel correlated to the NO⁺(X¹Σ⁺) and O(¹D) products.