Ro-vibrational Distribution of NO + Dissociated from NO 2 + Ions in the a 3 B 2 and b 3 A 2 States: A Slow "Impulsive" Dissociation Example Revealed from Threshold Photoelectron-Photoion Coincidence Imaging

To clarify the contentions about dissociative photoionization mechanism of nitrogen dioxide via the a B and b A ionic states, a new threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging has been conducted in the 12.8-14.0 eV energy range at the Hefei Light Source. The fine vibrational-resolved threshold photoelectron spectrum agrees well with the previous measurements. The ro-vibrational distributions of NO , as the unique fragment ion in the dissociation of NO in specific vibronic levels of a B and b A states, are derived from the recorded TPEPICO velocity images. A "cold" vibrational ( = 0) and "hot" rotational population is observed at the a B (0,3,0) and (0,4,0) vibronic levels, while the dissociation of NO in b A (0,0,0) leads to the NO fragment with both hot vibrational and rotational populations. With the aid of the quantum chemical calculations at the time-dependent B3LYP level, minimum energy paths on the potential energy surfaces of the a B and b A states clarify their adiabatic dissociation mechanisms near the thresholds, and this study proposes reliable explanations for the observed internal energy distributions of fragment ions. Additionally, this study provides valuable insights into the application of the classical "impulsive" model on an overall slow dissociation process.