New insight into dissociative photoionization of N₂O at ∼20 eV using threshold photoelectron–photoion coincidence velocity imaging

Highlights: • The unknown D²Π state was confessedly identified in threshold photoelectron spectrum of N₂O at ∼20 eV. • Kinetic energy distributions of NO⁺ fragments dissociated from D²Π and C²Σ⁺ states were observed very similar. • Dissociation mechanisms of N₂O⁺ in D²Π(v₂⁺) and C²Σ⁺(0,0,0) states were proposed respectively. - Abstract: Dissociative photoionization (DPI) of N₂O at ∼20 eV has been reinvestigated with threshold photoelectron–photoion coincidence (TPEPICO) velocity imaging. In threshold photoelectron spectrum, a shoulder peak at 20.045 eV is observed close to the ground vibrational level of C²Σ⁺ state at 20.100 eV. Through comparing the coincident mass spectra recorded at 20.045 and 20.100 eV, the assignment of the shoulder band is corrected to a vibrational excited D²Π ionic state from the previous conclusions of the vibrationless level of b⁴Π or hot band of C²Σ⁺ state. For the dominant photofragment of NO⁺ at 20.045 eV, TPEPICO time-sliced velocity image is measured to obtain the corresponding total kinetic energy and angular distributions. Interestingly, both the bimodal vibrational population and angular distribution of NO⁺ fragment from dissociation of N₂O⁺(D²Π) are very similar to those of N₂O⁺(C²Σ⁺) ions. With the aid of potential energy curves, the DPI mechanisms of N₂O via D²Π ionic state at 20.045 eV along the NO⁺(X¹Σ⁺) + N(²D) and NO⁺(X¹Σ⁺) + N(²P) dissociation channels are clarified, in which the internal conversion from D²Π to B²Π state is the rate-determined step