Ultrafast Mapping of Electronic and Nuclear Structure in the Photo Dissociation of Nitrogen Dioxide.

We investigate the photoinduced dissociation reaction of NO ₂ → NO + O upon electronic excitation of the X̃ ² A ₁ (D ₀ ) to the à ² B ₂ (D ₁ ) state by femtosecond X-ray absorption spectroscopy at the nitrogen K-edge. We obtain key insight into the chemical bond breaking event and its associated electronic structural dynamics. Calculations of the photoinduced reaction allow to assign the transient absorption features at time scales of 10-50 fs to wave packet motions in the excited D ₁ and ground D ₀ states, followed by the formation of the NO photoproduct with a 255 ± 23 fs time constant. Our analysis shows that there is no direct correlation between the 1s core levels and the electronic ground and excited states transition energies and the bond elongation of NO ₂ , while en route to dissociation toward the NO + O photoproducts, in the transient nitrogen K-edge spectra. However, simulations predict that for a sufficiently short UV pump pulse, the early wave packet dynamics in the D ₁ electronic excited state occurring within the first 35 fs along the bending and symmetric stretching modes can be directly mapped in the transient X-ray absorption spectra.