Soumen Ghosh

The Journal of Chemical Physics, Volume 154, Issue 19, May 2021.
Recent studies have indicated that nitramide (NH2NO2) may be formed more plentifully in the atmosphere than previously thought, while also being a missing source of the greenhouse gas nitrous oxide (N2O) via catalyzed isomerization. To validate the importance of NH2NO2 in the Earth’s atmosphere, the ground and first electronic excited states of NH2NO2 were characterized and its photochemistry was investigated using multireference and coupled cluster methods. NH2NO2 is non-planar and of singlet multiplicity in the ground state while exhibiting large out-of-plane rotation in the triplet first excited state. One-dimensional cuts of the adiabatic potential energy surface calculated using the MRCI+Q method show low-lying singlet electronic states with minima in their potential along the N–N and N–O bond coordinates. Due to vertical excitation energies in the 225–180 nm region, photochemical processes will not compete in the troposphere, causing N2O production to be the predicted major removal process of NH2NO2. In the upper atmosphere, photodissociation to form NH2NO + O (3P) is suggested to be a major photochemical removal pathway.

Conical intersections allow electronically excited molecules to return to their electronic ground state. Here, we observe the fastest electronic relaxation dynamics measured to date by extending attosecond transient-absorption spectroscopy (ATAS) to the carbon K-edge. We selectively launch wave packets in the two lowest electronic states (D₀ and D₁) of C₂H₄⁺. The electronic D₁ -> D₀ relaxation takes place with a short time constant of 6.8 ± 0.2 femtoseconds. The electronic-state switching is directly visualized in ATAS owing to a spectral separation of the D₁ and D₀ bands caused by electron correlation. Multidimensional structural dynamics of the molecule are simultaneously observed. Our results demonstrate the capability to resolve the fastest electronic and structural dynamics in the broad class of organic molecules. They show that electronic relaxation in the prototypical organic chromophore can take place within less than a single vibrational period.