For many years, the dynamics in high-excited states of polyatomic molecules has been predicted to be ultrafast due to the dense manifold of electronic states. However the production of ultrashort (sub 15fs) UV pulses needed to promptly excite molecules to their high excited states and the modeling of the induced dynamics occurring in a band of several strongly coupled high electronic states is still currently experimentally and theoretically challenging. In this joint experimental-theoretical study, the methyl azide molecule is excited by a 10fs UV pulse produced from the 5<sup>th</sup> harmonic of a 780nm laser pulse. The induced dynamics is then probed by a delayed 800nm IR pulse and the photoelectron is measured in coincidence with the photoion (PEPICO experiment). A fast sub 30fs decrease of the ion yield was observed as a function of the pump-probe delay, indicating that a fast dynamics occurs in the high-excited states of the methyl azide molecule. To shed light on this fast dynamics, we computed (i) the electronic structure (ii) the photoionization lifetimes and (iii) the nonradiative relaxation of the high-excited state accessed by the pump. We focus here on the theoretical part of this study and we show that the fast decrease of ion yield measured in the experiment is due to the relaxation of a mixed Rydberg-valence state to a valence state that is dark for the photoionization by the delayed IR pulse (see Figure). The dynamical simulations of the nonadiabatic relaxation carried out with the <i>ab-initio</i> multiple spawning method showed a fast transfer of population occurring in a timescale of around 20fs, in agreement with the experiment.

Probing the high-excited states dynamics in the methyl azide molecule: A joint experimental-theoretical study

256th National Meeting (2018)

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