|Title||Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling|
|Publication Type||Journal Article|
|Year of Publication||2017|
|Authors||Peters, WK, Couch, DE, Mignolet, B, Shi, X, Nguyen, QL, Fortenberry, RC, H. Schlegel, B, Remacle, F, Kapteyn, HC, Murnane, MM, Li, W|
|Journal||Proceedings of the National Academy of Sciences|
|Pagination||E11072 - E11081|
Highly excited electronic states are challenging to explore experimentally and theoretically-due to the large density of states and the fact that small structural changes lead to large changes in electronic character with associated strong nonadiabatic dynamics. They can play a key role in astrophysical and ionospheric chemistry, as well as the detonation chemistry of high-energy density materials. Here, we implement ultrafast vacuum-UV (VUV)-driven electron-ion coincidence imaging spectroscopy to directly probe the reaction pathways of highly excited states of energetic molecules-in this case, methyl azide. Our data, combined with advanced theoretical simulations, show that photoexcitation of methyl azide by a 10-fs UV pulse at 8 eV drives fast structural changes and strong nonadiabatic coupling that leads to relaxation to other excited states on a surprisingly fast timescale of 25 fs. This ultrafast relaxation differs from dynamics occurring on lower excited states, where the timescale required for the wavepacket to reach a region of strong nonadiabatic coupling is typically much longer. Moreover, our theoretical calculations show that ultrafast relaxation of the wavepacket to a lower excited state occurs along one of the conical intersection seams before reaching the minimum energy conical intersection. These findings are important for understanding the unique strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules. Although such observations have been predicted for many years, this study represents one of the few where such strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules have been conclusively observed directly, making it possible to identify the ultrafast reaction pathways.
|Short Title||Proc Natl Acad Sci USA|