Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling

Journal Article · Mon Nov 06 00:00:00 EST 2017 · Proceedings of the National Academy of Sciences of the United States of America

DOI:https://doi.org/10.1073/pnas.1712566114· OSTI ID:1408161

Peters, William K.  ^[1]; Couch, David E. ^[1]; Mignolet, Benoit ^[2]; Shi, Xuetao ^[3]; Nguyen, Quynh L. ^[1]; Fortenberry, Ryan C. ^[4]; Schlegel, H. Bernhard ^[3]; Remacle, Françoise ^[2]; Kapteyn, Henry C. ^[1]; Murnane, Margaret M. ^[1]; Li, Wen ^[3]

---

1. JILA, University of Colorado, Boulder, CO 80309,, Department of Physics, University of Colorado, Boulder, CO 80309,
2. Theoretical Physical Chemistry, Unité de Recherche Molecular Systems (UR MOLSYS) University of Liège, B4000 Liège, Belgium,
3. Department of Chemistry, Wayne State University, Detroit, MI 48202,
4. Department of Chemistry and Biochemistry, Georgia Southern University, Statesboro, GA 30460

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.

• Research Organization: Wayne State Univ., Detroit, MI (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: SC0012628
• OSTI ID: 1408161
• Alternate ID(s): OSTI ID: 1527121
• Journal Information: Proceedings of the National Academy of Sciences of the United States of America, Journal Name: Proceedings of the National Academy of Sciences of the United States of America Vol. 114 Journal Issue: 52; ISSN 0027-8424
• Publisher: Proceedings of the National Academy of Sciences
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
conical intersection
nonadiabatic dynamics
Rydberg–valence electronic states
PEPICO