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

Abstract

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 coupledmore » 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.« less

Authors:
ORCiD logo; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Wayne State Univ., Detroit, MI (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1408161
Alternate Identifier(s):
OSTI ID: 1527121
Grant/Contract Number:  
SC0012628
Resource Type:
Published Article
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Volume: 114 Journal Issue: 52; Journal ID: ISSN 0027-8424
Publisher:
Proceedings of the National Academy of Sciences
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; conical intersection; nonadiabatic dynamics; Rydberg–valence electronic states; PEPICO

Citation Formats

Peters, William K., Couch, David E., Mignolet, Benoit, Shi, Xuetao, Nguyen, Quynh L., Fortenberry, Ryan C., Schlegel, H. Bernhard, Remacle, Françoise, Kapteyn, Henry C., Murnane, Margaret M., and Li, Wen. Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling. United States: N. p., 2017. Web. doi:10.1073/pnas.1712566114.
Peters, William K., Couch, David E., Mignolet, Benoit, Shi, Xuetao, Nguyen, Quynh L., Fortenberry, Ryan C., Schlegel, H. Bernhard, Remacle, Françoise, Kapteyn, Henry C., Murnane, Margaret M., & Li, Wen. Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling. United States. doi:10.1073/pnas.1712566114.
Peters, William K., Couch, David E., Mignolet, Benoit, Shi, Xuetao, Nguyen, Quynh L., Fortenberry, Ryan C., Schlegel, H. Bernhard, Remacle, Françoise, Kapteyn, Henry C., Murnane, Margaret M., and Li, Wen. Mon . "Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling". United States. doi:10.1073/pnas.1712566114.
@article{osti_1408161,
title = {Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling},
author = {Peters, William K. and Couch, David E. and Mignolet, Benoit and Shi, Xuetao and Nguyen, Quynh L. and Fortenberry, Ryan C. and Schlegel, H. Bernhard and Remacle, Françoise and Kapteyn, Henry C. and Murnane, Margaret M. and Li, Wen},
abstractNote = {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.},
doi = {10.1073/pnas.1712566114},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 52,
volume = 114,
place = {United States},
year = {2017},
month = {11}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record
DOI: 10.1073/pnas.1712566114

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Cited by: 5 works
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Figures / Tables:

Fig. 1 Fig. 1: Femtosecond PEPICO spectroscopy of methyl azide Rydberg–valence states. A shows two laser pulses which interact with an effusive molecular beam; the pump pulse prepares an excited state, which is then ionized by the probe pulse. Ionized fragments and their coincident electrons are detected with timeand position-sensitive detectors. Bmore » shows a schematic of the dynamics in the electronic states. The pump pulse excites a wavepacket on a highly excited state (S8). If the second pulse probes the dynamics at time t = 0 (B, i), it ionizes the molecule to both the cation ground and first excited states. For pump–probe delay before 25 fs (B, ii), the wavepacket has time to evolve on the excited-state potential energy surface. Due to the corresponding increase in vibrational energy, the probe pulse can only ionize to the cation ground state. For longer pump–probe delays (B, iii), the wavepacket has completely relaxed to the dark S4 state, due to strong nonadiabatic coupling, and the ion yield drops to zero. C shows a schematic representation of the potential energy surfaces of the mixed Rydberg–valence state accessed by the pulse and the dark valence state, as well as the conical intersection seam through which the population transfer occurs. The motion of the initial wavepacket on these surfaces is also shown with a white line, as well as points corresponding to the dynamics described in B. D shows excitation orbitals computed with EOM-CCSD for the initially prepared excited state, S8, and the dark state, S4. The initial S8 state has mixed HOMO-excited and (HOMO-1) –excited character, with each component displaying mixed Rydberg and valence character. The dark S4 state is a (HOMO-1) –excited valence state. Isocontours are 0.03 |e|/Å3 for the mixed Rydberg–valence orbitals and 0.07 |e|/Å3 for the pure valence orbitals.« less

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