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Title: Scattering off molecules far from equilibrium

Abstract

We report that pump-probe gas phase X-ray scattering experiments, enabled by the development of X-ray free electron lasers, have advanced to reveal scattering patterns of molecules far from their equilibrium geometry. While dynamic displacements reflecting the motion of wavepackets can probe deeply into the reaction dynamics, in many systems, the thermal excitation embedded in the molecules upon optical excitation and energy randomization can create systems that encompass structures far from the ground state geometry. For polyatomic molecular systems, large amplitude vibrational motions are associated with anharmonicity and shifts of interatomic distances, making analytical solutions using traditional harmonic approximations inapplicable. More generally, the interatomic distances in a polyatomic molecule are not independent and the traditional equations commonly used to interpret the data may give unphysical results. Here, we introduce a novel method based on molecular dynamic trajectories and illustrate it on two examples of hot, vibrating molecules at thermal equilibrium. When excited at 200 nm, 1,3-cyclohexadiene (CHD) relaxes on a subpicosecond time scale back to the reactant molecule, the dominant pathway, and to various forms of 1,3,5-hexatriene (HT). With internal energies of about 6 eV, the energy thermalizes quickly, leading to structure distributions that deviate significantly from their vibrationless equilibrium. Themore » experimental and theoretical results are in excellent agreement and reveal that a significant contribution to the scattering signal arises from transition state structures near the inversion barrier of CHD. In HT, our analysis clarifies that previous inconsistent structural parameters determined by electron diffraction were artifacts that might have resulted from the use of inapplicable analytical equations.« less

Authors:
ORCiD logo [1];  [1];  [1];  [1]; ORCiD logo [1];  [1]; ORCiD logo [1];  [2];  [2];  [3];  [3];  [3];  [3];  [3]; ORCiD logo [2]; ORCiD logo [3]; ORCiD logo [1]
  1. Brown Univ., Providence, RI (United States)
  2. Univ. of Edinburgh, Scotland (United Kingdom)
  3. SLAC National Accelerator Lab., Menlo Park, CA (United States). Linac Coherent Light Source (LCLS)
Publication Date:
Research Org.:
Brown Univ., Providence, RI (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division; US Army Research Office (ARO); Carnegie Trust for the Universities of Scotland
Contributing Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
OSTI Identifier:
1596986
Grant/Contract Number:  
SC0017995; AC02-76SF00515; W911NF-17-1-0256; CRG050414
Resource Type:
Accepted Manuscript
Journal Name:
Journal of Chemical Physics
Additional Journal Information:
Journal Volume: 151; Journal Issue: 8; Journal ID: ISSN 0021-9606
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Yong, Haiwang, Ruddock, Jennifer M., Stankus, Brian, Ma, Lingyu, Du, Wenpeng, Goff, Nathan, Chang, Yu, Zotev, Nikola, Bellshaw, Darren, Boutet, Sébastien, Carbajo, Sergio, Koglin, Jason E., Liang, Mengning, Robinson, Joseph S., Kirrander, Adam, Minitti, Michael P., and Weber, Peter M. Scattering off molecules far from equilibrium. United States: N. p., 2019. Web. https://doi.org/10.1063/1.5111979.
Yong, Haiwang, Ruddock, Jennifer M., Stankus, Brian, Ma, Lingyu, Du, Wenpeng, Goff, Nathan, Chang, Yu, Zotev, Nikola, Bellshaw, Darren, Boutet, Sébastien, Carbajo, Sergio, Koglin, Jason E., Liang, Mengning, Robinson, Joseph S., Kirrander, Adam, Minitti, Michael P., & Weber, Peter M. Scattering off molecules far from equilibrium. United States. https://doi.org/10.1063/1.5111979
Yong, Haiwang, Ruddock, Jennifer M., Stankus, Brian, Ma, Lingyu, Du, Wenpeng, Goff, Nathan, Chang, Yu, Zotev, Nikola, Bellshaw, Darren, Boutet, Sébastien, Carbajo, Sergio, Koglin, Jason E., Liang, Mengning, Robinson, Joseph S., Kirrander, Adam, Minitti, Michael P., and Weber, Peter M. Thu . "Scattering off molecules far from equilibrium". United States. https://doi.org/10.1063/1.5111979. https://www.osti.gov/servlets/purl/1596986.
@article{osti_1596986,
title = {Scattering off molecules far from equilibrium},
author = {Yong, Haiwang and Ruddock, Jennifer M. and Stankus, Brian and Ma, Lingyu and Du, Wenpeng and Goff, Nathan and Chang, Yu and Zotev, Nikola and Bellshaw, Darren and Boutet, Sébastien and Carbajo, Sergio and Koglin, Jason E. and Liang, Mengning and Robinson, Joseph S. and Kirrander, Adam and Minitti, Michael P. and Weber, Peter M.},
abstractNote = {We report that pump-probe gas phase X-ray scattering experiments, enabled by the development of X-ray free electron lasers, have advanced to reveal scattering patterns of molecules far from their equilibrium geometry. While dynamic displacements reflecting the motion of wavepackets can probe deeply into the reaction dynamics, in many systems, the thermal excitation embedded in the molecules upon optical excitation and energy randomization can create systems that encompass structures far from the ground state geometry. For polyatomic molecular systems, large amplitude vibrational motions are associated with anharmonicity and shifts of interatomic distances, making analytical solutions using traditional harmonic approximations inapplicable. More generally, the interatomic distances in a polyatomic molecule are not independent and the traditional equations commonly used to interpret the data may give unphysical results. Here, we introduce a novel method based on molecular dynamic trajectories and illustrate it on two examples of hot, vibrating molecules at thermal equilibrium. When excited at 200 nm, 1,3-cyclohexadiene (CHD) relaxes on a subpicosecond time scale back to the reactant molecule, the dominant pathway, and to various forms of 1,3,5-hexatriene (HT). With internal energies of about 6 eV, the energy thermalizes quickly, leading to structure distributions that deviate significantly from their vibrationless equilibrium. The experimental and theoretical results are in excellent agreement and reveal that a significant contribution to the scattering signal arises from transition state structures near the inversion barrier of CHD. In HT, our analysis clarifies that previous inconsistent structural parameters determined by electron diffraction were artifacts that might have resulted from the use of inapplicable analytical equations.},
doi = {10.1063/1.5111979},
journal = {Journal of Chemical Physics},
number = 8,
volume = 151,
place = {United States},
year = {2019},
month = {8}
}

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