1
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Hervé du Penhoat MA, Souchaud A, Rajpal A, Vuilleumier R, Gaigeot MP, Tavernelli I, Fujii K, Yokoya A, Díaz-Tendero S, Politis MF. Ultrafast fragmentation of highly-excited doubly-ionized deoxyribose: role of the liquid water environment. Phys Chem Chem Phys 2024; 26:15693-15704. [PMID: 38766756 DOI: 10.1039/d4cp00489b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/22/2024]
Abstract
Ab initio molecular dynamics simulations are used to investigate the fragmentation dynamics following the double ionization of 2-deoxy-D-ribose (DR), a major component in the DNA chain. Different ionization scenarios are considered to provide a complete picture. First focusing on isolated DR2+, fragmentation patterns are determined for the ground electronic state, adding randomly distributed excitation energy to the nuclei. These patterns differ for the two isomers studied. To compare thermal and electronic excitation effects, Ehrenfest dynamics are also performed, allowing to remove the two electrons from selected molecular orbitals. Two intermediate-energy orbitals, localized on the carbon chain, were selected. The dissociation pattern corresponds to the most frequent pattern obtained when adding thermal excitation. On the contrary, targeting the four deepest orbitals, localized on the oxygen atoms, leads to selective ultrafast C-O and/or O-H bond dissociation. To probe the role of environment, a system consisting of a DR molecule embedded in liquid water is then studied. The two electrons are removed from either the DR or the water molecules directly linked to the sugar through hydrogen bonds. Although the dynamics onset is similar to that of isolated DR when removing the same deep orbitals localized on the sugar oxygen atoms, the subsequent fragmentation patterns differ. Sugar damage also occurs following the Coulomb explosion of neighboring H2O2+ molecules due to interaction with the emitted O or H atoms.
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Affiliation(s)
| | | | - Aashini Rajpal
- IMPMC, Sorbonne Université, UMR CNRS 7590, MNHN, Paris, France.
| | - Rodolphe Vuilleumier
- PASTEUR, Département de Chimie, École Normale Supérieure, PSL University, Sorbonne Université, CNRS, 75005 Paris, France
| | - Marie-Pierre Gaigeot
- Université Paris-Saclay, Univ Evry, CY Cergy Paris Université, CNRS, LAMBE, 91025 Evry-Courcouronnes, France
- Institut Universitaire de France (IUF), 75005 Paris, France
| | | | - Kentaro Fujii
- Institute for Quantum Life Science, National Institutes for Quantum Science and Technology, Chiba 263-8555, Japan
| | - Akinari Yokoya
- Institute for Quantum Life Science, National Institutes for Quantum Science and Technology, Chiba 263-8555, Japan
| | - Sergio Díaz-Tendero
- Departamento de Química, Universidad Autónoma de Madrid, Madrid, Spain
- Institute for Advanced Research in Chemistry (IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, Spain
- Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain
| | - Marie-Françoise Politis
- Université Paris-Saclay, Univ Evry, CY Cergy Paris Université, CNRS, LAMBE, 91025 Evry-Courcouronnes, France
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2
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Kuraoka T, Goto S, Kanno M, Díaz-Tendero S, Reino-González J, Trinter F, Pier A, Sommerlad L, Melzer N, McGinnis OD, Kruse J, Wenzel T, Jahnke T, Xue H, Kishimoto N, Yoshikawa K, Tamura Y, Ota F, Hatada K, Ueda K, Martín F. Tracing Photoinduced Hydrogen Migration in Alcohol Dications from Time-Resolved Molecular-Frame Photoelectron Angular Distributions. J Phys Chem A 2024; 128:1241-1249. [PMID: 38324399 PMCID: PMC10895665 DOI: 10.1021/acs.jpca.3c07640] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/20/2023] [Revised: 01/12/2024] [Accepted: 01/16/2024] [Indexed: 02/09/2024]
Abstract
The recent implementation of attosecond and few-femtosecond X-ray pump/X-ray probe schemes in large-scale free-electron laser facilities has opened the way to visualize fast nuclear dynamics in molecules with unprecedented temporal and spatial resolution. Here, we present the results of theoretical calculations showing how polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) can be used to visualize the dynamics of hydrogen migration in methanol, ethanol, propanol, and isopropyl alcohol dications generated by X-ray irradiation of the corresponding neutral species. We show that changes in the PA-MFPADs with the pump-probe delay as a result of intramolecular photoelectron diffraction carry information on the dynamics of hydrogen migration in real space. Although visualization of this dynamics is more straightforward in the smaller systems, methanol and ethanol, one can still recognize the signature of that motion in propanol and isopropyl alcohol and assign a tentative path to it. A possible pathway for a corresponding experiment requires an angularly resolved detection of photoelectrons in coincidence with molecular fragment ions used to define a molecular frame of reference. Such studies have become, in principle, possible since the first XFELs with sufficiently high repetition rates have emerged. To further support our findings, we provide experimental evidence of H migration in ethanol-OD from ion-ion coincidence measurements performed with synchrotron radiation.
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Affiliation(s)
- T. Kuraoka
- Department
of Physics, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
| | - S. Goto
- Department
of Physics, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
| | - M. Kanno
- Department
of Chemistry, Tohoku University, 6-3 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan
| | - S. Díaz-Tendero
- Departamento
de Química, Universidad Autónoma
de Madrid, Madrid 28049, Spain
- Condensed
Matter Physics Center (IFIMAC), Universidad
Autónoma de Madrid, Madrid 28049, Spain
- Institute
for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, Madrid 28049, Spain
| | - J. Reino-González
- Instituto
Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nano), Campus de Cantoblanco, Madrid 28049, Spain
| | - F. Trinter
- Molecular
Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, Berlin 14195, Germany
| | - A. Pier
- Institut
für Kernphysik, Goethe-Universität
Frankfurt, Max-von-Laue-Straβe 1, Frankfurt am
Main 60438, Germany
| | - L. Sommerlad
- Institut
für Kernphysik, Goethe-Universität
Frankfurt, Max-von-Laue-Straβe 1, Frankfurt am
Main 60438, Germany
| | - N. Melzer
- Institut
für Kernphysik, Goethe-Universität
Frankfurt, Max-von-Laue-Straβe 1, Frankfurt am
Main 60438, Germany
| | - O. D. McGinnis
- Institut
für Kernphysik, Goethe-Universität
Frankfurt, Max-von-Laue-Straβe 1, Frankfurt am
Main 60438, Germany
| | - J. Kruse
- Institut
für Kernphysik, Goethe-Universität
Frankfurt, Max-von-Laue-Straβe 1, Frankfurt am
Main 60438, Germany
| | - T. Wenzel
- Institut
für Kernphysik, Goethe-Universität
Frankfurt, Max-von-Laue-Straβe 1, Frankfurt am
Main 60438, Germany
| | - T. Jahnke
- Max-Planck-Institut
für Kernphysik, Saupfercheckweg 1, Heidelberg 69117, Germany
- European
XFEL, Holzkoppel
4, Schenefeld 22869, Germany
| | - H. Xue
- Department
of Chemistry, Tohoku University, 6-3 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan
| | - N. Kishimoto
- Department
of Chemistry, Tohoku University, 6-3 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan
| | - K. Yoshikawa
- Department
of Physics, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
| | - Y. Tamura
- Department
of Physics, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
| | - F. Ota
- Department
of Physics, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
| | - K. Hatada
- Department
of Physics, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
| | - K. Ueda
- Department
of Chemistry, Tohoku University, 6-3 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan
| | - F. Martín
- Departamento
de Química, Universidad Autónoma
de Madrid, Madrid 28049, Spain
- Instituto
Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nano), Campus de Cantoblanco, Madrid 28049, Spain
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3
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Crane SW, Lee JWL, Ashfold MNR, Rolles D. Molecular photodissociation dynamics revealed by Coulomb explosion imaging. Phys Chem Chem Phys 2023. [PMID: 37335247 DOI: 10.1039/d3cp01740k] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/21/2023]
Abstract
Coulomb explosion imaging (CEI) methods are finding ever-growing use as a means of exploring and distinguishing the static stereo-configurations of small quantum systems (molecules, clusters, etc). CEI experiments initiated by ultrafast (femtosecond-duration) laser pulses also allow opportunities to track the time-evolution of molecular structures, and thereby advance understanding of molecular fragmentation processes. This Perspective illustrates two emerging families of dynamical studies. 'One-colour' studies (employing strong field ionisation driven by intense near infrared or single X-ray or extreme ultraviolet laser pulses) afford routes to preparing multiply charged molecular cations and exploring how their fragmentation progresses from valence-dominated to Coulomb-dominated dynamics with increasing charge and how this evolution varies with molecular size and composition. 'Two-colour' studies use one ultrashort laser pulse to create electronically excited neutral molecules (or monocations), whose structural evolution is then probed as a function of pump-probe delay using an ultrafast ionisation pulse along with time and position-sensitive detection methods. This latter type of experiment has the potential to return new insights into not just molecular fragmentation processes but also charge transfer processes between moieties separating with much better defined stereochemical control than in contemporary ion-atom and ion-molecule charge transfer studies.
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Affiliation(s)
- Stuart W Crane
- School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK.
| | - Jason W L Lee
- Department of Chemistry, University of Oxford, Oxford, OX1 3TA, UK
| | | | - Daniel Rolles
- J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS 66506, USA
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4
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Wang E, Kling NG, LaForge AC, Obaid R, Pathak S, Bhattacharyya S, Meister S, Trost F, Lindenblatt H, Schoch P, Kübel M, Pfeifer T, Rudenko A, Díaz-Tendero S, Martín F, Moshammer R, Rolles D, Berrah N. Ultrafast Roaming Mechanisms in Ethanol Probed by Intense Extreme Ultraviolet Free-Electron Laser Radiation: Electron Transfer versus Proton Transfer. J Phys Chem Lett 2023; 14:4372-4380. [PMID: 37140167 DOI: 10.1021/acs.jpclett.2c03764] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
Ultrafast H2+ and H3+ formation from ethanol is studied using pump-probe spectroscopy with an extreme ultraviolet (XUV) free-electron laser. The first pulse creates a dication, triggering H2 roaming that leads to H2+ and H3+ formation, which is disruptively probed by a second pulse. At photon energies of 28 and 32 eV, the ratio of H2+ to H3+ increases with time delay, while it is flat at a photon energy of 70 eV. The delay-dependent effect is ascribed to a competition between electron and proton transfer. High-level quantum chemistry calculations show a flat potential energy surface for H2 formation, indicating that the intermediate state may have a long lifetime. The ab initio molecular dynamics simulation confirms that, in addition to the direct emission, a small portion of H2 undergoes a roaming mechanism that leads to two competing pathways: electron transfer from H2 to C2H4O2+ and proton transfer from C2H4O2+ to H2.
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Affiliation(s)
- Enliang Wang
- J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506-2604, United States
- Hefei National Research Center for Physical Sciences at the Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China
| | - Nora G Kling
- Physics Department, University of Connecticut, Storrs, Connecticut 06269-3046, United States
| | - Aaron C LaForge
- Physics Department, University of Connecticut, Storrs, Connecticut 06269-3046, United States
| | - Razib Obaid
- Physics Department, University of Connecticut, Storrs, Connecticut 06269-3046, United States
| | - Shashank Pathak
- J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506-2604, United States
| | - Surjendu Bhattacharyya
- J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506-2604, United States
| | - Severin Meister
- Max Planck Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - Florian Trost
- Max Planck Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - Hannes Lindenblatt
- Max Planck Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - Patrizia Schoch
- Max Planck Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - Matthias Kübel
- Institute of Optics and Quantum Electronics, Friedrich Schiller University Jena, D-07743 Jena, Germany
- Helmholtz Institute Jena, Fröbelstieg 3, 07743 Jena, Germany
| | - Thomas Pfeifer
- Max Planck Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - Artem Rudenko
- J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506-2604, United States
| | - Sergio Díaz-Tendero
- Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, 28049 Madrid, Spain
- Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain
- Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, Spain
| | - Fernando Martín
- Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, 28049 Madrid, Spain
- Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain
- Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nano), Campus de Cantoblanco, 28049 Madrid, Spain
| | - Robert Moshammer
- Max Planck Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
| | - Daniel Rolles
- J. R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506-2604, United States
| | - Nora Berrah
- Physics Department, University of Connecticut, Storrs, Connecticut 06269-3046, United States
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5
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Hydrogen migration in inner-shell ionized halogenated cyclic hydrocarbons. Sci Rep 2023; 13:2107. [PMID: 36747068 PMCID: PMC9902455 DOI: 10.1038/s41598-023-28694-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2022] [Accepted: 01/23/2023] [Indexed: 02/08/2023] Open
Abstract
We have studied the fragmentation of the brominated cyclic hydrocarbons bromocyclo-propane, bromocyclo-butane, and bromocyclo-pentane upon Br(3d) and C(1s) inner-shell ionization using coincidence ion momentum imaging. We observe a substantial yield of CH3+ fragments, whose formation requires intramolecular hydrogen (or proton) migration, that increases with molecular size, which contrasts with prior observations of hydrogen migration in linear hydrocarbon molecules. Furthermore, by inspecting the fragment ion momentum correlations of three-body fragmentation channels, we conclude that CHx+ fragments (with x = 0, …, 3) with an increasing number of hydrogens are more likely to be produced via sequential fragmentation pathways. Overall trends in the molecular-size-dependence of the experimentally observed kinetic energy releases and fragment kinetic energies are explained with the help of classical Coulomb explosion simulations.
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6
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Cencer M, Li C, Agarwal G, Gomes Neto RJ, Amanchukwu CV, Assary RS. Interactions of CO 2 Anion Radicals with Electrolyte Environments from First-Principles Simulations. ACS OMEGA 2022; 7:18131-18138. [PMID: 35664611 PMCID: PMC9161390 DOI: 10.1021/acsomega.2c01733] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 03/22/2022] [Accepted: 04/21/2022] [Indexed: 06/15/2023]
Abstract
Successful transformation of carbon dioxide (CO2) into value-added products is of great interest, as it contributes in part to the circular carbon economy. Understanding chemical interactions that stabilize crucial reaction intermediates of CO2 is important, and in this contribution, we employ atom centered density matrix propagation (ADMP) molecular dynamics simulations to investigate interactions between CO2 - anion radicals with surrounding solvent molecules and electrolyte cations in both aqueous and nonaqueous environments. We show how different cations and solvents affect the stability of the CO2 - anion radical by examining its angle and distance to a coordinating cation in molecular dynamics simulations. We identify that the strength of CO2 - interactions can be tailored through choosing an appropriate cation and solvent combination. We anticipate that this fundamental understanding of cation/solvent interactions can facilitate the optimization of a chemical pathway that results from selective stabilization of a crucial reaction intermediate.
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Affiliation(s)
- Morgan
M. Cencer
- Materials
Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
| | - Chenyang Li
- Materials
Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
| | - Garvit Agarwal
- Materials
Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
| | - Reginaldo Jose Gomes Neto
- Pritzker
School of Molecular Engineering, The University
of Chicago, Chicago, Illinois 60637, United States
| | - Chibueze V. Amanchukwu
- Pritzker
School of Molecular Engineering, The University
of Chicago, Chicago, Illinois 60637, United States
- Chemical
Sciences and Engineering Division, Argonne
National Laboratory, Lemont, Illinois 60439, United States
| | - Rajeev S. Assary
- Materials
Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
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7
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Yadav J, Safvan CP, Bhatt P, Kumari P, Kumar A, Rajput J. Hydrogen migration in triply charged acetylene. J Chem Phys 2022; 156:141101. [DOI: 10.1063/5.0086427] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We report on the direct experimental evidence of hydrogen migration in triply charged acetylene. The roaming hydrogen atom in a triply charged molecular ion is counter intuitive. The three body breakup channel [Formula: see text] is studied using the technique of recoil ion momentum spectroscopy. The triply charged ion was generated in collisions of the neutral parent with a slow highly charged Xe9+ ion. Three different dissociation pathways have been identified and separated, namely, concerted breakup in an acetylene configuration, concerted breakup in a vinylidene configuration, and sequential breakup via a [Formula: see text] intermediate, and the branching ratio for all three pathways are determined.
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Affiliation(s)
- Jatin Yadav
- Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India
| | - C. P. Safvan
- Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India
| | - Pragya Bhatt
- Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India
| | - Pooja Kumari
- Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India
| | - Aditya Kumar
- Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India
| | - Jyoti Rajput
- Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India
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8
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Zhang M, Guo Z, Mi X, Li Z, Liu Y. Ultrafast Imaging of Molecular Dynamics Using Ultrafast Low-Frequency Lasers, X-ray Free Electron Lasers, and Electron Pulses. J Phys Chem Lett 2022; 13:1668-1680. [PMID: 35147438 DOI: 10.1021/acs.jpclett.1c03916] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
The requirement of high space-time resolution and brightness is a great challenge for imaging atomic motion and making molecular movies. Important breakthroughs in ultrabright tabletop laser, X-ray, and electron sources have enabled the direct imaging of evolving molecular structures in chemical processes, and recent experimental advances in preparing ultrafast laser and electron pulses resulted in molecular imaging with femtosecond time resolution. This Perspective presents an overview of the versatile imaging methods of molecular dynamics. High-order harmonic generation imaging and photoelectron diffraction imaging are based on laser-induced ionization and rescattering processes. Coulomb explosion imaging retrieves molecular structural information by detecting the momentum vectors of fragmented ions. Diffraction imaging encodes molecular structural and electronic information in reciprocal space. We also present various applications of these ultrafast imaging methods in resolving laser-induced nuclear and electronic dynamics.
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Affiliation(s)
- Ming Zhang
- State Key Laboratory for Mesoscopic Physics and Collaborative Innovation Center of Quantum Matter, School of Physics, Peking University, Beijing 100871, China
| | - Zhengning Guo
- State Key Laboratory for Mesoscopic Physics and Collaborative Innovation Center of Quantum Matter, School of Physics, Peking University, Beijing 100871, China
| | - Xiaoyu Mi
- State Key Laboratory for Mesoscopic Physics and Collaborative Innovation Center of Quantum Matter, School of Physics, Peking University, Beijing 100871, China
| | - Zheng Li
- State Key Laboratory for Mesoscopic Physics and Collaborative Innovation Center of Quantum Matter, School of Physics, Peking University, Beijing 100871, China
- Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China
- Yangtze Delta Institute of Optoelectronics, Peking University, Nantong 226010, China
| | - Yunquan Liu
- State Key Laboratory for Mesoscopic Physics and Collaborative Innovation Center of Quantum Matter, School of Physics, Peking University, Beijing 100871, China
- Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China
- Center for Applied Physics and Technology, HEDPS, Peking University, Beijing 100871, China
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9
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Guerra C, Kumar S, Aguilar-Galindo F, Díaz-Tendero S, Lozano AI, Mendes M, Oller JC, Limão-Vieira P, García G. Total Electron Detachment and Induced Cationic Fragmentation Cross Sections for Superoxide Anion (O 2-) Collisions with Benzene (C 6H 6) Molecules. Int J Mol Sci 2022; 23:ijms23031266. [PMID: 35163189 PMCID: PMC8835784 DOI: 10.3390/ijms23031266] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2021] [Revised: 01/17/2022] [Accepted: 01/21/2022] [Indexed: 02/06/2023] Open
Abstract
In this study, novel experimental total electron detachment cross sections for O2- collisions with benzene molecules are reported for the impact energy range (10-1000 eV), as measured with a transmission beam apparatus. By analysing the positively charged species produced during the collision events, relative total ionisation cross sections were derived in the incident energy range of 160-900 eV. Relative partial ionisation cross sections for fragments with m/z ≤ 78 u were also given in this energy range. We also confirmed that heavier compounds (m/z > 78 u) formed for impact energies between 550 and 800 eV. In order to further our knowledge about the collision dynamics governing the fragmentation of such heavier molecular compounds, we performed molecular dynamics calculations within the framework of the Density Functional Theory (DFT). These results demonstrated that the fragmentation of these heavier compounds strongly supports the experimental evidence of m/z = 39-42, 50, 60 (u) cations formation, which contributed to the broad local maximum in the total ionisation observed from 550 to 800 eV. This work reveals the reactivity induced by molecular anions colliding with hydrocarbons at high energies, processes that can take place in the interstellar medium under various local conditions.
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Affiliation(s)
- Carlos Guerra
- Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas, Serrano 113-bis, 28006 Madrid, Spain;
| | - Sarvesh Kumar
- Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal; (S.K.); (A.I.L.); (M.M.); (P.L.-V.)
| | - Fernando Aguilar-Galindo
- Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián, Spain;
| | - Sergio Díaz-Tendero
- Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, 28049 Madrid, Spain
- Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain
- Institute for Advanced Research in Chemical Science (IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, Spain
- Correspondence: (S.D.-T.); (G.G.)
| | - Ana I. Lozano
- Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal; (S.K.); (A.I.L.); (M.M.); (P.L.-V.)
| | - Mónica Mendes
- Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal; (S.K.); (A.I.L.); (M.M.); (P.L.-V.)
| | - Juan C. Oller
- Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Avenida Complutense 22, 28040 Madrid, Spain;
| | - Paulo Limão-Vieira
- Laboratório de Colisões Atómicas e Moleculares, CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal; (S.K.); (A.I.L.); (M.M.); (P.L.-V.)
| | - Gustavo García
- Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas, Serrano 113-bis, 28006 Madrid, Spain;
- Centre for Medical Radiation Physics, University of Wollongong, Wollomgong, NSW 2522, Australia
- Correspondence: (S.D.-T.); (G.G.)
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10
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Barreiro-Lage D, Nicolafrancesco C, Kočišek J, Luna A, Kopyra J, Alcamí M, Huber BA, Martín F, Domaracka A, Rousseau P, Díaz-Tendero S. Controlling the diversity of ion-induced fragmentation pathways by N-methylation of amino acids. Phys Chem Chem Phys 2022; 24:941-954. [PMID: 34913940 DOI: 10.1039/d1cp04097a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
We present a combined experimental and theoretical study of the fragmentation of singly and doubly N-methylated glycine (sarcosine and N,N-dimethyl glycine, respectively) induced by low-energy (keV) O6+ ions. Multicoincidence mass spectrometry techniques and quantum chemistry simulations (ab initio molecular dynamics and density functional theory) allow us to characterise different fragmentation pathways as well as the associated mechanisms. We focus on the fragmentation of doubly ionised species, for which coincidence measurements provide unambiguous information on the origin of the various charged fragments. We have found that single N-methylation leads to a larger variety of fragmentation channels than in no methylation of glycine, while double N-methylation effectively closes many of these fragmentation channels, including some of those appearing in pristine glycine. Importantly, the closure of fragmentation channels in the latter case does not imply a protective effect by the methyl group.
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Affiliation(s)
- Darío Barreiro-Lage
- Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, Madrid 28049, Spain.
| | - Chiara Nicolafrancesco
- Normandie Univ, ENSICAEN, UNICAEN, CEA, CNRS, CIMAP, Caen 14000, France. .,Synchrotron SOLEIL, LOrme des Merisiers, St Aubin, BP 48, Gif sur Yvette Cedex 91192, France
| | - Jaroslav Kočišek
- J. Heyrovsky Institute of Physical Chemistry v.v.i., The Czech Academy of Sciences, Dolejskova 3, Prague 18223, Czech Republic
| | - Alberto Luna
- Centro de Computación Científica, Universidad Autónoma de Madrid, Madrid 28049, Spain
| | - Janina Kopyra
- Faculty of Exact and Natural Sciences, Siedlce University of Natural Sciences and Humanities, 3 Maja 54, Siedlce 08-110, Poland
| | - Manuel Alcamí
- Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, Madrid 28049, Spain. .,Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nano), Campus de Cantoblanco, Madrid 28049, Spain.,Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, Madrid 28049, Spain
| | - Bernd A Huber
- Normandie Univ, ENSICAEN, UNICAEN, CEA, CNRS, CIMAP, Caen 14000, France.
| | - Fernando Martín
- Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, Madrid 28049, Spain. .,Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nano), Campus de Cantoblanco, Madrid 28049, Spain.,Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, Madrid 28049, Spain
| | - Alicja Domaracka
- Normandie Univ, ENSICAEN, UNICAEN, CEA, CNRS, CIMAP, Caen 14000, France.
| | - Patrick Rousseau
- Normandie Univ, ENSICAEN, UNICAEN, CEA, CNRS, CIMAP, Caen 14000, France.
| | - Sergio Díaz-Tendero
- Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, Madrid 28049, Spain. .,Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, Madrid 28049, Spain.,Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, Madrid 28049, Spain
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11
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Chiarinelli J, Barreiro-Lage D, Bolognesi P, RICHTER R, Zettergren H, Stockett MH, Díaz-Tendero S, Avaldi L. Electron and ion spectroscopy of the cyclo-alanine-alanine dipeptide. Phys Chem Chem Phys 2022; 24:5855-5867. [DOI: 10.1039/d1cp05811h] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The VUV photoionisation and photofragmentation of cyclo-alanine-alanine (cAA) has been studiedin a joint experimental and theoretical work. The photoelectron spectrum and the photoelectron-photoion coincidence (PEPICO) measurements, which enable a control...
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12
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Mishra D, Reino-González J, Obaid R, LaForge AC, Díaz-Tendero S, Martín F, Berrah N. Ultrafast molecular dynamics in ionized 1- and 2-propanol: from simple fragmentation to complex isomerization and roaming mechanisms. Phys Chem Chem Phys 2021; 24:433-443. [PMID: 34897321 DOI: 10.1039/d1cp04011a] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Upon photoexcitation, molecules can undergo numerous complex processes, such as isomerization and roaming, leading to changes in the molecular and electronic structure. Here, we report on the time-resolved ultrafast nuclear dynamics, initiated by laser ionization, in the two structural isomers, 1- and 2-propanol, using a combination of pump-probe spectroscopy and coincident Coulomb explosion imaging. Our measurements, paired with quantum chemistry calculations, identify the mechanisms for the observed two- and three-body dissociation channels for both isomers. In particular, the fragmentation channel of 2-propanol associated with the loss of CH3 shows possible evidence of methyl roaming. Moreover, the electronic structure of this roaming methyl fragment could be responsible for the enhanced ionization also observed for this channel. Finally, comparison with similar studies done on ethanol and acetonitrile helps establish a correlation between the length of the alkyl chain and the likelihood of hydrogen migration.
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Affiliation(s)
- Debadarshini Mishra
- Department of Physics, University of Connecticut, Storrs, Connecticut, 06269, USA.
| | - Juan Reino-González
- Departamento de Química, Universidad Autónoma de Madrid, Módulo 13, 28049 Madrid, EU, Spain
| | - Razib Obaid
- Department of Physics, University of Connecticut, Storrs, Connecticut, 06269, USA.
| | - Aaron C LaForge
- Department of Physics, University of Connecticut, Storrs, Connecticut, 06269, USA.
| | - Sergio Díaz-Tendero
- Departamento de Química, Universidad Autónoma de Madrid, Módulo 13, 28049 Madrid, EU, Spain.,Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, EU, Spain.,Institute for Advanced Research in Chemical Sciences(IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, EU, Spain
| | - Fernando Martín
- Departamento de Química, Universidad Autónoma de Madrid, Módulo 13, 28049 Madrid, EU, Spain.,Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, EU, Spain.,Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nano), Campus de Cantoblanco, 28049 Madrid, EU, Spain.,Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián, EU, Spain
| | - Nora Berrah
- Department of Physics, University of Connecticut, Storrs, Connecticut, 06269, USA.
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13
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Ota F, Abe S, Hatada K, Ueda K, Díaz-Tendero S, Martín F. Imaging intramolecular hydrogen migration with time- and momentum-resolved photoelectron diffraction. Phys Chem Chem Phys 2021; 23:20174-20182. [PMID: 34473148 DOI: 10.1039/d1cp02055b] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Imaging ultrafast hydrogen migration with few- or sub-femtosecond time resolution is a challenge for ultrafast spectroscopy due to the lightness and small scattering cross-section of the moving hydrogen atom. Here we propose time- and momentum-resolved photoelectron diffraction (TMR-PED) as a way to overcome limitations of existing methodologies and illustrate its performance in the ethanol molecule. By combining different theoretical methods, namely molecular dynamics and electron scattering methods, we show that TMR-PED, along with a judicious choice of the reference frame for multi-coincidence detection, allows for direct imaging of single and double hydrogen migration in doubly-charged ethanol with both few-fs and Å resolutions, all the way from its birth to the very end. It also provides hints of proton extraction following H2 roaming. The signature of hydrogen dynamics shows up in polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) as moving features that allow for a straightforward visualization in space.
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Affiliation(s)
- Fukiko Ota
- Department of Physics, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan.
| | - Shigeru Abe
- Department of Physics, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan.
| | - Keisuke Hatada
- Department of Physics, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan.
| | - Kiyoshi Ueda
- Department of Chemistry, Tohoku University, 6-3 Aramaki Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan.
| | - Sergio Díaz-Tendero
- Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, 28049 Madrid, EU, Spain. .,Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, EU, Spain.,Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, EU, Spain
| | - Fernando Martín
- Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, 28049 Madrid, EU, Spain. .,Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, EU, Spain.,Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nano), Campus de Cantoblanco, 28049 Madrid, EU, Spain
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14
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Wei L, Lam CS, Zhang Y, Ren B, Han J, Wang B, Zou Y, Chen L, Lau KC, Wei B. Isomerization Dynamics in the Symmetric and Asymmetric Fragmentation of Ethane Dications. J Phys Chem Lett 2021; 12:5789-5795. [PMID: 34137607 DOI: 10.1021/acs.jpclett.1c01276] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
Hydrogen- or proton-migration-induced isomerization has recently been of concern for its critical role in the dissociation of organic molecules of astrophysical or biological relevance. Herein we present a combined experimental and theoretical study of the two-body C-C bond breakdown dissociation of ethane dication. For the asymmetric fragmentation channel CH2+ + CH4+, the kinetic energy release measurements and ab initio quantum chemical calculations demonstrate that the reaction pathway involving hydrogen-migration-induced isomerization of [CH3-CH3]2+ to [CH2-CH4]2+ can be accessed via the lowest triplet state rather than the ground singlet state of ethane dication. Interestingly, it is found that a considerable proportion of the yield of symmetric fragmentation CH3+ + CH3+, which is usually considered from a direct Coulomb explosion and seemingly independent of isomerization, could come from the dissociation of ethane dication in the ground singlet state with the involvement of [CH3-CH3]2+ isomerization to intermediate [H2C(H2)CH2]2+ of the diborane-like double-bridged structure.
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Affiliation(s)
- Long Wei
- Institute of Modern Physics, Key Laboratory of Nuclear Physics and Ion-Beam Application (MOE), Fudan University, Shanghai 200433, China
| | - Chow-Shing Lam
- Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, China
| | - Yu Zhang
- College of Data Science, Jiaxing University, Jiaxing 314001, China
| | - Baihui Ren
- Institute of Modern Physics, Key Laboratory of Nuclear Physics and Ion-Beam Application (MOE), Fudan University, Shanghai 200433, China
| | - Jie Han
- Institute of Modern Physics, Key Laboratory of Nuclear Physics and Ion-Beam Application (MOE), Fudan University, Shanghai 200433, China
| | - Bo Wang
- Institute of Modern Physics, Key Laboratory of Nuclear Physics and Ion-Beam Application (MOE), Fudan University, Shanghai 200433, China
| | - Yaming Zou
- Institute of Modern Physics, Key Laboratory of Nuclear Physics and Ion-Beam Application (MOE), Fudan University, Shanghai 200433, China
| | - Li Chen
- Institute of Modern Physics, Key Laboratory of Nuclear Physics and Ion-Beam Application (MOE), Fudan University, Shanghai 200433, China
| | - Kai-Chung Lau
- Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, China
| | - Baoren Wei
- Institute of Modern Physics, Key Laboratory of Nuclear Physics and Ion-Beam Application (MOE), Fudan University, Shanghai 200433, China
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15
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Das R, Pandey DK, Nimma V, P M, Bhardwaj P, Chandravanshi P, Shameem K M M, Singh DK, Kushawaha RK. Strong-field ionization of polyatomic molecules: ultrafast H atom migration and bond formation in the photodissociation of CH 3OH. Faraday Discuss 2021; 228:432-450. [PMID: 33576353 DOI: 10.1039/d0fd00129e] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Strong-field ionization induces various complex phenomena like bond breaking, intramolecular hydrogen migration, and bond association in polyatomic molecules. The H-atom migration and bond formation in CH3OH induced by intense femtosecond laser pulses are investigated using a Velocity Map Imaging (VMI) spectrometer. Various laser parameters like intensity (1.5 × 1013 W cm-2-12.5 × 1013 W cm-2), pulse duration (29 fs and 195 fs), wavelength (800 nm and 1300 nm), and polarization (linear and circular) can serve as a quantum control for hydrogen migration and the yield of Hn+ (n = 1-3) ions which have been observed in this study. Further, in order to understand the ejection mechanism of the hydrogen molecular ions H2+ and H3+ from singly-ionized CH3OH, quantum chemical calculations were employed. The dissociation processes of CH3OH+ occurring by four dissociative channels to form CHO+ + H3, H3+ + CHO, CH2+ + H2O, and H2O+ + CH2 are studied. Using the combined approach of experiments and theory, we have successfully explained the mechanism of intramolecular hydrogen migration and predicted the dissociative channels of singly-ionized CH3OH.
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Affiliation(s)
- Rituparna Das
- Physical Research Laboratory, Ahmedabad, India. and Indian Institute of Technology, Gandhinagar, Gujarat 382355, India
| | - Deepak K Pandey
- Department of Physics, Institute of Infrastructure Technology Research and Management, Ahmedabad, 380026, India
| | | | - Madhusudhan P
- Physical Research Laboratory, Ahmedabad, India. and Indian Institute of Technology, Gandhinagar, Gujarat 382355, India
| | - Pranav Bhardwaj
- Physical Research Laboratory, Ahmedabad, India. and Indian Institute of Technology, Gandhinagar, Gujarat 382355, India
| | | | | | - Dheeraj K Singh
- Department of Physics, Institute of Infrastructure Technology Research and Management, Ahmedabad, 380026, India
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16
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Pathak S, Obaid R, Bhattacharyya S, Bürger J, Li X, Tross J, Severt T, Davis B, Bilodeau RC, Trallero-Herrero CA, Rudenko A, Berrah N, Rolles D. Differentiating and Quantifying Gas-Phase Conformational Isomers Using Coulomb Explosion Imaging. J Phys Chem Lett 2020; 11:10205-10211. [PMID: 33206545 DOI: 10.1021/acs.jpclett.0c02959] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Conformational isomerism plays a crucial role in defining the physical and chemical properties and biological activity of molecules ranging from simple organic compounds to complex biopolymers. However, it is often a significant challenge to differentiate and separate these isomers experimentally as they can easily interconvert due to their low rotational energy barrier. Here, we use the momentum correlation of fragment ions produced after inner-shell photoionization to distinguish conformational isomers of 1,2-dibromoethane (C2H4Br2). We demonstrate that the three-body breakup channel, C2H4+ + Br+ + Br+, contains signatures of both sequential and concerted breakup, which are decoupled to distinguish the geometries of two conformational isomers and to quantify their relative abundance. The sensitivity of our method to quantify these yields is established by measuring the relative abundance change with sample temperature, which agrees well with calculations. Our study paves the way for using Coulomb explosion imaging to track subtle molecular structural changes.
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Affiliation(s)
- Shashank Pathak
- J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506, United States
| | - Razib Obaid
- Department of Physics, University of Connecticut, Storrs, Connecticut 06269, United States
| | - Surjendu Bhattacharyya
- J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506, United States
| | - Johannes Bürger
- Department of Physics, Ludwig Maximilian University of Munich, Munich 80539, Germany
| | - Xiang Li
- J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506, United States
| | - Jan Tross
- J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506, United States
| | - Travis Severt
- J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506, United States
| | - Brandin Davis
- Department of Physics, University of Connecticut, Storrs, Connecticut 06269, United States
| | - René C Bilodeau
- Department of Physics, University of Connecticut, Storrs, Connecticut 06269, United States
- Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
| | | | - Artem Rudenko
- J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506, United States
| | - Nora Berrah
- Department of Physics, University of Connecticut, Storrs, Connecticut 06269, United States
| | - Daniel Rolles
- J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, Kansas 66506, United States
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17
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Formation of H 3+ from ethane dication induced by electron impact. Commun Chem 2020; 3:160. [PMID: 36703403 PMCID: PMC9814254 DOI: 10.1038/s42004-020-00415-9] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2020] [Accepted: 10/19/2020] [Indexed: 01/29/2023] Open
Abstract
Hydrogen migration plays an important role in the chemistry of hydrocarbons which considerably influences their chemical functions. The migration of one or more hydrogen atoms occurring in hydrocarbon cations has an opportunity to produce the simplest polyatomic molecule, i.e. H3+. Here we present a combined experimental and theoretical study of H3+ formation dynamics from ethane dication. The experiment is performed by 300 eV electron impact ionization of ethane and a pronounced yield of H3+ + C2H3+ coincidence channel is observed. The quantum chemistry calculations show that the H3+ formation channel can be opened on the ground-state potential energy surface of ethane dication via transition state and roaming mechanisms. The ab initio molecular dynamics simulation shows that the H3+ can be generated in a wide time range from 70 to 500 fs. Qualitatively, the trajectories of the fast dissociation follow the intrinsic reaction coordinate predicted by the conventional transition state theory. The roaming mechanism, compared to the transition state, occurs within a much longer timescale accompanied by nuclear motion of larger amplitude.
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