Ultrafast Vibrational Relaxation Dynamics in XUV-Excited Polycyclic Aromatic Hydrocarbon Molecules

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Ultrafast Vibrational Relaxation Dynamics in XUV-Excited Polycyclic Aromatic Hydrocarbon Molecules

A. Boyer, M. Hervé, V. Despré, P. Castellanos Nash, V. Loriot, A. Marciniak, A. G. G. M. Tielens, A. I. Kuleff, and F. Lépine

Phys. Rev. X 11, 041012 – Published 18 October 2021

Abstract

Unraveling ultrafast molecular processes initiated by energetic radiation provides direct information on the chemical evolution under extreme conditions. A prominent example is interstellar media where complex molecules such as polycyclic aromatic hydrocarbons (PAHs) are excited by energetic photons. Until recently, ultrafast dynamics following such excitations remained largely unexplored due to the lack of relevant technologies. Here, we use time-resolved mass spectrometry combining ultrashort femtosecond XUV and IR pulses, to investigate the dynamics induced by high-energy photon excitation in PAHs. We demonstrate that excited cations relax through a progressive loss of vibrational selectivity, created at the early-stage dynamics, and which represents the first steps of a complete intramolecular vibrational energy redistribution. This process is in competition with the recently revealed correlation-band dynamics. These results might have direct consequences for the development of XUV molecular physics and other fields such as astrochemistry.

Received 8 January 2021
Revised 29 June 2021
Accepted 10 August 2021

DOI:https://doi.org/10.1103/PhysRevX.11.041012

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Ultrafast phenomena

Atomic, Molecular & Optical

Authors & Affiliations

A. Boyer ¹, M. Hervé ¹, V. Despré ², P. Castellanos Nash ³, V. Loriot ¹, A. Marciniak ¹, A. G. G. M. Tielens³, A. I. Kuleff ^2,4, and F. Lépine¹

¹Univ Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F-69622, Villeurbanne, France
²Theoretische Chemie, PCI, Universität Heidelberg, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany
³Leiden Observatory, Leiden University, PO Box 9513, NL-2300RA Leiden, Netherlands
⁴ELI-ALPS, Wolfgang Sandner utca 3, H-6728 Szeged, Hungary

Popular Summary

Polycyclic aromatic hydrocarbon (PAH) molecules may harbor about 15 percent of the carbon in the Universe and are therefore key players in the carbon chemistry of the interstellar medium. Fully understanding the role that they play requires a better accounting of how these molecules respond to extreme ultraviolet (XUV) radiation in space, which can induce fragmentation or structural changes. Here, we show that XUV excitation of PAH molecules leads to complex dynamics that combine the interactions between the large number of electrons and nuclei constituting these molecules.

Until recently, XUV-induced processes could only be investigated at large synchrotron facilities that can reveal the evolution of photoreactivity as a function of photon energy. Having access to the initial ultrafast processes at the heart of this interaction has only recently become possible thanks to the development of tabletop high-order harmonic generation (HHG) laser sources.

In our work, we use HHG sources of femtosecond XUV and infrared pulses, combined with time-resolved mass spectrometry, to probe the dynamics induced by high-energy photon excitation of PAH molecules. We find that excited cations progressively relax through a series of vibrational states that represent the first steps in a complete redistribution of intramolecular vibrational energy.

Our results bring new insight into the internal dynamics of highly excited PAH molecules, which in turn could lead to improved astrochemistry models and better intel on measurements of interstellar cloud temperatures and molecular abundances.

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Issue

Vol. 11, Iss. 4 — October - December 2021

Subject Areas

Atomic and Molecular Physics

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Images

Figure 1
(a) Mass spectrum of anthracene after interaction with the XUV. (b) Cation yield as a function of the XUV-IR delay with a global fit (orange curve) and its breakdown in the two components: the exponential decay depletion (gray dashed line) and population (yellow dashed line). (c) Dication yield with exponential fit (d) $C_{2} H_{x}$ loss yield as a function of the XUV-IR delay with a global fit (orange curve) and its breakdown in the two components: the exponential decay (yellow dashed line) and the increase exponential (purple dashed line). (e) Same as (d) but for loss of $C_{4} H_{x}$ .
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Figure 2
Fragmentation yield for the 2H-loss channel, as a function of the XUV-IR delay with a global fit and its breakdown into exponential decay (gray dashed curve) and population (yellow dashed curve) components, for six PAHs. (a) Anthracene in orange. (b) Pyrene in red. (c) Tetracene in sky blue. (d) Pentacene in blue. (e) Coronene in purple. (f) HBC in green. The color theme for the different PAHs is the same for the entire paper and the additional data shown in the Supplemental Material [21]. For the smallest PAH presented here, the abundance of other fragments contributes to the accurate determination of the timescales by using a multidimensional fitting procedure.
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Figure 3
Extracted timescales for the nonadiabatic relaxation dynamics in the CB (blue diamonds) and the vibrational relaxation dynamics (green dots) as function of the size of the PAHs (anthracene to HBC) described by the number of valence electrons. We note that in the case of coronene and HBC, the experimental timescale of the vibrational relaxation dynamics is very inaccurate (shown by the shaded green area). The CB timescales are fitted using a logarithmic law and vibrational dynamics data are fitted with a $1 / N$ law as discussed in the text. We note that the $1 / N$ fitting is constrained in order to give only positive values to be consistent with our interpretation. The fitting curves are presented as blue and green dashed lines, respectively.
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Figure 4
Schematic of the IR probing of the two mechanisms in XUV ionized PAHs: CBs relaxation through conical intersections (CI) and nuclear relocalization due to vibrational coupling (vibrational dynamics). Both mechanisms are observed in the time-dependent fragmentation yield, as soon as the fragmentation threshold is overcome. The potential energy surfaces are schematically represented along one nuclear coordinate (right-hand part of the figure), while the density of states (DOS) populated by the XUV pulse is presented on the left-hand part.
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Figure 5
Velocity map imaging measurements performed on pyrene. (a) Raw VMI image of $C_{2} H_{X}$ loss in pyrene and (b) corresponding kinetic energy distribution of $(Py − C_{2} H_{x})^{+}$ ions.
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