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VUV Photoionization Dynamics of the C60 Buckminsterfullerene: 2D-Matrix Photoelectron Cite This: Phys

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VUV Photoionization Dynamics of the C60 Buckminsterfullerene: 2D-Matrix Photoelectron Cite This: Phys Showcasing research from the groups of Dr Helgi Rafn Hrodmarsson and Prof. Harold Linnartz at the Laboratory As featured in: for Astrophysics (Leiden University) and Dr Gustavo Garcia Volume 22 Number 25 7 July 2020 and Dr Laurent Nahon at Synchrotron SOLEIL. Pages 13821–14382 PCCP VUV photoionization dynamics of the C60 Physical Chemistry Chemical Physics rsc.li/pccp buckminsterfullerene: 2D-matrix photoelectron spectroscopy in an astrophysical context This work presents the threshold photoelectron spectrum of C60, recorded using synchrotron VUV photoionization spectrometry. The results are presented in a two-dimensional photoelectron matrix which holds a wealth of spectroscopic ISSN 1463-9076 PAPER Abdelouahab El Kharbachi, Magnus H. Sørby et al . Pseudo-ternary LiBH4 ·LiCl·P2 S5 system as structurally disordered bulk electrolyte for all-solid-state lithium information of astrophysical relevance. This work touches batteries + upon subjects such as the symmetry of C60 , the diffuse interstellar bands, the heating of the ISM, and the absolute See Helgi Rafn Hrodmarsson et al ., Phys . Chem . Chem . Phys ., and partial photoionization cross sections of C . 60 2020, 22 , 13880. rsc.li/pccp Registered charity number: 207890 PCCP View Article Online PAPER View Journal | View Issue VUV photoionization dynamics of the C60 buckminsterfullerene: 2D-matrix photoelectron Cite this: Phys. Chem. Chem. Phys., 2020, 22, 13880 spectroscopy in an astrophysical context† Helgi Rafn Hrodmarsson, *ab Gustavo A. Garcia, a Harold Linnartz b and Laurent Nahon a We present the photoionization dynamics of the C60 buckminsterfullerene from threshold up to 14.0 eV recorded with VUV synchrotron radiation at the DESIRS beamline at the SOLEIL synchrotron. The recorded data is obtained using a double-imaging photoelectron photoion coincidence spectrometer and is presented as a two-dimensional photoelectron matrix which contains a wealth of spectroscopic data. We present these data in an astrophysical context which relates to (i) the threshold photoelectron spectrum which is compared to data relevant to the diffuse interstellar bands (DIBs), (ii) the kinetic photoelectron distribution at the Lyman-a line which is relevant to the dominant heating source in the Creative Commons Attribution 3.0 Unported Licence. ISM, and (iii) the absolute photoionization cross section of C60 up to approx. 10.5 eV. The photoelectron Received 3rd March 2020, spectrum implies that the symmetry of the ground state is different than previous theoretical models Accepted 23rd April 2020 have predicted, and this result is discussed in context of recent experimental and theoretical findings. DOI: 10.1039/d0cp01210f Also presented are partial photoionization cross sections of the first two photoelectron bands and their anisotropy parameters. These data are compared with previous theoretical values and discussed where rsc.li/pccp appropriate. This article is licensed under a largest pure carbon species identified in diffuse clouds16,17 and 1. Introduction + thus, the detection of C60 substantially stretches the size-limit 1 The discovery of the buckminsterfullerene C60 was in sorts a for carbon-containing molecules in the diffuse ISM which Open Access Article. Published on 23 April 2020. Downloaded 10/1/2021 2:55:31 PM. prescient affair. The experimental conditions under which C60 could hold the key to unravelling the rest of the DIB mystery was formed, were devised in order to understand the formation which has remained unsolved for almost a century at the time mechanisms of long-chain carbon molecules whose presence of writing.18–21 had been confirmed in various interstellar regions in the The question of how C60 forms in interstellar space has 2–5 22 preceding years. It was predicted that C60 should be as given rise to some convincing hypotheses, especially in light abundant in space as well, but it took over three decades before of the apparent inverse relationship between the abundance of C60 could be unambiguously identified; first in various types of polycyclic aromatic hydrocarbons (PAHs) and C60 at decreasing nebulae,6–8 and later on in H II regions,9 circumstellar shells,10 distances towards stars.23 Among the formation mechanisms post-asymptotic giant-branch (post-AGB) objects,11 and various for interstellar fullerenes that have gained much traction are other Photon Dominated Regions (PDRs).12 Furthermore, with (i) dehydrogenation of hydrogenated amorphous carbon + 24 the identification of its cation, C60 , as the carrier of initially nanoparticles and (ii) the formation of graphene through two and later four diffuse interstellar bands (DIBs) in the near- photo-stripping H-atoms off larger PAHs. In the second sce- infrared of the diffuse interstellar medium (ISM),13–15 research nario the bare carbon skeleton then undergoes further photo- into this molecule is forcing us to reevaluate our understanding excitation via VUV photons and self-stabilizes via IR photon of interstellar molecular complexity. For a long time C3 was the emission or carbon loss leading to pentagonal defects; a pre- requisite for the sheet to fold into a fullerene.23 Indeed, mass- a Synchrotron SOLEIL, L’Orme des Merisiers, St Aubin, BP 48, Gif sur Yvette, resolved laboratory data have shown that upon UV irradiation, France. E-mail: [email protected], [email protected]; PAHs undergo H-stripping to reveal bare carbon skeletons Tel: +31629413569 b which form stable cluster ions, indicative of fullerene Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 25 NL-2300 RA Leiden, The Netherlands formation. For this reason, several recent studies have been † Electronic supplementary information (ESI) available. See DOI: 10.1039/ focusing on PAH pentagon formation schemes, to further exploit 26,27 d0cp01210f bowling mechanisms under interstellar conditions. 13880 | Phys. Chem. Chem. Phys., 2020, 22, 13880--13892 This journal is © the Owner Societies 2020 View Article Online Paper PCCP With the tenuous connection between PAHs and fullerenes work, which focuses on the photoionization dynamics of C60 in space, it is worth noting that fullerenes may contribute to the (in)directly addresses these questions. ionization balance of the ISM and the photoelectric heating of The use of photoelectron spectroscopy in tandem with PDRs in a similar way as PAHs have been found to be.28 In and synchrotron radiation has been somewhat of a hallmark in 39,46–52 of itself C60 has been proposed as a diagnostic tool to estimate the history of spectroscopic investigations of C60, but so the temperature and/or UV flux inside molecular clouds29 but far, the threshold photoelectron spectrum (TPES) has still such data is dependent on measurements of solid C60 rather eluded closer inspection. By utilizing the double-imaging than gas-phase measurements as those relative to DIBs photoelectron photoion coincidence (i2PEPICO) technique, a studies.13–15 wealth of information besides the TPES can be retrieved The ground state electronic structure of neutral C60 pos- regarding the electronic structure and properties of C60 and sesses an icosahedral Ih symmetry. The highest occupied here we will present a 2D photoelectron dynamics matrix, from molecular orbital (HOMO) is fully occupied which results in a which are extracted the first measured TPES of C60, the electron closed-shell ground state that is both fully symmetric and kinetic energy distributions, an estimation of the absolute nondegenerate with ten valence electrons in an electronic photoionization cross section, and partial photoionization quintet level labelled 6hu. The relevant portion of the valence yields from threshold and above the Lyman-a limit (up to electronic configuration of the first three photoelectron bands 14 eV), and discuss them within the context of the presence 30,31 + of C60 are: of C60 and C60 in space. ÀÁ ÀÁ 10 10 ... 9h ðÞ6t 6ðÞ5h 10ðÞ6g 8 10h ð6g Þ8 ðÞ6h 10 |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}g 2u u u |fflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflffl}g g |fflfflffl{zfflfflffl}u Band 3 Band 2 Band 1 2. Experimental Experiments were performed at the DESIRS VUV beamline53 at Creative Commons Attribution 3.0 Unported Licence. Upon ionization the icosahedral symmetry is broken and a the Synchrotron SOLEIL facility. Horizontally polarized radia- hole in the HOMO results in the cation possessing a hu tion in the photon energy range 7–14 eV was generated by an symmetric fivefold degenerate ground state which is Jahn– undulator (OPHELIE2)54 whose higher harmonics are subse- Teller (JT) active.32 The electron density relevant to the JT- quently suppressed by four to five orders of magnitude with a active modes is akin to a band along an imaginary waistline gas filter filled with Krypton which is transparent in the 33 55 of the C60 structure and when spontaneous symmetry break- aforementioned photon energy range. The purified photon ing takes place upon ionization, the electron density around the beam was then dispersed by a 6.65 normal incidence mono- molecule is best described by an oblate spheroid. Thus, a singly chromator equipped with a 200 grooves per mm grating. The + 2 2 2 12 13 charged C60 ion possesses a Hu ground state and Hg and Gg photon flux delivered was between 10 and 10 photons per s This article is licensed under a low-lying excited states which are all capable of JT-distortions and was calibrated with an AXUV100, IRD Si photodiode. For 34 along hg and gg normal coordinates. the slit width used for the experiments, the photon energy + For C60 , one can discern JT-active modes from the Ih resolution scales from 6 meV at 7.3 eV to 23 meV at 14.0 eV. 35 Open Access Article. Published on 23 April 2020. Downloaded 10/1/2021 2:55:31 PM. character table. These are Hg,Gg, and Ag. Though the Ag Downstream the monochromator, the photon beam intersected mode does not reduce the symmetry, it does shift the total a supersonic molecular beam at a right angle inside the double 56 energy of the molecule.
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