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COINCIDENCE STUDY OF DOUBLE ELECTRON EMISSION ASSOCIATED WITH K-SHELL PHOTOIONIZATION OF Ceo
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Sanja Lj. Koricaa, Axel Reinkösterb
Uwe Beckerjc
University Union - Nikola Tesla, Faculty for Ecology and or Environmental Protection, Belgrade, Republic of Serbia +
w Fritz-Haber-Institut, Department of Molecular Physics,
^ Berlin, Federal Republic of Germany,
0 e-mail: [email protected],
^ ORCID iD: http://orcid.org/0000-0002-7915-9430
< b Fritz-Haber-Institut, Department of Molecular Physics, Berlin, Federal Republic of Germany
1 c Fritz-Haber-Institut, Department of Molecular Physics, ¡^ Berlin, Federal Republic of Germany
£ DOI: 10.5937/vojtehg66-16527; https://doi.org/10.5937/vojtehg66-16527
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^ FIELD: Molecular Physics, Photoelectron-Photoion Spectroscopy
ARTICLE TYPE: Original Scientific Paper ARTICLE LANGUAGE: English
Abstract:
< The (multiple) photoionization and subsequent fragmentation of the C60 ej molecule was studied with the synchrotron radiation after removing
2 electrons from the inner K-shell. Our intention was especially focused on — the dynamics of the subsequent fragmentation. In addition to 'normal' i (non-coincident) electron and ion time-of-flight spectroscopy, we
investigated this topic with the help of an electron-electron-coincidence § measurement. Our experiment shows that in these processes Cq+60 ions
o with charge states up to 3+ and several smaller Cq+60.2m fragments are
formed. In addition, the broad peak besides the C(1s) line, usually referred to as the 'plasmon'peak, has been observed.
Key words: molecular physics, photoelectron-photoion spectroscopy, plasmon excitation.
Introduction
Since the discovery of C60 molecule (Kroto et al, 1985, pp.162-163), (Kratschmer et al, 1990, pp.354-358) many studies were performed to investigate its fundamental properties. Due to its high symmetry, C60
ACKNOWLEDGMENT: The authors are indebted to the Deutsche Forschungsgemainschaft (DFG) and to the Bundesministerium für Bildung und Forschung (BMBF) for the financial support.
represents an ideal cluster with many possible applications. Its properties § are mainly driven by its unique molecular structure like a spherical shell (Kusmany et al, 1993), (Korica et al, 2005, pp.132031-132035). So far, S only a little is known about the C60 fragmentation after K-shell ionization (Aksela et al, 1995, pp.2112-2115), (Karvonen et al, 1997, pp.34663472), (Leiro et al, 2003, pp.205-213). For the main fragmentation channel, the (successive) emission of neutral C2 has been proposed for a low charge state (Scheier et al, 1994, pp.77-93), although the emission of other neutral carbon atoms/small clusters is also present (Lykke, 1995, pp.1354-1357). Even triply charged fullerene ions appear to be rather stable (Bernard et al, 2003, pp.196-200). Highly charged C60 ions often decay through fission processes leading to a multiply charged fullerene and at least one other charged carbon atom or cluster, or they undergo multi-fragmentation processes leading exclusively to small, charged and neutral carbon clusters (Reinkoster et al, 2003, pp.263-267).
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Experiment |
The measurements were performed at the HASYLAB undulator beam line BW3 in Hamburg and at the BESSYII dipole beamline TGM4 in Berlin using monochromatized synchrotron radiation whose wavelength can be scanned with a resolution set to an appropriale value. Measurements of Ne/Ar-resonances were carried out to check the accuracy of the monochromator settings.The photon beam crosses an effusive beam of C60 molecules, provided by an oven heated to 500 °C. Outgoing electrons are detected in time-of-flight (TOF) electron spectrometers at two different angles with respect to the electric vector of the ionizing radiation(Fig1.(a)). Appropriate voltages can be applied to the TOF-analysers to keep a constant resolution of the electron spectra for different photon energies. Some measurements were recorded in the coincidence mode. Additional fullerene ion data were accumulated using a multi-hit capable ion spectrometer with a pulsed electrical separation field (Fig. 1(b)). The positively charged C60 ions or fragments are separated according to their mass-per-charge ratio by a pulsed field (pulse amplitude = 820 V, duration o = 10 js, repetition rate =12 kHz, rise time < 15 ns, field length = 5 mm). ® The ions are accelerated into a potential of -2800 V (field length = 4 mm) followed by a 200 mm long field-free drift tube. After passing the drift tube, the ions hit the detector surface which is held at a constant voltage of o -3300 V. The distance from the drift tube to the detector is 5 mm. The ^ detector consists of a Z-stack of MCPs with an active diameter of 40 mm.
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Figure 1 - Sketch of the experimental set-up. (a)beam source with the electron time-offlight (TOF) spectrometer. (b)schematic view and picture of the ion spectrometer Рис. 1 - Изображение экспериментального устройства (а)источник света с TOF спектрометром. (б) ионный спектрометр Слика 1 - Приказ експерименталног уре^а]а: (а) извор снопа са TOF спектрометром, (б) рнски спектрометар
Results and discussion
Inner shell ionization and fragmentation of C60 molecule
Figure 2 shows an example of the ion spectrum recorded at the photon energy of 390 eV. The inset shows the low-mass region of the spectrum. A rough classification of different processes is indicated by different coloured areas. In these processes, Cq+60 ions with charge states up to 3+ and several smaller Cq+60 -2m fragments have been observed. This is in accordance with the previous claim that the main fragmentation channel is the emission of neutral C2. Large singly charged carbon clusters (such as C+30, C+29, or C+20) have no particular stability and one ought to expect additional species with similar sizes (such as C+31 or C+21), which have not been observed. For all photon energies used, no small charged carbon fragments resulting from fission or multi-fragmentation processes are observed.
Figure 2 - Ion spectrum recorded with a photon energy of 390 eV Рис. 2 - Ионный спектр, записанный на энергии фотона 390 eV Слика 2 - Jонски спектар снимъен на енерг^и фотона од 390 eV
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Figure 3 - Contributions of singly, doubly, and triply charged fullerene ions for several photon energies. Different scans are marked by different symbols. The results of the coincident electron spectrum are marked by 'e'. The vertical lines indicate the C(1s) threshold (A), electron shake-up levels (A), and plasmon excitation energies (A) (Leiro et al, 2003, pp.205-213). The high amount of triply charged fullerene can be explained by double Auger and electron shake-off processes, observed in the electron-electron-
coincidence map (Fig.4). Рис. 3 - Вклад однозарядных, двухзарядных и трехзарядных ионов фуллерена по нескольким значениям энергии фотонов. Разные сканы отмечены различными символами. Результаты совпадающего электронного спектра обозначены «е». Вертикальные линии обозначают C (1s) порог (A), электронные уровни sheik-up
(A) и энергию возбуждения плазмы (A) (Leiro et al, 2003, pp. 205-213). Высокий процент трехзарядных фуллеренов можно объяснить двойными Оже- процессом и электронным sheik-up процессом, которые видны на карте совпадений
электронов (рис.4).
Слика 3 - Доприноси jедноструко, двоструко и троструко наелектрисаних jона фулерена за неколико вредности енерги'е фотона. Различити скенови су означени различитим симболима. Резултати коинцидентног електронског спектра означени су са 'e'. Вертикалне лин^е означаваjу C(1s) границу (A), електронске ше^-ап нивое (A) и енерги'е плазмонских ексцитаци'а (A) (Leiro et al, 2003, pp.205-213). Висок проценат троструко наелектрисаних фулерена може се обjаснити двоструким Ожеовим и електронским шеjк-оф процесима, щи се могу видети на електрон-електрон коинцидентноj мапи (слика 4).
Figure 3 shows contributions of singly, doubly and triply charged § fullerene ions for several photon energies. These results indicate that, above the carbon K-shell of C60, the main products are doubly and triply S charged fullerenes. C+60 is the most abundant ion in the low energy region. The relative C2+60 yield compared to the C+60 yield first increases with increasing photon energy and stagnates above «350 eV at a nearly constant level. The yield of triply charged C3+60 is similar to the yield of C2+58 . Different doubly charged C2+ 60-2m fragments appear step by step |j with increasing photon energy. The yields of singly charged fragments exhibit an enhancement in certain photon energy regions; at high photon energies, these yields decrease and are only slightly visible.
So the obtained ion yield spectroscopy of gas phase C60 is corroborated by the corresponding photoelectron measurements (Korica et al, 2018). The continuous intensity distribution in the photoelectron spectra can be either the result of direct double photoionization or double-Auger decay. The quality of the former K-shell photoelectron ® measurements was insufficient to disentangle these two contributions o experimentally (Aksela et al, 1995, pp.2112-2115), (LeBrun et al, 1994, pp.3965-3968), (Bruhwiler et al, 1993, pp.3721-3724), (Krummacher et al, 1993, pp.8424-8429). In general, the disentanglement of the two processes on the basis of normal ion or electron spectroscopy is not unambiguously possible.
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Electron-electron coincidence spectroscopy of excited C60
Electron-electron-coincidence measurements were carried out to get a deeper understanding about the fundamental processes causing o the many-electron emission in C60. Here, a separation of different underlying processes can be better achieved. (Fig. 4).
Shake-off electrons are abundant at low-kinetic energies. Therefore, even at such high photon energies, shake-off processes are important to understand the yields of the multiply charged C60 ions besides the Auger and double Auger processes. In the case that two shake-off electrons p leave the C60 molecule, the energy sharing is very asymmetric. The faster of the two shake-off electrons contributes signifcantly to the broad peak besides the C(1s) main line, usually referred to as the 'plasmon' peak (Hertel et al, 1992, pp.784-787), (Leiro et al, 2003, pp.205-213); this to possibility has been unrevealed so far. S
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Figure 4 - Electron-electron coincidences as a function of the kinetic energy of the two detected electrons. The coincidence map was recorded with a photon energy of 380 eV. Рис. 4 - Карта совпадений электронов как функция кинетической энергии двух обнаруженных электронов. Карта совпадений записана на нергии фотона 380 eV. Слика 4 - Електрон-електрон коинциденце као функци^а кинетичке енерги^е два детектована електрона. Коинцидентна мапа сним^ена jе на енергир фотона од
380 eV.
Conclusion
We have studied the photoionization of the C60 molecule above the C(1s) threshold, in the photon energy range hv=(330-390)eV. A careful analysis of the spectra yielded a surprising and unexpected result.
Clear hints have been found that the major contribution to the triply charged ion yield is the direct double photoionization of C60. However, in contrast to most atoms and molecules, it is driven by the plasmon excitation associated with the K-shell photoionization of the fullerenes. Whereas the K-shell satellites are still bound core excited ionic states of the C60 molecule, plasmon excitations at higher binding energies are already in the double electron emission continuum. This causes a specific intensity distribution and explains the origin of the broad resonance features in the continuum part of the spectrum and an unusual high amount of triply charged fullerenes of 40%.
References
Aksela, S., Nömmiste, E., Jauhiainen, J., Kukk, E., Karvonen, J., Berry, H.G., Sorensen, S.L. & Aksela, H. 1995. Photofrag mentation of C60 Molecules following Resonance Excitation and Ionization near the C 1s Edge. Physical Review Letters, 75(11), pp.2112-2115. Available at: https://doi.org/10.1103/physrevlett.75.2112.
Bernard, J., Martin, S., Chen, L., Cederquist, H., Salmoun, A., & Bredy, R. 2003. A single ion (Ar q+ q =1,3 or C 60 3+ ) in a conic electrode electrostatic trap. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 205, pp.196-200. Available at: https://doi.org/10.1016/s0168-583x(03)00553-6.
Brühwiler, P.A., Maxwell, A.J., Rudolf, P., Gutleben, C.D., Wästberg, B., & Märtensson, N. 1993. C1s autoionization study of electron hopping rates in solid C60. Physical Review Letters, 71(22), pp.3721-3724. Available at: https://doi.org/10.1103/physrevlett.71.3721.
Hertel, I.V., Steger, H., de Vries, J., Weisser, B., Menzel, C., Kamke, B., & Kamke, W. 1992. Giant plasmon excitation in free C60 and C70 molecules studied by photoionization. Physical Review Letters, 68(6), pp.784-787. Available at: https://doi.org/10.1103/physrevlett.68.784.
Karvonen, J., Nömmiste, E., Aksela, H., & Aksela, S. 1997. Photoion spectra of C60 molecules at resonance excitation and ionization energies near the C 1s edge. Journal of Chemical Physics, 106(9), pp.3466-3472. Available at: https://doi.org/10.1063Z1.473442.
Korica, S., Reinköster, A., & Becker, U. 2018. Strong enhancement of double Auger decay following Plasmon excitation in C60. Vojnotehnicki glasnik/Military Technical Courier, 66(3), pp.483-494. Available at: https://doi.org/10.5937/vojtehg66-16269.
Korica, S., Rolles, D., Reinköster, A., Langer, B., Viefhaus, J., Cvejanovic, S., & Becker, U. 2005. Partial cross sections and angular distributions of resonant and nonresonant valence photoemission of C60. Physical Review A, 71(1), pp.132031-132035. Available at:
https://doi.org/10.1103/physreva.71.013203.
Krätschmer, W., Lamb, L.D., Fostiropoulos, K., & Huffman, D.R. 1990. Solid C60: a new form of carbon. Nature, 347(6291), pp.354-358. Available at: https://doi.org/10.1038/347354a0.
Kroto, H.W., Heath, J.R., O'Brien, S.C., Curl, R.F., & Smalley, R.E. 1985. C60: Buckminsterfullerene. Nature, 318(6042), pp.162-163. Available at: https://doi.org/10.1038/318162a0.
Krummacher, S., Biermann, M., Neeb, M., Liebsch, A., & Eberhardt, W. 1993. Close similarity of the electronic structure and electron correlation in gasphase and solid C60. Physical Review B, 48(11), pp.8424-8429. Available at: https://doi.org/10.1103/physrevb.48.8424.
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Gemmell, D.S., Kanter, E.P., & Bauer, W. 1994. Ionization and Multifragmentation of C60 by High-Energy, Highly Charged Xe Ions. Physical Review Letters, 72(25), pp.3965-3968. Available at: ™ https://doi.org/10.1103/physrevlett.72.3965.
ER, Leiro, J.A., Heinonen, M.H., Laiho, T., & Batirev, I.G. 2003. Core-level XPS
spectra of fullerene, highly oriented pyrolitic graphite, and glassy carbon. Journal of Electron Spectroscopy and Related Phenomena, 128(2-3), pp.205-213.
0 Available at: https://doi.org/10.1016/s0368-2048(02)00284-0. ^ Lykke, K.R. 1995. Fragmentation of C60: Experimental detection of C, C2, ^ C3, and C4 by xuv postionization. Physical Review A, 52(2), pp.1354-1357.
1 Available at: https://doi.org/10.1103/physreva.52.1354.
Й Reinkoster, A., Siegmann, B., Werner, U., & Lutz, H.O. 2003. Ion-impact
induced excitation and fragmentation of C60. Radiation Physics and Chemistry, oE 68(1-2), pp.263-267. Available at: https://doi.org/10.1016/s0969-806x(03)00295-* 0.
Scheier, P., Dunser, B., Worgotter, R., Lezius, M., Robl, R., & Mark, T.D. 1994. Appearance and ionization energies of singly, doubly and triply charged C60 and its fragment ions produced by electron impact ionization. International w Journal of Mass Spectrometry and Ion Processes, 138, pp.77-93. Available at: https://doi.org/10.1016/0168-1176(94)04034-6.
о ИССЛЕДОВАНИЕ ДВОЙНОЙ ЭЛЕКТРОННОЙ ЭМИССИИ,
СВЯЗАННОЙ С ФОТОИОНИЗАЦИЕЙ К-ОБОЛОЧКИ С,
Саня Л. Корицаа, Аксел Райнкостер6, Уве Бекерв
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> Институт им. Фрица Габера, Отделение молекулярной физики,
г. Берлин, Федеративная Республика Германия б Институт им. Фрица Габера, Отделение молекулярной физики,
г. Берлин, Федеративная Республика Германия в Институт им. Фрица Габера, Отделение молекулярной физики, г. Берлин, Федеративная Республика Германия
ОБЛАСТЬ: молекуларная физика, фотоэлектрон-фотоионная
спектроскопия ВИД СТАТЬИ: оригинальная научная статья ЯЗЫК СТАТЬИ: английский
Резюме:
(Многогранная) фотоионизация и сопроводительная
фрагментация молекулы Сбо исследовалась с помощью
синхротронного излучения после выброса электрона из
внутренней К-оболочки. Наше исследование было сосредоточено на динамике сопроводительной фрагментации. Кроме «нормальной» (несовпадающей) электронной и ионной ££ спектроскопии, применялись и другие методы измерений, так например, было проведено измерение электронного совпадения. Наш эксперимент показал, что в течение этих процессов ^ формируются Оч+ео ионы с зарядом до трех + и несколько небольших Оч+60-2т фрагментов. Кроме того, был выявлен широкий пик рядом с основной линией О (1в), так называемым «плазмоном».
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Ключевые слова: молекулярная физика, фотоелектрон- о фотоионная спектроскопия, возбуждение плазмона.
Саъа Корицаа, Аксел Ра]нкостерб, Уве Бекерв
Сажетак:
КОИНЦИДЕНТНА СТУДША ДВОСТРУКЕ ЕЛЕКТРОНСКЕ ЕМИСШЕ ПОВЕЗАНЕ СА ФОТОиОНИЗАЦШОМ К-^УСКЕ С60
Универзитет Унион - Никола Тесла, Факултет за екологи]у и .
заштиту животне средине, Београд, Република Срби]а + о
Институт Фриц Хабер, Одсек за молекуларну физику, р
Берлин, Савезна Република Немачка 1 Институт Фриц Хабер, Одсек за молекуларну физику, Берлин, Савезна Република Немачка Институт Фриц Хабер, Одсек за молекуларну физику, Ё
Берлин, Савезна Република Немачка
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ОБЛАСТ молекуларна физика, фотоелектронско-фоторнска ^
спектроскопи]а ВРСТА ЧЛАНКА: оригинални научни чланак иЕЗИК ЧЛАНКА: енглески
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Фотоjонизациjа (вишеструка) и пратеЯа фрагментац^а молекула О60 проучавана jе уз помоЯ синхротронског зрачеъа након избациваъа електрона из унутрашъе К-л>уске. Наше | истраживаъе било jе посебно фокусирано на динамику пратеЯе фрагментац^е. Поред „нормалне" (некоинцидентне) електронске и jонске ТОФ спектроскопц'е, ову тему изучавали смо и уз помоЯ £ електрон-електрон коинцидентног мерена. Наш експеримент показухе да се у овим процесима формираjу Оч+60 jони са наелектрисаъем до 3+ и неколико маъих С+60-2т фрагмената. & Поред тога, поред главне О(1в) лин^е уочен jе широки пик, тзв. я плазмон.
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Къучне речи: физика молекула, фотоелектронска-фотоjонска спектроскоп^а, плазмонска ексцитац^а.
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Paper received on / Дата получения работы / Датум приема чланка: 11.02.2018. Manuscript corrections submitted on / Дата получения исправленной версии работы / Датум достав^а^а исправки рукописа: 12.04.2018.
Paper accepted for publishing on / Дата окончательного согласования работы / Датум коначног прихвата^а чланка за об]ав^ива^е: 14.04.2018.
© 2018 The Authors. Published by Vojnotehnicki glasnik / Military Technical Courier (www.vtg.mod.gov.rs, втг.мо.упр.срб). This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/rs/).
© 2018 Авторы. Опубликовано в «Военно-технический вестник / Vojnotehnicki glasnik / Military Technical Courier» (www.vtg.mod.gov.rs, втг.мо.упр.срб). Данная статья в открытом доступе и распространяется в соответствии с лицензией «Creative Commons» (http://creativecommons.org/licenses/by/3.0/rs/).
© 2018 Аутори. Обjавио Воjнотехнички гласник / Vojnotehnicki glasnik / Military Technical Courier (www.vtg.mod.gov.rs, втг.мо.упр.срб). Ово jе чланак отвореног приступа и дистрибуира се у складу са Creative Commons licencom (http://creativecommons.org/licenses/by/3.0/rs/).
