Photo-ionization and fragmentation of Sc3N@C80 following excitation above the Sc K-edge

Citation for published version: Obaid, R, Schnorr, K, Wolf, TJA, Takanashi, T, Kling, NG, Kooser, K, Nagaya, K, Wada, S, Fang, L, Augustin, S, You, D, Campbell, EEB, Fukuzawa, H, Schulz, CP, Ueda, K, Lablanquie, P, Pfeifer, T, Kukk, E & Berrah, N 2019, 'Photo-ionization and fragmentation of Sc3N@C80 following excitation above the Sc Kedge', The Journal of Chemical Physics, vol. 151, no. 10, pp. 104308. https://doi.org/10.1063/1.5110297

We have investigated the ionization and fragmentation of a metallo-endohedral fullerene, Sc 3 N@C 80 using ultrashort (10 fs) x-ray pulses. Following selective ionization of a Sc (1s) electron (hν = 4.55 keV), an Auger cascade leads predominantly to either a vibrationally cold multiply charged parent molecule or multi-fragmentation of the carbon cage following a phase transition. In contrast to previous studies, no intermediate regime of C 2 evaporation from the carbon cage is observed. A time-delayed, hard x-ray pulse (hν = 5.0 keV) was used to attempt to probe the electron transfer dynamics between the encapsulated Sc species and the carbon cage. A small but significant change in the intensity of Sc-containing fragment ions and coincidence counts for a delay of 100 fs compared to 0 fs, as well as an increase in the yield of small carbon fragment ions, may be indicative of incomplete charge transfer from the carbon cage on the sub-100 fs timescale. a) Electronic mail: razib.obaid@uconn.edu

I. INTRODUCTION
# Endohedral (or internally doped) fullerenes, are intriguing systems that bridge the gap between $ molecular and nano-systems 1,2 . Like C 60 , there is much to learn about their behavior when they % are excited with photons, in particular in the hard x-ray regime. Endohedral fullerenes are of great & interest to study due to their unique properties, including electron transfer between the encaged ' species and the carbon cage 2 , and their potential use in molecular electronics and organic photo-! voltaics. The understanding derived from the photoionization of carbon nanomaterials can provide ! insight towards optimizing their properties for use in these applications 3 . Endohedral fullerenes ! have been studied with optical lasers 4,5 and synchrotron radiation [6][7][8][9] . Synchrotron studies, which !! allow for core-level ionization of the encapsulated species, have indicated that there is a much !" higher fragmentation propensity if the inner species becomes highly excited 7 , especially compared !# to the level of fragmentation observed following excitation/ionization of valence electrons by op-!$ tical lasers 5 . The most intriguing aspect of the endohedral systems compared to empty fullerenes !% is the mutual influence of the electrons from the cage and the electrons from the encapsulated !& moiety, which may significantly modify the dynamics 10 .

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Like synchrotron radiation, x-ray free electron lasers (FEL) can access the core levels of the " inner moiety of the endohedral fullerene 11 . The brilliance and photon energy tunability of the " FEL allows for site-specific photon absorption, and because they have short pulse durations, also " allows, in principle, for following the dynamics of the molecule 11,12  with the cage 20,21 . A cartoon schematic of this mechanism is shown in Fig. 1  "pump" and the "probe" pulses were produced by the same electron bunch using a variable length % undulator scheme 25  photon absorption. Meanwhile, the probe pulse was chosen to be about 5.0 keV for two reasons: %# 1) the photon energy of 5.0 keV sufficiently allows for ionization to highly charge states, above %$ 4+, and 2) the difference in photon energies between the two pulses needed to be large enough %% such that the pump and the probe could be monitored independently by an in-line spectrometer 27 . 60 Hz, and the duration of these two pulses were estimated to be about 10 fs each, as measured us-&$ ing a spectrometer consisting of an analyzer flat crystal of silicon 29 prior to the experiment. Since &% these measurements require their own chamber, the pulse duration was measured ex situ before the && beamtime. The temporal jitter between the two pulses was estimated to be a few fs due to the elec-&' tron bunch spacing, as measured previously for this scheme 25 . Using an in-line spectrometer 27 , ' the shot-to-shot pulse energy was monitored to be about 105 μJ for the pump and 110 μJ for the ' probe, and the energy fluctuation between the two pulses were found to be about 20% of the mean ' for each arm, as also shown in Fig. S1.

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The experimental setup is shown schematically in Fig. 1. The Sc 3 N@C 80 sample (97% pu-'" rity) was procured from SES Research. The sample was converted to an effusive vapor by using '# a sample dispenser oven 16 which evaporated Sc 3 N@C 80 into its vapor phase at 970 -1000 K.

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The orientation of the oven was horizontal with respect to the spectrometer axis, and along the '% polarization of the x-ray pulses. We estimate the target density from the oven to be about 2 × 10 8 The ions created were extracted by a uniform electric field of 550 V/cm at the interaction region, and were subsequently detected on a microchannel plate (MCP) detector. The ion hits on the MCP were recorded using a digitizer and a software discriminator 30 . More details of the ion TOF spectrometer are described in detail elsewhere, see 24 .
At a fluence of about 40 µJ/ µm 2 for each pulse (corresponding to an intensity of about 4 × 10 17 # W/cm 2 ), the ionization mechanism is expected to be step-wise. First there is a photoionization $ (P) event followed by an intra-atomic Auger (A) decay until a stable charge state is reached. The stability of the Sc 3 N@C 80 molecular complex depends on the oxidation state of the cage. It ! has been determined that electron sharing with the Sc 3 N stabilizes the C 80 cage, which attains the " stable icosahedral form by accepting about 6.3 electrons from the inner moiety 38 . This induces a # partial charge of 2.4+ on each Sc atom inside the cage, for the overall neutral Sc 3 N@C 80 . ions per shot (ca. 96 % of all shots that yield an ion signal), as illustrated in Fig. 1 most stable charge on the parent molecule is 2+ and 3+. This may be due to the stabilizing nature !# of the π electron delocalization in the system following ionization, which corresponds to a larger !$ gap between the HOMO and LUMO orbital for the resultant multiply charged parent molecule 39 . ison of the two sets of data, the normalized ion yield is slightly higher for the small carbon species ## and, more significantly, the scandium-containing small fragments at 100 fs delay compared to 0 #$ fs. Due to technical reasons of the two pulse scheme 25 , the only way single pulse spectra could be in coincidence. in coincidence with Sc + are C + n with n spanning n = 1 − 18. Note that it is not possible to rule out '# the contribution of doubly or triply charged species with the same mass/charge ratio as the singly '$ charged species. However a consideration of the peak shapes in the mass spectra would indicate '% that the contribution of multiply-charged species is very low.
'& In Fig. 3 (b), we also see a larger intensity for coincidences between Sc + and C + n for the 100 fs '' delay data, particularly for the smallest carbon species. There is a more obvious difference in the coincidences between Sc + and scandium-containing fragment ions with a very significant decrease in the Sc + -Sc + coincidence rate for a delay of 100 fs and increase in the Sc + -ScC + 2 and Sc + - to identify due to the higher level of multi-body fragmentation, further blurring out the ion-ion coincidence islands. There are also some indications of carbon fragment ions being produced in coincidence with other carbon fragment ions, such as C + + C + 2 , C + + C + 3 , C + + C + 4 , C + + C + 5 , C + + C + 7 and others. the emission of a high energy electron. If we consider the role of the cage which encapsulates the moiety, then we would see that C 4− 80 , which is the charge state of the cage after charge rearrangement followed by removal of 2 electrons from a single Sc, is as stable as C 6− 80 , which is the charge states attained by the cage in the neutral system 39 . This is analogous to stating that Sc 3 N@C 2+ 80 is ! as stable as the neutral system due to the interplay of the charge stabilization by the π orbital and " the strain effect due to Coulomb repulsion. Triply and quadruply charged species most likely arise with the carbon cage, this will provide enough internal excitation to induce a phase transition in " the hot molecule 41 leading to rapid cage break-up and the characteristic mass spectrum of small " carbon fragment ions that we observe. Extensively studied fragmentation patterns using multipho- The statistical break-up of internally excited fullerene cages had been modeled previously us- ing both a percolation model 44 and a maximum entropy model 41 . In the bond percolation model, # a power-law distribution for the small cage fragment species is predicted. The fragment ion inten-# sities for both the 0 fs and 100 fs delay data have been plotted on a ln-ln plot in Fig. 4. Figures   #   4 (a) and (b) show that the data fits reasonably well to the expected power law behavior for all #! fragments from C + 3 to C + 15 , as given by: S n ∝ n −γ . Here S n is the yield of carbon fragments, C + n ,

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and n is the number of carbon atoms, while γ is the fit parameter. The goodness of the fit, R 2 , ## for the blue fit lines for Fig. 4 (a) and (b) are 0.63, 0.82 respectively. For the fragmentation of #$ a cage system such as C 60 , it has been shown previously that due to finiteness and the periodic #% nature of the system, γ ≈ 1.3 is obtained for a fullerene system undergoing a phase transition by #& emitting small carbon molecular fragments 44 . This value of γ is a consequence of the amount of #' energy transferred to the fullerene in the highly charged ion collisions 44 after averaging over the $ impact parameter dependence of the energy transfer. The gradient will change depending on how $ much internal energy is present in the system. For very high amounts of internal energy, γ will be $ higher since the fragment distribution will shift more to smaller mass ions. Since, neither from the $! consideration of C + and C + 2 nor from their omission in the overall ln-ln fit, we did not obtain value $" of γ higher than 1.3 for any of the delays, it can be said that the measured distribution is consistent $# with the power law behavior expected from statistical break-up as predicted by the percolation $$ model, in analogy with nuclear multi-fragmentation 45,46 .

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The maximum entropy model as applied to C 60 shows similar behavior but is able to reproduce $& variations in the relative intensities, including the lower than predicted yield for C + and C + 2 by $' taking the ionization energies and binding energies of all possible fragments into consideration 41 .

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If we again turn our attention to the coincidence data in Fig. 3 (b), we see small but significant &% differences between the 0 fs and the 100 fs data which may be indicative of two-photon processes && and the timescale of intramolecular electron transfer events. The increase in the intensity of co- incidences between Sc + and small scandium-containing fragment ions compared to coincidences ' between two singly-charged scandium ions could be due to the dynamics of electron transfer from ' the carbon cage to the photoionized Sc ions. At 0 fs delay, all direct or Auger induced photoion-' ization can be expected to occur on the 10 fs timescale. Any two-photon absorption is likely to '! produce two charged Sc species within the cage giving a relatively high probability to detect a Sc + '" -Sc + coincidence signal for 0 fs delay (taking into consideration the energy and charge equili-'# bration that will take place as the system is undergoing the rapid phase transition and break-up). ond photon, and therefore giving a higher probability to detect more small charged carbon species '' in coincidence with Sc + on the second (P)-(A) cycle. The higher yield of coincidences between ! Sc + and scandium-containing fragment ions for 100 fs delay may be caused by the initiation of the ! cage break-up on this timescale after the first photon absorption. In the total count of Sc + , as seen ! in Fig. 2 (c), we do not see a significant difference between the two delays. K-shell ionization of !! two neighboring Sc is more likely to give Sc + -Sc + coincidences than in the single photon case.

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We can see from Fig. S3 that the single pulse Sc + -Sc + peak is much lower in intensity than the !# pump-probe coincidence data in Fig. 3. This is reasonably convincing evidence that a majority of !$ the intensity of the Sc + -Sc + coincidence peak in Fig. 3   fs provided additional evidence for a "slow" (> 10 fs) electron transfer between the carbon cage ! and the multiply-ionized Sc ions. Due to the "all or nothing" nature of the energy transfer to the ! cage, the mass spectra look quite different to the typical bimodal fragment distributions that are ! normally observed for fragmenting fullerene species. The absence of highly charged species is also ! ! an unusual feature for K-shell excitation studies and is thought to be a consequence of the particu-! " lar geometry of the endohedral species and the highly statistical nature of fullerene fragmentation.

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Although the current data is of a rather preliminary nature and requires further experimental and ! $ theoretical study to fully unravel the complex dynamics of the studied system, we have shown