Synopsis Photoionization of endohedral Sc 3 N@C + 80 fullerene ions is investigated using synchrotron radiation in the energy range 280 to 420 eV. In the product channels Sc 3 N@C 2+ 80 and Sc 3 N@C 2+ 78 clear signatures of the cross section contributions arising from L-shell photoabsorption by the encapsulated Sc atoms have been detected. Photon-induced reactions of Sc 3 N@C + 80 ions have been investigated in the energy range 280 -420 eV employing the Photon-Ion Spectrometer at PETRA III

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... E) [1] . The measurements cover photoabsorption near the K edge of carbon, the L edge of scandium and the K edge of nitrogen. Several reaction channels were studied. As an example, Fig. 1 shows the preliminary cross section for single ionization of Sc 3 N@C + 80 ions. Very clear signatures mainly arising from photoabsorption by Sc atoms near the L edge are visible on top of the contribution of the C 80 cage. Similar results were obtained for the Sc 3 N@C 2+ 78 product channel which shows a slightly different spectral shape. Only the single-ionization channels with and without fragmentation showed significant contributions arising from photoabsorption by one of the encapsulated Sc atoms. This is quite surprising considering that about 400 eV are deposited in the Sc 3 N@C + 80 molecule when a photon is absorbed by an encapsulated scandium atom with promotion of an L-shell electron. Previous studies on Xe 4d photoabsorption by Xe@C + 60 at much lower photon energies of 60 to 150 eV showed strong contributions of double ionization associated with multiple fragmentation [2] . A recent experiment with Lu 3 N@C q+ 80 , q = 1, 2, 3, exploring photoabsorption near the Lu 3d edge [3] suggested that the whole fullerene molecule explodes when a 1500 to 1700 eV photon is absorbed by one of the Lu atoms. In contrast to that, the present findings for Sc 3 N@C + 80 indicate that an initial vacancy in the L shell of one of the Sc atoms predominantly results in net single ionization of the atom and subsequently also the whole molecule. Redistribution of 2p and 2s absorption oscillator strengths [4] may then be the reason for different spectral shapes of the measured excess cross sections. We thank G. Hartmann, F. Scholz, J. Seltmann, and J. Viefhaus for their assistance in using beamline P04. 1) Photon-induced reactions of Sc3N@C80 + endohedral fullerene ions in the photon energy range 280 to 420 eV have been investigated at beamline P04 using the PIPE endstation. This energy range covers photoabsorption in the region around the K edge of carbon, the L edge of scandium and the K edge of nitrogen. Several reaction channels were studied: plain single, double and triple ionization as well as single ionization accompanied by the loss of a C2 unit. Only the single-ionization channels with and without fragmentation showed significant contributions beyond the empty-cage cross section arising from photoabsorption by one of the encapsulated atoms. This is quite surprising considering the results of previous experiments with endohedral fullerenes. The figure below shows a preliminary cross section result for plain single ionization of Sc3N@C80 + ions. Very clear signatures arising mainly from photoabsorption by Sc atoms near the L edge are visible above the contribution of the C80 cage. Similarly high quality results were obtained for the Sc3N@C78 2+ product channel which shows a slightly different spectral shape. Exploratory single-ionization measurements using Sc3N@C80 2+ primary ions were performed and also showed a clear signature of the encapsulated atoms. The statistical quality of the measured Sc3N@C80 3+ product-ion yield obtained in this test experiment was not sufficient, however, to draw conclusions on the detailed shape of the excess cross section caused by the encapsulated atoms. 2) We used (very expensive) enriched samples of Sc3N@C80 (>95%) but still, the signal count rates were too low to cover more than one incident-ion charge state during the beamtime. We encountered a problem with electrical connections of the evaporation oven used in the ion source which had to be fixed during the beamtime. 3) The originally proposed experiment was slightly extended to clarify existing issues with the photon energy calibration: photoabsorption by Ne atoms and Ne + ions near the K edge was therefore remeasured. 4) The results obtained in this beamtime will be published in one of the leading journals covering atomic processes with photons. The main goal was to see and to quantify influences of the Sc and N atoms encapsulated inside a C80 cage on the photoabsorption by the endohedral fullerene molecule Sc3N@C80. This goal was clearly reached with the surprising result, that straight single ionization of the whole complex is the main reaction channel while strong fragmentation and multiple ionization were expected to dominate considering that a potential energy of approximately 400 eV could be deposited inside the carbon cage. Because of low signal rates only one primary ion charge state, Sc3N@C80 + , could be investigated. The planned measurements on negative and multiply charged positive ions have to be addressed in future beamtimes. 5) Jens Viefhaus and Jörn Seltmann will be included as co-authors in a publication addressing photoabsorption by Ne and Ne + near the K edge (see item 3). 395 400 405 410 415 420 100 150 200 250 hν + Sc 3 N@C 80 1+ → Sc 3 N@C 80 2+ + 1enormalized cross section (set to 100 at 395 eV) fit representing empty-cage photoionization Cross section ( arb. units ) Photon energy ( eV ) contributions from Sc L-edge and N K-edge photoabsorption Figure 1. Cross section for single photoionization of Sc 3 N@C + 80 in the vicinity of the scandium L edge. The data are normalized to 100 Mb at 392 eV. References [1] S. Schippers et al. 2014 J. Phys. B 47 115602 [2] R. A. Phaneuf et al. 2013 Phys. Rev. A. 88 053402 [3] J. Hellhund et al. 2015 Phys. Rev. A. 92 013413 [4] A. Müller et al. 2008 Phys. Rev. Lett. 101 133001 1