Ultrafast soft X-ray emission spectroscopy of surface adsorbates using an X-ray free electron laser

Journal of Electron Spectroscopy and Related Phenomena 187 (2013) 9–14

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Ultrafast soft X-ray emission spectroscopy of surface adsorbates using an X-ray free electron laser T. Katayama a , T. Anniyev a , M. Beye a,b , R. Coffee c , M. Dell’Angela d , A. Föhlisch b,e , J. Gladh f , S. Kaya a , O. Krupin c,g , A. Nilsson a,f,h,i , D. Nordlund i , W.F. Schlotter c , J.A. Sellberg a,f , F. Sorgenfrei d , J.J. Turner c , W. Wurth d , H. Öström f , H. Ogasawara a,i,∗ a

SIMES, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA Institute for Methods and Instrumentation in Synchrotron Radiation Research, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Wilhelm-Conrad-Röntgen Campus, Albert-Einstein-Str. 15, 12489 Berlin, Germany c Linac Coherent Light Source, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA d University of Hamburg and Center for Free Electron Laser Science, Luruper Chaussee 149, D-22761 Hamburg, Germany e Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany f Department of Physics, AlbaNova University Center, Stockholm University, SE-10691, Sweden g European XFEL GmbH, Albert-Einstein-Ring 19, 22761 Hamburg, Germany h SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA i SSRL, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA b

a r t i c l e

i n f o

Article history: Received 2 February 2013 Received in revised form 19 March 2013 Accepted 20 March 2013 Available online 3 April 2013 Keywords: X-ray emission spectroscopy Surface science Free electron laser Ultrafast

a b s t r a c t We report on an experimental system designed to probe chemical reactions on solid surfaces on a subpicosecond timescale using soft X-ray emission spectroscopy at the Linac Coherent Light Source (LCLS) free electron laser (FEL) at the SLAC National Accelerator Laboratory. We analyzed the O 1s X-ray emission spectra recorded from atomic oxygen adsorbed on a Ru(0 0 0 1) surface at a synchrotron beamline (SSRL, BL13-2) and an FEL beamline (LCLS, SXR). We have demonstrated conditions that provide negligible amount of FEL induced damage of the sample. In addition we show that the setup is capable of tracking the temporal evolution of electronic structure during a surface reaction of submonolayer quantities of CO molecules desorbing from the surface. Published by Elsevier B.V.

1. Introduction The fundamental understanding of heterogeneous catalysis requires a detailed knowledge of the dynamics of elementary processes on the atomic scale, such as adsorption, surface reactions involving different intermediates, and desorption, as schematically shown in Fig. 1. During a surface reaction there are important electron and energy transfer processes between the different adsorbates and the catalytic substrates, which determine many of the important reaction steps [1]. The ultimate goal here is to understand on a fundamental level, i.e. microscopic, how chemical bonds are broken and reformed during catalytic reactions. Specifically, we would like to obtain a molecular-level understanding of reactivity, visualize how electrons are transferred and how different molecular states are rearranged during the course of surface events. Such knowledge provides the basis for the understanding of

∗ Corresponding author. Tel.: +1 650 926 4010; fax: +1 650 926 4100. E-mail address: [email protected] (H. Ogasawara). 0368-2048/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.elspec.2013.03.006

chemical trends and predicting chemical reactivity for many different catalytic surfaces. The elementary steps of surface chemical reactions typically take place on picosecond or sub-picosecond time-scales and the lifetime of transient intermediate states can be very short. Therefore the equilibrium concentration of these intermediate states is often low and it becomes a challenge to observe these under steadystate conditions. One way to overcome this issue is to perform pump-probe experiments where a reaction is initiated via an ultrashort laser pulse at a well-defined point in time and monitored by another ultrashort laser pulse after a well-defined time span [2]. Such experiments have been performed for surface reactions, where the adsorbate dynamics have been followed in real-time during the course of reactions [3,4]. For this purpose, ultrafast optical laser based techniques have be used, which is ideal to monitor nuclear dynamics [5,6] and hot-electron dynamics [7]. In order to dynamically follow the transient evolution of the electronic structure of adsorbates in sub-monolayer quantities, one has to overcome problems with a small signal-to-background ratio. Ultimately it is desirable to extend such studies to use element-specific

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T. Katayama et al. / Journal of Electron Spectroscopy and Related Phenomena 187 (2013) 9–14

Fig. 1. Schematic figure showing the elementary steps in a catalytic reaction involving molecular oxygen and hydrogen. The reaction can be stimulated by laser pump and probed with XES using the FEL soft X-ray pulse.

and/or site-specific electronic structure probes of the electronic structure that can monitor the chemical bonding changes in real time and follow how molecules are transformed around the center where the interesting chemistry takes place. Ultrafast X-ray pulses from an X-ray FEL have allowed such a dream to now become a step closer to reality by using core level X-ray absorption and emission spectroscopies [8] as probes of chemical reactions on surfaces at FEL facilities. X-ray Emission Spectroscopy (XES) has the unique ability to provide an atom-specific probe of the electronic structure. The atomic sensitivity arises from the creation of a core hole on an atom during the X-ray absorption process, which can only be filled by valence electrons in close proximity to the excited atom. The final state of the X-ray emission process is single valence hole state similar to that in valence band photoemission but here the valence electronic structure is projected onto a specific atom [9,10], thus eliminating the background from a metallic substrate. Accessing the 1s core levels of C, N and O in the soft X-ray regime with spectroscopy opens up new possibilities to study time-resolved changes in the electronic structure of surface reactions and catalysis for chemically important species. Due to the electronic transition dipole selection rule, XES observes the partial density of states for pcomponents of occupied valence orbital projected onto the selected atom. Thus we can experimentally probe the hybridized orbitals of the adsorbate–substrate bonding by the atomic-site-specific projection similar to the linear combination of atomic orbital (LCAO) approach [9,10]. In addition XES can be used as a mean to obtain fluorescence-detected X-ray absorption spectra (XAS), providing a complementary atom-by-atom electronic structure probe of the unoccupied density of states. Furthermore, many spectroscopic techniques such as photoelectron spectroscopy are based on electron detection that will have severe limitations for ultrafast studies where the large peak intensity will build up space charge on the surface [11–16]. In XES and XAS both the excitation source and the detected signal are photons so the spectra will not be distorted due to the development of space charge. In the present paper we describe the implementation of an experimental system for the ultrafast studies of surface reactions using free-electron laser based XES and XAS. One of the essential questions that needs to be addressed is if the ultrashort FEL pulse will modify the spectroscopic signature due to nonlinear phenomena. Here we show that the same spectroscopic signatures are obtained for the adsorbate systems O/Ru(0 0 0 1) and CO/Ru(0 0 0 1) at the FEL as with conventional synchrotron radiation based XES. 2. End station The ultrafast soft X-ray surface science experimental end station (see Fig. 2) consists of preparation and analysis vacuum chambers

with an operating pressure of 1 × 10−10 Torr or lower. The preparation chamber is equipped with an electron beam heater and an ion-gun for sample cleaning, as well as gas dosers for sample preparation. The analysis chamber is equipped with a grating spectrometer for X-ray emission spectroscopy measurements and an electron energy analyzer (VG-Scienta, R3000) for photoelectron spectroscopy measurements, which is mounted perpendicular to the beam propagation direction at a 45◦ angle with respect to the horizontal, which is also the fixed direction of the electric field vector of the incoming light at LCLS. The experimental station is mounted on an optical table (Advanced Deign Consulting USA, Inc., OPT-1000-6) that can be translated longitudinally, laterally and vertically with respect to the axis of the incoming beam. The pitch, roll, and yaw can also be adjusted (see Fig. 2). The Ru(0 0 0 1)-p(2 × 1)-O phase [17] was prepared on a 10 mm × 10 mm commercial Ru(0 0 0 1) single-crystal (Matek, Germany) by exposing the surface to 5 × 10−8 Torr of molecular oxygen while cooling from 900 K to 400 K, which corresponded to 15 Langmuir. The CO saturated Ru(0 0 0 1) phase was prepared by exposing the clean surface to 1 × 10−8 Torr of CO for >3 Langmuir at room temperature. During the data acquisition, CO was re-dosed through a variable leak valve with an attached tube (5 mm ID outlet) directed toward the surface. The X-ray emission spectrum is recorded by the slit-less grating spectrometer based on an elliptically shaped grating (40 mm × 100 mm) optimized in negative diffraction order (see Fig. 2 inset). The grating is coated with Ni to enhance the reflectivity at less grazing incidence angles. Since the entrance slit is removed, then the incident beam on the sample becomes the source point. Energy resolution in the X-ray emission spectrometer is defined by the source size as well as grating aberrations, slope errors and image errors. The spectrometer is based on a Rowland grating setup, which images the source vertically onto a 2D-detector. Although various setups are used today [18] they are all subject to the relations given by the Rowland condition. This condition gives a simple spherical focusing term and minimized coma aberration with unit magnification and constant wavelength dispersion both in negative and positive order. For first order diffraction, the reciprocal wavelength dispersion is given by  source size) = d/R, where d is the grating period, which is equal to the inverse of the grating line density, and R is the spherical radius of the grating. Large deviations from these values are associated with efficiency loss, either by grating efficiency or solid angle acceptance. We optimized the performance of the spectrometer to obtain the source size limited energy resolution by optimizing the line density and spherical radius of grating. For the photon energy of the oxygen Kedge, line densities of 1100 lines/mm and R about 5000 mm give a ˚ typical dispersion about 0.02 A/␮m or 40 meV/␮m. By replacing the spherical shape with an elliptical shape, the illuminated section of the grating can be made a few times longer without blowing up the image error contribution. A high collection angle of 1 millisteradian is achieved with a grating illumination length of 100 mm at an incident angle of 5◦ from the grating surface. The 2D-detector assembly is based on multichannel plates (MCP) coated with CsI for enhanced efficiency [19] with a negatively biased electrode placed in front of the MCP. Digital cameras, Allied Vision Technology Dolphin F-145C for measurements at SSRL and Adimec OPAL-1000 for measurements at LCLS, are used to record the image from a phosphor screen behind the MCP. The soft X-ray beam can be focused down to 50 ␮ along the horizontal direction and less than 10 ␮ along the vertical direction using a Kirkpatrick-Baez pair of focusing mirrors at both the synchrotron beamline (SSRL, BL13-2) [20] and the FEL beamline (LCLS, SXR) [21–23], which is required to obtain good energy resolution, yielding a typical resolving power of about 1000.

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Fig. 2. The ultrafast soft X-ray surface science experimental end station designed for optical pump and X-ray probe soft X-ray emission spectroscopy and photoelectron spectroscopy measurements. The system runs under ultra high vacuum conditions. The system is equipped with a soft X-ray grating spectrometer and an electron spectrometer. A manipulator allows for positioning and transfer of the sample(s) between the preparation chamber and the analysis chamber. The chamber frame is mounted on an optical table, which is capable of moving in all six degrees of freedom by motors. The inset shows a schematic view of soft X-ray grating spectrometer.

3. X-ray induced sample damage Even with the relative low peak and average photon flux achieved at a synchrotron, sample damage can still occur on the surface, which has to be taken into account. In order to produce reliable data from the high intensity FEL, measurements have to be performed to ensure that the sample is unaffected from beam induced damage during the experiment. Excitation by X-ray photons and secondary electrons can lead to dissociation or desorption of surface adsorbates. Absorption of X-rays from the FEL in the metal substrate involves the production of a secondary electron cascade on a sub-100 fs time scale, which can result in the ablation of several atomic layers near the surface [24]. Grazing incidence geometry is preferred since this enhances the surface as well as reduces sample damage issues by increasing the reflectivity of the metal and dispersing the beam spatially over the surface to reduce the flux density. Beam induced accumulative structural damage can be minimized by continuous movement of the sample [10,25,26]. In order to deliver a completely fresh area at the interaction point, the sample needs to be translated along the Z-axis by the beam footprint size after each exposure to the FEL beam. The sample was scanned along the Z-axis at a speed of 0.4 mm/s, which produces negligible damage in the surface layer. Furthermore, the probed electronic structure can be modified during the duration of the X-ray pulse. In particular, high peak intensities exceeding 1017 W/cm2 [27–29] achievable at LCLS can induce double core hole ionization, which can happen if the core ionization event occurs more frequently than the decay event of the core holes. Double core holes can give rise to X-ray emission transitions in a completely different energy region. A more likely rapid Auger decay process from the core holes can also generate multiple

valence hole states. If these multiple valence hole states have a lifetime of similar magnitude as the duration of the FEL pulse, the electronic structure probed with XES can be altered, which would result in an appearance of both single and multiple valence hole final states in the XES spectra. To control the X-ray fluence at the sample position, the incident beam in our setup is stretched on the sample due to the grazing incidence geometry. This decreases the peak intensity significantly and we anticipate that the effect of double core-holes will be negligible. In order to investigate the existence of such electronic structure damage, we compared O 1s XES from adsorbed atomic oxygen on a Ru(0 0 0 1) surface measured at the FEL (LCLS SXR) to that measured at a synchrotron facility (SSRL BL13-2) where the peak intensity is much lower. Fig. 3 shows XES measured using the monochromatic beam in the positive first diffracted order on BL13-2 at SSRL (photon flux was approximately 1 × 104 photon in a 60 ps pulse) and using the monochromatic beam in the negative first diffracted order at SXR at LCLS (1 × 109 photon in a 100 fs pulse). The observed count rate on SXR at LCLS was 0.45 counts/pulse. The incident angle of the beam to the sample surface was 1◦ at LCLS and ∼4◦ at SSRL, in which the beam footprint on the sample in two coordinates (X-axis and Z-axis in Fig. 2) was estimated to be 550 ␮m (X) × 60 ␮m (Z) at LCLS and 150 ␮m (X) × 60 ␮m (Z) at SSRL. We can clearly recognize that the XES spectra from the two measurements are almost identical. A minor spectral difference between the two measurements can be explained by a worse spectrometer resolution at LCLS due to an enlarged beam size along the sagittal direction. The absence of difference in spectral features supports the idea that there is no significant structural damage and no additional initial states are produced upon the FEL excitation.

Intensity [arb. unit]

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Intensity [arb. unit]

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a

a

b

b

510

520

530

540

Photon Energy [eV] Fig. 3. X-ray emission spectra of atomic oxygen on Ru(0 0 0 1). The spectra were obtained using synchrotron radiation on BL13-2 at SSRL (a) by accumulating 5 min and FEL radiation from SXR at LCLS (b) by accumulating 50,000 FEL shots (i.e. about 7 min at 120 Hz FEL repetition rate). Spectra were recorded using excitation energy of 531.3 eV, which was 0.7 eV above the oxygen K-edge absorption maximum. The energy bandwidth of the incoming photons and the XES spectrometer resolution were estimated to be 0.3 eV and 0.8 eV, respectively.

The spectrum has been discussed in literature in terms of the formation of bonding and antibonding states between the O 2p states and the Ru 4d states [10,30]. The peak at 525 eV corresponds to the bonding states and the shoulder at 528 eV to the antibonding states which spill over the Fermi level and show up as intensity at the onset of the XAS spectrum. To increase the data collection rate, we attempted to record XES spectra by operating the SXR LCLS beamline monochromator in the non-monochromatic mode, which delivers an order of magnitude higher average and peak flux. However, there is a trade-off between a gain in data collection rate due to high average flux and electronic structure modification due to high peak flux. We observed a plasma plume from ablated species due to the high peak flux. In addition, an intense background due to Bremsstrahlung radiation from ablated plasma species is observed in the lower photon energy region from the oxygen X-ray emission line, see curve (a) in Fig. 4. We could avoid the ablation of sample and the Bremsstrahlung radiation emission by reducing the peak power of the FEL with a gas attenuator, see curve (b) in Fig. 4. Under this condition, we gained a factor of five in the data collection rate in the non-monochromatic mode compared to the monochromatic mode. The broadening of spectral features in curve (b) in Fig. 4 compared to curve (b) in Fig. 3 can be explained by a different beam profile at the sample position: the vertical beam size and position at the sample position were different between the monochromatic and non-monochromatic operations. The broad spectral feature centered around 539 eV in curve (b) in Fig. 4 corresponds to the elastic scattering peak of the incident X-rays. Neither the non-monochromatized beamline focus nor the appropriate focus position of the slit-less grating spectrometer was re-optimized for this measurement. This influences the energy resolution due to the removal of the entrance slit in the design. 4. Optical laser pump-X-ray probe X-ray emission Photoexcitation of substrate electrons stimulates dynamical processes on surfaces, such as desorption, dissociation, etc. Photostimulated desorption of CO from Ru(0 0 0 1) has been reported [5,31–34]. The process involves heating the substrate electronic

500

520

540

Photon Energy [eV] Fig. 4. X-ray emission spectra of atomic oxygen on Ru(0 0 0 1) recorded using the pink beam of the LCLS. Curves (a) and (b) show the spectra recorded without and with an order of magnitude attenuation of the beam, respectively. 539 eV was used as an excitation energy near the oxygen K-edge, with a band width of 8 eV FWHM. The spectra were normalized arbitrarily to have the same height at 524 eV.

temperature and the subsequent increase of the substrate phonon and CO-substrate vibrational temperature, resulting in desorption [5,33,34]. We used the 400 nm optical laser to pump the system into the excited state in order to stimulate the desorption and the soft X-rays from the FEL to probe the temporally evolved electronic structure of CO adsorbed on Ru(0 0 0 1) [35]. A Ti:sapphire laser system synchronized with the FEL e-beam delivers laser pulses of 50 fs in duration with a pulse energy of 1 mJ at a wavelength of 800 nm, which can be doubled or tripled to 400 or 267 nm, respectively. The repetition rate of the optical laser can be set to multiples/fractions of the FEL repetition rate. In the present experiment a FEL repetition rate of 60 Hz and an optical laser repetition rate of 30 Hz was used, which allowed us to record un-pumped background spectra on the same sample as the transient spectra were recorded. A stabilized optical fiber is used to synchronize the experimental laser with the electron beam that is used to produce the FEL radiation [36]. The laser beam is brought into the chamber collinearly with the FEL beam. At the interaction point, the laser beam is focused to 100 × 100 ␮m in order to coincide spatially with the FEL spot. In the present experiments a ␤-barium borate (BBO) crystal was used to frequency double the optical laser to 400 nm. Substantially higher desorption yield was observed using 400 nm compared to 800 nm at comparable fluences in a previous study of associative desorption of C + O → CO [37] as well as in desorption of molecularly adsorbed CO in the preparatory experiments performed at the laser laboratory of Stockholm University [38]. The polarization and fluence of the optical laser pulses can be set using half-wave plates placed before and after the BBO doubling crystal. Temporal jitter between the FEL and the optical laser, which determines the temporal resolution of the pump-probe experiment, is ∼300 fs between the FEL and the laser. Recently, tools have been implemented to increase the resolution to below 100 fs [39]. The spatial overlap was aligned using a phosphor screen, which allows optical alignment of both the visible and X-ray beams. A needle tip antenna was used to coarsely align the temporal overlap between the FEL and the optical laser pulses by monitoring photocurrents induced by the optical laser and the FEL, which could be overlapped to within 5 ps. The temporal overlap was set to within 0.25 ps using ultrafast X-ray induced changes in the optical reflectivity of a Si3 N4 film with a thickness of 1 ␮m [40,41].

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Acknowledgements

Fig. 5. A 2D map shows the time evolution of resonantly excited X-ray emission spectra of CO on Ru(0 0 0 1) after excitation with 400 nm, 50 fs pump pulses. The fluence of the pump pulses was tuned to be high enough to desorb CO. The spectra were recorded at various pump-probe delays using resonant excitation of oxygen in CO (near the O 1s → * resonance at 533 eV), thus the oxygen site-projected transient electronic structure was recorded.

We recorded a series of XES for excitation energies near the O 1s → ␲* resonance (533 eV) as a function of delay time between the pump and the probe. Intense optical laser irradiation heats the substrate and can cause ablation of the metal surface layers [42]. The enhanced scattering of the optical laser beam from such defects was used to detect such damage. The incident optical laser fluence was adjusted in order not to exceed the damage threshold of the Ru crystal, yet high enough to ensure that a significant fraction of CO molecules desorbs from each laser shot. By accumulating multiple sets of delay scans for over 10 h, we were able to collect enough counts and data points to map transient X-ray emission spectra on a sub-picosecond time scale. Fig. 5 shows the pump-probe O 1s X-ray emission spectra of CO as a function of delay time, mapping the temporal evolution of O 2p components in the valence electronic states. There are features observed at 524.9, 523.5 and 521.2 eV due to the X-ray emission from 1, 5, and 4 states of carbon–oxygen chemical bond and at 527.3 eV due to the d state related to the  interaction with the substrate [10,43,44]. We observe see that spectral changes after the excitation at time = 0 and that spectral features from short-lived transient species [35]. The detailed understanding of the time evolution of the electronic structure of transient species will provide us with understanding on how the chemical reaction proceeds after the photoinjection of energy into the system [35,45]. 5. Conclusions We have demonstrated the atom specific probing of electronic structure of adsorbed atoms and molecules on the surface at submonolayer coverages and transient response on the subpicosecond time scale through soft X-ray emission spectroscopy at a FEL facility. The FEL provides several orders of magnitude higher peak photon flux compared to synchrotron facilities. Anticipated electronic structure damage, which occurs on faster time scales than the FEL pulse length, has been avoided by controlling the flux density on the sample, and a negligible amount of electronic structure modification is shown for the Ru(0 0 0 1)-p(2 × 1)-O phase. The high throughput X-ray emission spectrometer ensures a high enough count rate to allow the accumulation of statistically valid data. Photo-stimulated dynamics of adsorbed CO on a Ru surface was studied for a proof-of-principle experiment of pump-probe X-ray emission spectroscopy. 400 nm laser pump and oxygen 1s X-ray emission probe study tracked the modification of electronic states projected onto the oxygen atom of CO on a sub-picosecond time scale.

This research was carried out on the SXR Instrument at the Linac Coherent Light Source (LCLS) and the SSRL (Stanford Synchrotron Radiation Lightsource) at the SLAC National Accelerator Laboratory. The SXR Instrument is funded by a consortium whose membership include the LCLS, Stanford University through the Stanford Institute for Materials Energy Sciences (SIMES), Lawrence Berkeley National Laboratory (LBNL), University of Hamburg through the BMBF priority program FSP 301, and the Center for Free Electron Laser Science (CFEL). The LCLS and SSRL are funded by the US Department of Energy’s Office of Basic Energy Sciences. We gratefully acknowledge the support of LCLS and SSRL staff. This work was further supported by the Japan Science and Technology Agency, the Swedish research council (VR), the Alexander von Humboldt foundation and the VolkswagenStiftung. References [1] A.C. Luntz, Chemical Bonding at Surfaces and Interfaces, Elsevier, Amsterdam, 2008, p. 143. [2] A.H. Zewail, World Scientific (1994). [3] J.A. Misewich, T.F. Heinz, P. Weiand, A. Kalamarides, in: H.-L. Dai, W. Ho (Eds.), Laser Spectroscopy and Photochemistry on Metal Surfaces Part II, World Scientific, Singapore, 1995. [4] T. Fauster, H. Petek, M. Wolf, Dynamics at Solid State Surfaces and Interfaces, Wiley-VCH Verlag GmbH & Co KGaA, 2012, p. 115. [5] M. Bonn, C. Hess, S. Funk, J.H. Miners, B.N.J. Persson, M. Wolf, G. Ertl, Phys. Rev. Lett. 84 (2000) 4653. [6] F. Fournier, W. Zheng, S. Carrez, H. Dubost, B. Bourguignon, J. Chem. Phys. 121 (2004) 4839. [7] M. Lisowski, P. Loukakos, U. Bovensiepen, J. Stähler, C. Gahl, M. Wolf, Applied Physics A: Materials Science & Processing 78 (2004) 165. [8] M. Beye, F. Sorgenfrei, W.F. Schlotter, W. Wurth, A. Föhlisch, PNAS 107 (2010) 16772. [9] A. Nilsson, J. Electron Spectrosc. 126 (2002) 3. [10] A. Nilsson, L.G.M. Pettersson, Surf. Sci. Rep. 55 (2004) 49. [11] R. Clauberg, A. Blacha, J. Appl. Phys. 65 (1989) 4095. [12] T.L. Gilton, J.P. Cowin, G.D. Kubiak, A.V. Hamza, J. Appl. Phys. 68 (1990) 4802. [13] X.J. Zhou, B. Wannberg, W.L. Yang, V. Brouet, Z. Sun, J.F. Douglas, D. Dessau, Z. Hussain, Z.X. Shen, J. Electron Spectrosc. 142 (2005) 27. [14] S. Passlack, S. Mathias, O. Andreyev, D. Mittnacht, M. Aeschlimann, M. Bauer, J. Appl. Phys. 100 (2006) 024912. [15] S. Hellmann, K. Rossnagel, M. Marczynski-Bühlow, L. Kipp, Phys. Rev. B 79 (2009) 035402. [16] A. Pietzsch, A. Föhlisch, M. Beye, M. Deppe, F. Hennies, M. Nagasono, E. Suljoti, W. Wurth, C. Gahl, K. Döbrich, A. Melnikov, New J. Phys. 10 (2008) 033004. [17] H. Pfnür, G. Held, M. Lindroos, D. Menzel, Surf. Sci. 220 (1989) 43. [18] J. Nordgren, J. Guo, J. Electron Spectrosc. 110,111 (2000) 1. [19] G.W. Fraser, M.A. Barstow, M.J. Whiteley, A. Wells, Nature 300 (1982) 509. [20] http://www-ssrl.slac.stanford.edu/beamlines/bl13-2/ [21] W.F. Schlotter, J.J. Turner, M. Rowen, P. Heimann, M. Holmes, O. Krupin, M. Messerschmidt, S. Moeller, J. Krzywinski, R. Soufli, M. Fernandez-Perea, N. Kelez, S. Lee, R. Coffee, G. Hays, M. Beye, N. Gerken, F. Sorgenfrei, S. Hau-Riege, L. Juha, J. Chalupsky, V. Hajkova, A.P. Mancuso, A. Singer, O. Yefanov, I.A. Vartanyants, G. Cadenazzi, B. Abbey, K.A. Nugent, H. Sinn, J. Luning, S. Schaffert, S. Eisebitt, W.S. Lee, A. Scherz, A.R. Nilsson, W. Wurth, Rev. Sci. Instrum. 83 (2012) 043107. [22] J. Chalupsky, P. Bohacek, V. Hajkova, S.P. Hau-Riege, P.A. Heimann, L. Juha, J. Krzywinski, M. Messerschmidt, S.P. Moeller, B. Nagler, M. Rowen, W.F. Schlotter, M.L. Swiggers, J.J. Turner, Nucl. Instrum. Methods A 631 (2011) 130. [23] P. Heimann, O. Krupin, W.F. Schlotter, J. Turner, J. Krzywinski, F. Sorgenfrei, M. Messerschmidt, D. Bernstein, J. Chalupsky, V. Hajkova, S. Hau-Riege, M. Holmes, L. Juha, N. Kelez, J. Luning, D. Nordlund, M.F. Perea, A. Scherz, R. Soufli, W. Wurth, M. Rowen, Rev. Sci. Instrum. 82 (2011) 093104. [24] B. Iwan, J. Andreasson, E. Abreu, M. Bergh, C. Caleman, J. Hajdu, N. Tîmneanu, Proceedings of SPIE 8077 (2011) 807705. [25] N. Wassdahl, A. Nilsson, T. Wiell, H. Tillborg, L.C. Duda, J.H. Guo, N. Mårtensson, J. Nordgren, J.N. Andersen, R. Nyholm, Phys. Rev. Lett. 69 (1992) 812. [26] T. Schiros, K.J. Andersson, L.G.M. Pettersson, A. Nilsson, H. Ogasawara, J. Electron Spectrosc. 177 (2010) 85. [27] S.M. Vinko, O. Ciricosta, B.I. Cho, K. Engelhorn, H.K. Chung, C.R.D. Brown, T. Burian, J. Chalupsky, R.W. Falcone, C. Graves, V. Hajkova, A. Higginbotham, L. Juha, J. Krzywinski, H.J. Lee, M. Messerschmidt, C.D. Murphy, Y. Ping, A. Scherz, W. Schlotter, S. Toleikis, J.J. Turner, L. Vysin, T. Wang, B. Wu, U. Zastrau, D. Zhu, R.W. Lee, P.A. Heimann, B. Nagler, J.S. Wark, Nature 482 (2012) 59. [28] L. Young, E.P. Kanter, B. Krässig, Y. Li, A.M. March, S.T. Pratt, R. Santra, S.H. Southworth, N. Rohringer, L.F. DiMauro, G. Doumy, C.A. Roedig, N. Berrah, L.

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