Surface structure determination by x-ray standing waves at a free-electron laser

Abstract

We demonstrate the structural sensitivity and accuracy of the x-ray standing wave technique at a high repetition rate free-electron laser, FLASH at DESY in Hamburg, by measuring the photoelectron yield from the surface SiO 2 of Mo/Si multilayers. These experiments open up the possibility to obtain unprecedented structural information of adsorbate and surface atoms with picometer spatial and femtosecond temporal resolution. This technique will substantially contribute to a fundamental understanding of chemical reactions at catalytic surfaces and the structural dynamics of superconductors.

Spiers Memorial Lecture: prospects for photoelectron spectroscopy

“…142 The structural sensitivity and accuracy of the XSW technique has also been demonstrated using a fs pulsed source of photons from a FEL, FLASH at DESY in Hamburg. 145 Photoelectron yield was measured from Si/Mo multilayers terminated with surface SiO 2 layers of various thicknesses, allowing determination of the oxide layer thickness to an accuracy of a few percent. 145 This opens up the prospect of combining the fs temporal resolution of a FEL with a high level of structural accuracy, in order to better understand the structural dynamics of surfaces.…”

“…145 Photoelectron yield was measured from Si/Mo multilayers terminated with surface SiO 2 layers of various thicknesses, allowing determination of the oxide layer thickness to an accuracy of a few percent. 145 This opens up the prospect of combining the fs temporal resolution of a FEL with a high level of structural accuracy, in order to better understand the structural dynamics of surfaces. Because XSW does not require lateral long-range order (unlike, for example, low energy electron diffraction), the technique could provide a route to measuring the structural evolution occurring during chemical reactions at surfaces, significantly enhancing our understanding of heterogeneous catalysis.…”

“…139 As full-field photoelectron diffraction is now demonstrated in static experiments, there is a realistic prospect that tr-ARPES/tr-MM can be used with fs core-level excitation to simultaneously probe how the both the electronic states and the coherent lattice motion evolve after a pump pulse, by exploiting photoelectron diffraction and X-ray standing waves. 138,145,191…”

Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens

“…[6][7][8][9] Soft and hard x-ray pulses from FELs give access to fs core-level dynamics 7,[10][11][12] and allow "locking-in" to the coupled coherent lattice motion, exploiting photoelectron diffraction and x-ray standing waves. [13][14][15][16] The recent advances of full-field imaging momentum microscopes (MMs) have given a new drive to this rapidly developing field with extreme ultraviolet (XUV) or x-ray pulses from an FEL [7][8][9][16][17][18][19] or vacuum ultraviolet (VUV) pulses from HHG laboratory sources. [20][21][22][23][24] Emerging applications of tr-MM are ultrafast molecular orbital imaging 19,25,26 and the tracking of transient changes of topological properties and orbital textures of out-ofequilibrium states of matter.…”

Photoemission from the gas phase using soft x-ray fs pulses: An investigation of the space-charge effects

“…Advancements with the use of the pump-andprobe technique are strictly related to the development of stable and intense sources with ultrashort (fs-ps) pulse duration. While the pump signal is often provided by a pulsed optical laser, the list of employed sources for the probe radiation includes the multiplication of a laser fundamental frequency through non-linear crystals [7,10] or high-harmonic generation (HHG) [13][14][15], synchrotron light [9,16,17], free-electron lasers (FELs) [18][19][20][21][22]. This collection of available sources covers a photon-energy range extending from the near ultraviolet to the hard X-rays.…”