C. Kunz

Investigation of carbon contamination of mirror surfaces exposed to synchrotron radiation

Abstract In a systematic investigation gold coated silicon substrates were irradiated with filtered and unfiltered synchrotron radiation under varying conditions. The thickness of the contaminant was measured with a VUV reflectometer through the observation of interference structures. In addition the overall photocurrent which varies with the thickness of the contaminant could be measured. All results show that the crackiing process is caused predominantly by the photoelectrons emitted and not by the primary photons. This is specifically supported by the observation of two different growth rates: an initial fast rate up to about 50 A thickness due to the high photoyield of gold and an asymptotic growth rate of about 10% the initial rate due to the much smaller yield of carbon. A rate equation deduced from a simple model describes the dependence of the growth rate on residual gas pressure and temperature in good qualitative agreement with the experimental results.

Messung charakteristischer Energieverluste von Elektronen an leichtoxydierbaren Metallen im Ultrahochvakuum

Energy losses of 34 keV electrons penetrating thin foils of Li, Na, K, Rb, Cs, Ca, Sr, Ba, Mg, and Al are measured. The metals are evaporated and investigated in UHV (~10-9 Lorr) with an apparatus constructed especially for this purpose. For all the alkali metals free electrons like behaviour is found with a surface and a volume plasma loss. Ca and Sr show more complicated structures. For most of the volume losses half width, dispersion and cut-off is measured. Surface losses are investigated for oxide-free surfaces with respect to angular distribution, dispersion and total excitation probability, and also the displacement due to oxidation is measured. A narrow loss line from the excitation of a surface wave at the Al/Mg interface is found at 12.8 eV in good agreement with the theoretically predicted value.

Über die Winkelabhängigkeit der charakteristischen Energieverluste an Al, Si, Ag

ZusammenfassungThe angular dependence of the amount of the energy losses, of the width of the loss peaks, and of their intensities has been measured, and general agreement with the plasma theory is found for the 15 eV Al loss and the 17 eV Si loss. In addition, there is a remarkable dependence of the width of the Al loss peaks on the mean crystalline size. The intensities of the 3.6 eV Ag loss and the 6 eV Al loss show a more rapid decrease with scattering angle than is required by ordinary plasma theory. The variation of the intensities of these losses with foil thickness has been investigated and gives evidence that both of these losses must be due to surface excitation.Im Rahmen unserer Untersuchungen über die unelastische Elektronenstreuung werden im folgenden Messungen zur Winkelabhängigkeit der charakteristischen Energieverluste mitgeteilt. Zunächst sollen die benötigten theoretischen Ergebnisse kurz zusammengestellt werden.

Soft X-ray scattering from rough surfaces: experimental and theoretical analysis

Specularly reflected and scattered radiation was measured at lambda = 50, 100, 200 A from artificially roughened flat mirrors and from thick vacuum evaporated Au samples. The results of the roughened samples are in reasonable agreement with Beckmanns scalar theory using an exponential autocorrelation function. The angle-dependent scattering distributions at different angles of incidence and different wavelengths are described with a unique mean roughness and autocorrelation length. Some small discrepancies remain because the scalar theory does not take into account the optical constants of the scattering surface. The use of vector theory in its simplest analytical form is not successful; however, it provides a means to qualitatively correct the scalar theory for the influence of the actual optical constants in agreement with our experiments. The thick Au films are roughened by surface crystallization and yield completely different scattering distributions. These results could not be fitted to scalar theory, either with an exponential or a Gaussian autocorrelation function.

Grazing incidence optics for soft x‐ray microscopy

We have developed and brought into operation a new type of scanning soft x‐ray microscope basing on a grazing incidence mirror. A ring shaped ellipsoidal mirror serves as a nonimaging but concentrating optical element to produce a microfocus. The specimen is mechanically scanned, and the image is recorded a pixel at a time. Transmission, total photoelectron yield, photoelectron spectroscopy, fluorescence, and stimulated desorption of ions and neutrals can provide information in the range of photon energies between 20 and 1300 eV. While the smallest spatially resolved structure, limited by the mirror imperfections, has a size of 0.4 μm, the minimum diameter of selected areas necessary for photoelectron microspectroscopy presently is about 4 μm.

Output diagnostics of the grazing incidence plane grating monochromator bumble bee (15–1500 eV)☆

The Bumble Bee is a bakeable UHV compatible plane grating monochromator with a fixed exit beam and the capability to suppress higher order radiation in a wide energy range. The instrument was built to be used in connection with UHV reflectometer and has a differential pumping section between the optical components and the sample allowing a pressure of 10−5 Torr in the experimental chamber without influencing the UHV in the monochromator. The monochromator is not optimized for resolution. Due to its location at a beamline with a short source distance we achieve only medium resolving power in the order of E/ΔE ≈ 200. The primary goal is the suppression of higher orders, fortunately the thus selected operating parameters for the coupled rotations of the optical components also give nearly the highest available output. The instrument is characterized in great detail. The performance of the instrument is discussed and compared with extensive theoretical calculations.

Performance of the flipper monochromator at the wiggler/undulator at HASYLAB

Abstract After almost ten years of successful operation at a bending magnet beamline of HASYLAB, the Flipper monochromator was transferred to a 32-pole wiggler/undulator. With new grazing incidence optics the original energy range of 15–450 eV could be extended to more than 1500 eV. The gain in intensity is approximately two orders of magnitude. Undulator effects are observed between 36 and 900 eV and contribute considerably to this intensity gain. Monochromators of the Flipper type are ideally matching the emission characteristics of undulators. The main problem with wigglers at high energy storage rings is the heat load on the optical elements.

The adsorption site of oxygen on Si(100) determined by SEXAFS

We present a SEXAFS (surface extended X-ray absorption fine structure) investigation of oxygen adsorbed on Si(100) at the oxygen K absorption edge ( E K =535 eV), in the photon energy range 500–800 eV. The oxygen uptake on Si(100) at room temperature reaches a saturation at about one monolayer coverage, where the (2×1) reconstruction of the clean surface is lost. Our results demonstrate the following: the oxygen atoms occupy two types of bridge positions, namely between Si atoms belonging to the first layer and between Si atoms belonging to different topmost layers. The bridge site is characterized by a bond distance of 1.65 A and a Si−O−Si bond angle of ∼120°. No evidence of other types, of adsorption geometries is found nor the presence of adsorbed molecular oxygen is observed.

A new VUV-reflectometer for UHV-applications☆

Abstract A VUV-reflectometer for HASYLAB at DESY, Hamburg, is described. The necessary motions of the sample and detector inside vacuum are transferred by special rotation-translation feedthroughs. These make the apparatus quite simple and allow various kinds of accurate optical measurements, e.g. determination of optical constants, grating efficiencies, and scattering from surfaces.

Angular dependent photoelectric yield and optical constants of Al between 40 and 600 eV

Abstract Total photoelectric yield measurements were performed on ∼ 800 A thick Al films evaporated in ultrahigh vacuum on glass substrates. Fitting of the photoelectric yield as a function of angle of incidence using Peppers equations yielded optical constants in excellent agreement with those obtained from reflectance measurements on the same films. This establishes a convenient method for determining optical constants and yields in addition values for film thickness and electron escape depths.