Hochauflösende Bildgebung und NEXAFS-Spektroskopie mit weicher Röntgenstrahlung aus laserinduzierten Plasmen

Abstract

Soft x-ray microscopy and absorption spectroscopy are extremely useful tools for high-resolution imaging and chemical analysis of samples in various scientific fields. However, due to the required high photon flux of soft x-ray radiation, up to now, both methods are almost exclusively performed at synchrotron sources. To overcome this limitation there have been successful efforts to employ laboratory-scale sources based on gas discharge or laser-produced plasmas using liquid and solid laser targets. A serious drawback of these sources is the almost unavoidable generation of debris particles, which can damage optical elements close to the plasma. The operation of liquid jet targets requires also great experimental and technical effort. Thus, there is still the need for further optimization and simplification of lab-scale soft x-ray sources to pave the way for their wider dissemination. Here, the development of a laser-produced plasma source from a pulsed gas jet is presented, enabling the construction of a clean, compact and long-term stable soft x-ray source. As its photon yield is smaller due to the lower particle density within the laser focus, several ways to increase the source brilliance have been examined in detail. In particular, it was investigated how higher gas pressures, shorter laser pulses and smaller emission angles affect the plasma properties. Based on this source a compact transmission x-ray microscope operating in the "water window" ( = 2.3 – 4.4nm) was built, characterized and applied for imaging of various test and biological objects. The resolving power of the microscope is about 50 to 60 nm. In addition, an advanced laboratoryscale NEXAFS spectrometer has been established, allowing for the reliable analysis of different absorption edges at photon energies between 250 to 1250 eV. So far, NEXAFS experiments with the table-top spectrometer have been performed in a high vacuum system, excluding a number of interesting samples from spectroscopic investigations. To extend the application possibilities of the spectrometer, the setup was modified to measure samples also under atmospheric conditions using two silicon nitride membranes as vacuum windows. Finally, different schemes have been suggested to merge the soft x-ray microscope and the NEXAFS spectrometer to a combined laboratory-scale spectromicroscope, which shall enable spatially resolved elemental mapping in the future. It has already been shown that a test object can be imaged at two wavelengths below and above the titanium L-edge.