Sub-femtosecond XUV pulse generation and measurement

The insight in always smaller microscopic units of matter as well as in always shorter courses of chemical, physical or atomic processes gives important information used for basic research. This is a driving force for the development of techniques allowing such informative insight. The resolution of time resolved measurements is limited by the exposure time, a fact, which is well known from photography, but acts as a big challenge on ultrashort-timescale. A lot of processes in the atomar or sub-atomar regime happen on a sub-femtosecond timescale (1 fs = 10EXP-15 s). To make snapshots of these events - e.g. with so called 'pump-probe' experiments, electromagnetic pulses having a duration below one femtosecond are required. Nature, however, sets a limit to such pulses in the visible range, as they can be produced by laser-light. At a wavelength of 780 nm, for instance, one oscillation cycle lasts for 2.7 fs, which turns out to be, due to quantum-mechanical considerations, the under limit of the pulse-duration. Already before the year 2000 man was able to generate pulse-durations below two oscillation cycles using ultrashort-laser techniques. In order to reach even shorter pulse-durations, one has two move towards shorter wavelengths in the XUV (extreme ultraviolet) respectively x-ray regime. Chapter 1, 'Motivation', points out these problems, several processes in the sub-femtosecond regime and the necessity of shorter wavelengths. A reliable and well known method of such a frequency conversion, providing the preservation of temporal and spatial coherence like from the driving laser radiation, is the so-called 'High Order Harmonic Generation (HHG)'. It is the main principle of generating sub-femtosecond pulses used in this work will be discussed in chapter 2, 'Generation of sub-femtosecond pulses'. Therefore, the applied laser system will be presented, the principle of High Order Harmonic Generation will be explained, and the possibility of generating sub-femtosecond pulses will be discussed based on experimental results. Chapter 3, 'Measurement of sub-femtosecond pulses', is addressed to the measurement on this time scale which is not feasible by conventionary methods. Problems which would arise at measurements using the autocorrelation method will be explained as well as the principle of crosscorrelation between XUV-pulses and visible light, which was applied at this work. The non-linear process of photo-ionization, measurement of photo-electron spectra, spectral processes happening at the temporal overlap between XUV und fundamental laser pulses and the calculation of the pulse duration based on experimental results will be shown. In chapter 4, 'Time-resolved attosecond spectroscopy', experiments, that are already carried out or planned, using sub-femtosecond pulses, especially in the field of spectroscopy will be presented. A discussion about the importance, usability on technical applications of the results will form chapter 5, 'Conclusion'. (author)