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Abstract

The method of laser spectroscopy, widely applied in physics and chemistry, has been extended into the soft x-ray region for the first time within this thesis. By using ultrabrilliant light from FLASH (free electron laser in Hamburg) the 2S_1/2-2P_1/2 transition in Li-like Fe23+ ions was measured by direct resonant excitation from the ground state at roughly 48.6 eV. The ions were trapped and produced in the Tesla-EBIT (electron beam ion trap), a novel cryogenic EBIT specifically designed and built up for that purpose within this thesis. This proof-of-principle experiment achieved a relative statistical accuracy on the 20 parts-per-million level for the transition energy comparable to the best conventional measurements reported so far. Exploiting all possibilities offered by FLASH and the upcoming XFEL this method promises a further gain in accuracy up to three orders of magnitude, leading to unprecedented insights into basic aspects of QED and atomic structure theory in strong fields. In addition to laser spectroscopic studies conventional high precision measurements by grating spectroscopy where performed on ground state transitions in Be- and Li-like iron ions. For the 2S_1/2-2P_1/2 transition in Fe23+ for example a so far unmatched precision in the soft x-ray regime of 8 parts-per-million in wavelength was achieved.