J. Kutzner

Laser based soft-x-ray pulses for photoelectron spectroscopy of surfaces

Light pulses in the extreme ultraviolet (EUV) are generated in the photon energy range hν from 20 to 120 eV in the conversion media argon, neon, and helium by a tabletop high-repetition-rate 30-fs Ti:sapphire laser system. This high-harmonic radiation is applied in the energy range from 20 to 50 eV for photoelectron spectroscopy studies of clean and adsorbate covered Ni(111) and Pt(111) surfaces. The photoemission spectra of the clean Pt(111) surface show secondary electron emission structures that influence the cross-section analysis of CO-induced states. Taking these Pt resonances into consideration, the well-known 4σ and 5σ CO shape resonances are observed at photon energies of 37 and 28 eV. In contrast to previously published experimental data, a resonance is also observed at hν=31 eV for the CO 1π state. On the basis of theoretical approaches this signal structure is identified as an autoionization resonance, experimentally detected for the first time to our knowledge.

Adaptive Shaping of High-Power Broadband Femtosecond Laser Pulses

We demonstrate the implementation of a feedback controlled pulse shaping device in a femtosecond high-power Ti:sapphire laser system. The laser system consists of a mirror dispersion controlled oscillator and a multipass amplifier with a pairing double prism compressor. The system provides pulses with a duration of 30 fs and an energy of up to 1.2 mJ per pulse at 1 kHz repetition rate. The phase distorted output pulses are phase modulated with a high resolution spatial light modulator (SLM). The pulse shaper consists of an all-reflective zero-dispersion compressor equipped with a liquid crystal array. For adaptive compression of the amplified pulses a feedback loop is implemented. A two-photon process is used to monitor the temporal pulse characteristics. To achieve the shortest possible pulse an evolutionary algorithm controls the pulse shaper utilizing the two-photon signal as feedback. With this set-up transform limited pulses are achieved. Detailed investigations of algorithm parameters and their effect on convergence behaviour have been performed and are compared with the experimental findings.