Applications of Short Extreme Ultraviolet Pulses to the Spectroscopy of Atoms and Molecules
Sammanfattning: This thesis is devoted to the characterisation in time and frequency, and to spectroscopic application, of extreme ultraviolet (XUV) pulses. High-order harmonic generation or wave mixing processes were used to produce the pulses in the XUV spectral region. Time-frequency characterisation of femtosecond XUV pulses generated as high-order harmonics is important both for applications in XUV spectroscopy and in gaining a deeper understanding of the high-order harmonic generation process. A method of characterising the harmonics generated with respect to the time-dependent frequency (chirp) and the pulse duration has been developed. The method is based on two-photon non-resonant ionisation of argon using XUV and infrared pulses. Lifetime measurements were performed in He, CO and N2. In He, the 13th and 14th harmonics of a tuneable picosecond distributed feedback dye laser were used to excite the 1s2p and 1s3p 1P1 state. The states were subsequently ionised by a delayed visible or ultraviolet probe. In N2, the 8th harmonic was used in a similar set-up to determine the lifetimes of the state and of several 1Pu states. These lifetimes were found to be between 100 ps and 2.7 ns. In CO, the same harmonic was used for lifetime measurements of the 4ppL1P state for three different isotopomers. Complementary measurements were performed in the frequency domain using the nanosecond XUV system at the Laser Centre of Vrije Universiteit in Amsterdam. Finally, the influence of the relative polarisation of the pump and probe fields was investigated in two-photon resonant ionisation of He using picosecond pulses, as well as in non-resonant two-colour ionisation of Ar in the femtosecond regime.
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