ausblenden:
Schlagwörter:
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Zusammenfassung:
The implementation of pump-probe experiments with ultrashort laser pulses enables the
study of dynamical processes in atoms or molecules, which may provide a deeper inside in
their physical origin. The application of this method to systems as nitrous oxide, which is
not only a simple example for polyatomic molecules but which also plays a crucial role in
the greenhouse effect, promises interesting and beneficial findings.
This thesis presents, on the one hand, the technical extension of an existing experimental
setup for high-harmonic generation (HHG) and ultra-fast laser physics by an extreme
ultraviolet (XUV) spectrometer for the in-situ observation of the harmonic spectrum
during ongoing measurements. The present setup enables the production of short laser
pulse trains in the XUV spectral range with durations of a few hundred attoseconds (1 as
= 10−18 s) via HHG and supports to perform XUV-IR pump-probe experiments using
the infrared (IR) driving field with durations of a few femtoseconds. Moreover, a reaction
microscope is implemented, which enables the coincident detection of several charged
particles emerging from an ionization or dissociation process and to reconstruct their
full 3-D-momentum vectors. With this technique it is possible to perform time-resolved
momentum spectroscopy of few-particle quantum systems. Here, the design and the
calibration of the XUV spectrometer is presented as well as a first application to the
analysis of experimental data by providing information on the produced photon energies.
On the other hand, the results of an XUV-pump IR-probe measurement on nitrous oxide
(N2O) are discussed. With the broad harmonic spectrum (∼ 17 − 45 eV) it is possible
to address several states of the singly and doubly ionized cation. One reaction channel
is the single ionization into a stable state of N2O+. Here, the coincidently measured
photoelectron energies allow the observation of sidebands, which served to estimate the
pulse durations of the involved XUV pulse trains as well as of the fundamental IR pulses.
Additionally, single ionization of nitrous oxide can lead to a dissociation into a charged and
a neutral fragment. The four respective dissociation channels are compared by presenting
their branching ratios, kinetic energy release (KER) distributions and their dependencies
on the time delay between pump and probe pulse. In the production of the dication,
there are two competitive processes: direct double ionization considering photon energies
above the double-ionization threshold, and autoionization of singly ionized and excited
molecules in the case of photon energies near the double-ionization threshold. In both
cases, the ionization leads to a Coulomb explosion into two charged fragments, where the
N − N bond or the N − O bond may dissociate. The influence of the IR-probe field on
the ionization yield and the KER was investigated for both dissociation channels and
compared. In addition, the corresponding photoelectron energy spectra are presented,
which show indications for autoionizing states being involved, and their dependence on
the delay and the KER of the respective ions is analyzed.
