Mehrphotonen-Ionisationsprozesse mit intensiven Laserpulsen

When free atoms interact with intense laser fields, there are three
main processes of importance: high harmonic generation (HHG),
above-threshold ionization (ATI) and non-sequential double
ionization (NSDI). All three of these multi-photon processes are
physically strongly related to each other, they all share a common
behaviour of the electron dynamics: First, an electron is ionized,
then it is accelerated in the external field, and when returning to
the parent ion, it can exert its kinetic energy to initialize the
processes mentioned above. The subject of this thesis is the
investigation of ATI and NSDI of atoms and simple molecules. Aside
from two different species of matter, there are also two different
kinds of laser pulses available for comparison. The femtosecond
lasers used here differ in their specifications: one of them
delivers ”ordinary“ femtosecond pulses, consisting of many optical
cycles of the underlying electric field (many-cycle pulses), while
the other one delivers extremely short pulses, only consisting of
very few optical cycles (few-cycle pulses). The latter case is the
reason that a new physical quantity, the absolute phase or
CEO-phase (Carrier-Envelope Offset), comes into play. This quantity
is defined as the relative phase of the evelope of the pulse and
the underlying field. The experiments of this thesis compare the
behaviour of multi-photon effects under these different conditions.
It can be shown, that not only qualitatively new effects appear
depending on the absolute phase, but it is also possible to gain
insight into inner-atomic processes with attosecond precision. Most
of the observed processes can be explained by means of an
intuitive, semi-classical model. Nevertheless, pure quantum-effects
were also observed. One example is interference effects of ATI
spectra with few-cycle pulses in analogy to the double-slit
experiment of matter waves. As another example, the level-structure
of the investigated atoms dominates the features of the energy
distribution of the ATI-electrons. As a useful application of the
discussed phase measurement, it was investigated how to apply the
special features of ATI to implement a new stabilization scheme for
the absolute phase of a few-cycle pulse laser system. In the frame
of this work, the lasers used have been significantly improved.
Those activities are also described in the thesis.