Femtosekunden-Spektroskopie biologischer Systeme mittels kohärenter Kontrolle

This doctoral thesis presents new approaches for the
characterisation of ultrafast energy flow in complex systems, based
on concepts of coherent control. By initiating a photoreaction with
femtosecond pulses whose temporal phase and amplitude are shaped in
such a manner that specific molecular vibrations and states are
addressed, the energy flow can be steered at will. The comparison
between the ensuing energy flow patterns following shaped and
unshaped excitation pulses constitutes a differential measurement
of the function of the controlled vibrations and states within the
photoreaction. Coherent control as a spectroscopic tool is first
applied to biological systems, specifically the light harvesting
complex LH2 from the photosynthetic purple bacterium
Rhodopseudomonas acidophila, and the isolated carotenoid donor of
the same complex. The pump-probe method using shaped excitation
pulses is shown to be successful for the first time in controlling
the natural function of a biological system, namely the flow of
excitation energy in the complex network of states in LH2. By means
of a closed-loop optimisation of parametrised excitations, a
bending mode in the carotenoid donor can be identified as being
responsible for steering the energy flow. This bu vibrational mode
couples the carotenoid S2-S1 states; its frequency is determined to
be 160±25cm-1. Furthermore the deactivation of the carotenoid S2
state in LH2 and in solution is studied with pump-probe and
pump-deplete-probe spectroscopy. Here it is shown that there exists
an alternative singlet state S*T (1Bu-) involved in the
deactivation process, though only in LH2. Its function as a
precursor of ultrafast triplet population and as a donor for
photosynthetic energy transfer is characterised with a novel
evolutionary target analysis of conventional pump-probe spectra.
Secondly, coherent control as a measurement technique is applied to
another extremely complex system, in this case a material dominated
by non-linear interactions with instantaneous dynamics: Propagation
of femtosecond pulses in optical fibres that are only a few
micrometers in diameter to generate a supercontinuum of optical
frequencies. Here shaped pump pulses succeed in resolving for the
first time the sequential steps leading to the enormous spectral
broadening. Open-loop variations of precompression allows the
evolution and fission of optical solitons to be followed, while
closed-loop optimisations render observable the coupling of
solitons with phase-matched visible frequencies. On atoms, finally,
open-loop control of interfering pathways from the ground to the
excited state by application of strongly modulated spectra seeks to
establish a direct link between coherent control experiments and
theory. The novel phenomenon of a Fresnel zone plate in the time
domain is first developed in theory and then successfully realised
in experiment.