Systematic Study on Coherent Control of Electronic Population and Vibrational Coherence in the One-Photon Regime

Abstract

In the scope of this thesis, a systematic study about coherent control with shaped femtosecond pulses in the one-photon regime was performed. This regime is especially important in nature as photochemical and -physical reactions are driven by sun light, i.e. at very low intensities. Apart from the relevance of these conditions in nature, coherent control experiments at low intensity are well suited to gain understanding of the underlying processes as well as to control them by using shaped laser light fields. For the experimental shaping in the visible spectral range, a liquid-crystal spatial light modulator (LCM) was utilized. Here, it was shown for the first time that such an LCM introduces noise on the tailored spectral phase. The detailed characterization of the noise implied that molecular properties like the mobility of the liquid crystals cause the noise. Reliable pulse shapes on a pulse-to-pulse basis were achieved by externally cooling the LCM. Due to appropriate data averaging, coherent control experiments were successfully performed, where even small differences in the shaped pulses are important. These control experiments aimed at the enhancement of electronic population and vibrational coherence in the ground and excited state in dependence on the temporal shape of the excitation pulses for various excitation spectra. The temporal shape of the excitation pulse was tailored to linearly chirp pulses, multipulses, whose interpulse distance matches the period of the dominant molecular mode, and the sum of both as chirped multipulses. While the ideal choice to enhance the population and the vibrational coherence in the ground state is a resonant negatively chirped multipulse, the excited state is enhanced best with a blue-detuned positively chirped multipulse. These transient absorption experiments were performed on a prototype chromophore. However, the results should be applicable to other systems. These kinds of control experiments should be transferred to DNA bases and prototype molecules, which can be easily addressed theoretically. As many organic molecules absorb light in the ultraviolet wavelength regime, an experimental set-up for the shaping of femtosecond pulses in the spectral range between 250 nm and 350 nm was developed and characterized in detail. This setup provides the basis for future experiments with organic samples.