Development of Ultraviolet Photon Echo Experiments for the Study of Molecular Dynamics in Solution

This thesis work concerns the development and applications of ultrafast optical techniques and in particular photon echo in the UV range (< 300 nm). Development of these techniques in the UV range is important for studying molecular dynamics in solution, since standard UV dyes are small molecules and are therefore, appropriate for theoretical and experimental characterization. The UV techniques are also important for the investigation of protein dynamics, since most proteins contain UV – absorbing amino acids. The processes we studied include electronic dephasing, non-polar solvation dynamics and tryptophan dynamics in water. The UV three-pulse photon echo set-up which we developed is presented in the first step of this thesis. Pulse compression is performed in two stages by the prism-based compression on visible and UV pulses. The sample is excited at 287 nm with 20 nJ UV pulses with a temporal width of about 50 fs. Frequency Resolved Optical Gating (FROG) technique is implemented to characterize the UV pulses. In the study of non-polar solvation dynamics, solvent dependent ps decays are detected in transient grating (TG) and photon echo peak shift (PEPS) experiments, which were attributed to solvation dynamics and rotational diffusion. Fluorescence up-conversion (UC) measurements did not show similar dynamics, which highlights the high sensitivity of TG and PEPS techniques with respect to the non-polar solvation. Sub-100 fs homogeneous dephasing times are measured for p-terphenyl (pTP), diphenylacetylene (DPA) and tryptophan in solutions. Homogeneous dephasing times can not be detected with conventional ultrafast optical methods, such as fluorescence UC and pump-probe techniques. Our results emphasize the role of intramolecular processes as the primary dynamics causing electronic dephasing in the case of non-polar solvation. Excited state dynamics of tryptophan in water is investigated by a combination of TG, UV pump – broadband UV probe and fluorescence UC techniques. It is suggested that a Stokes shift with characteristic times of τ1 ≅ 0.16 ps and τ2 ≅ 1 ps results in stronger coupling of the excited state population to a higher state, resulting in an increase of the TG and pump-probe signals. These measurements highlight the higher sensitivity of TG technique to detect molecular dynamics in solution compared to the pump-probe technique. At the next part of the thesis work, the dependence of the PEPS traces (depletion at short times and long time offset variation) on experimental parameters and, in particular, the pulse chirp was investigated. A model is presented which describes these effects in terms of phasematching and triangular beam geometry. It is concluded that a precise control on the experimental parameters and, in particular, on the chirp of the pulses is necessary, in order to obtain reliable dynamics on the UV PEPS traces. Finally, the experimental aspects of extending two-dimensional photon echo (2D-PE) experiments into the UV range are discussed. The new 2D-PE set-up, which we developed using a compact design is presented. This set-up is capable of supporting the necessary phase stability for UV 2D-PE experiments.


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