Ultrafast Coherent Control Spectroscopy

Abstract

Coherent control of quantum systems is currently a very active area of research in physics and chemistry. The goal of coherent control is to prepare molecules in specic quantum states that can lead to different chemical reactions, e.g. fragmentation and isomerization. One approach is the control of interference between multiple quantum pathways via their phases from the initial to the final state which consequently excites one molecular state or molecule over another with shaped pulses. The other approach is to generate different reaction products with pulses that match specic transient Franck Condon windows and transfer the wavepacket in a precise phase of vibrational motion to a new electronic state. Recently, there are increasing applications of coherent control towards cellular imaging. It is especially benecial for distinguishing broadband fluorophores with similar two-photon absorption crosssections, e.g. for free and enzyme-bound nicotinamide adenine dinucleotide (NADH). In this thesis, we discriminate between samples containing either free NADH or enzyme-bound NADH solutions with pulses that have a phase jump at a given frequency within the excitation bandwidth. This parameter scan is sensitive to as low as 3% of binding. The same idea can be generalized to other two-photon fluorescence systems, and a closed-loop feedback control approach should allow even wider application. We also develop two-dimensional (2D) Fourier transform spectroscopy in the deep UV (262 nm) to study DNA bases excited state relaxation dynamics. We compare 2D spectroscopy measurements in the deep UV for monomeric adenine and uracil in aqueous solutions. Both molecules show excited state absorption on short timescales and ground state bleach extending for over 1 ps. While the 2D spectrum for uracil shows changes in the center of gravity during the first few hundred femtoseconds, the center of gravity of the 2D spectrum for adenine does not show similar changes. We discuss our results in light of ab initio electronic structure calculations.