Coherent charge dynamics in molecules on the attosecond to few-femtosecond timescale

Abstract

Electronic coherences in molecules are typically expected to survive for a few femtoseconds. This limits the timescale of charge migration, which is the electronic motion following valence excitation that is the result of electronic wavepackets, i.e. coherent superpositions of excited states. It is of fundamental and practical interest to molecular physics and quantum chemistry to understand how coherent charge migration results in long term outcomes of chemical reactions or in processes such as radiation damage. It may even be possible to bias reaction outcomes to desired products if the states with a wavepacket and their coherences can be controlled, which is the aim of quantum control. In this doctoral thesis, two experiments are also discussed in detail which aim to improve understanding of decohering interactions on this timescale. The first is a two colour all X-ray pump-probe experiment using an X-ray free electron laser to investigate charge migration of inner valence holes following ionisation of isopropanol vapour. Two hole wavepackets are studied, each corresponding to cluster of states which can be understood as satellites of a single one-hole state broken down due to coupling to two-hole one-particle states (breakdown in the molecular orbital picture of ionisation). Hole lifetimes for these states of 5.1 ± 3.2 fs and 1+2−1 fs were measured, and accompanying theoretical investigations are referred to which elucidate the role of the initial spread in nuclear geometries and nuclear motion on decoherence. The second experiment is in liquid isopropanol, this time looking at the role of the liquid environment of electron trajectories following ionisation via high harmonic generation. This experiment has tangential interest to strong field physics, as the processes involved in high harmonic generation in liquid are investigated. Harmonics up to 50 eV are generated, the highest reported to-date in the condensed phases, and it is found that scattering of electrons from neighbouring molecules dampens but does not exclude recombination. Accompanying these experiments is the advancement of experimental tools for those and future investigations, especially for the study of liquid systems. For this purpose, flat liquid targets of order 1 μm thickness as well as the tools for transient absorption spectroscopy using a soft X-ray high harmonic generation source are developed.