The Attosecond Electron Dynamics of Charge Migration and Charge Transfer: Opening the Door to Attochemistry

Abstract

The field of attochemistry is concerned with the few- and subfemtosecond electronic dynamics of molecules and their interactions with the motion of the nuclei. Resolving these dynamics experimentally has only become possible in the last 5-10 years, with the studies presented in this thesis being some of the first examples. The three reported experiments employ attosecond transient absorption spectroscopy on the silicon L_{2,3} and iodine N_{4,5} edges to study the electronic and vibrational dynamics of the strong-field ionised and excited molecules. Their results provide unique insights on various ultrafast molecular phenomena with unprecedented single-femtosecond temporal resolution. These experiments are supported by comprehensive quantum-mechanical theoretical treatments, exploring so-far-unseen aspects of the interactions of the nuclear and electronic degrees of freedom beyond the Born-Oppenheimer approximation. In the first study, the evolution of a vibrational wavepacket in the silane cation (SiH4+), created upon strong-field ionisation, is fully resolved. Its evolution is tracked from creation up to and beyond dissociation, thereby observing the competition between the adiabatic and nonadiabatic interactions that mediate a Jahn-Teller distortion; the structural deformation of an electronically degenerate molecular system. The captured bifurcating wavepacket is found to follow two different dissociation mechanisms. One takes the form of a ballistic pathway that involves the excitation of an umbrella-bending motion of the molecule, which breaks one of the Si-H bonds in 23 fs and produces a vibrational wavepacket in the SiH3+ fragment that anharmonically collapses within 200 fs. The second pathway is initially dominated by the scissoring mode which traps the system, from which it decays statistically with a 140 fs time-constant. The second experimental investigation sheds light on the multifaceted, ultrafast dynamics associated with the phenomenon of charge migration; the oscillatory redistribution of electron density within a molecular system. A wavepacket consisting of multiple vibrational and electronic states of the neutral silane molecule (SiH4) is created through strong-field excitation. The superposition of electronic states gives rise to a quantum beating with a period of merely 1.3 fs, the fastest coherent electronic motion that could be captured to date. Moreover, the attosecond temporal resolution of the experiment allows the vibrational-electronic wavepacket to be observed undergoing a revival after 45 fs. Quantitative agreement with fully-quantum simulations of the experimental observables allow these observations to unearth a general relationship between the periodicity of the vibrational dynamics and the wavepacket revival. In a third experiment on the cation of trifluoroiodomethane (CF3I), the electron dynamics of a charge transfer reaction are uncovered. Three archetypal elements could be disentangled from the transient spectra of the system; nuclear rearrangement (taking 9 fs), a change in electronic character (with a timescale of 2.4 fs), and a completely unexpected temporal delay of 1.4 fs are observed to mediate the transfer of charge from the fluorine-centred donor B state to the iodine-centred acceptor E state. Furthermore, thanks to the multipicosecond delay range of the experiment, the picosecond-spanning vibrational dynamics of the X state could be observed, demonstrating an interesting interplay between spin-orbit coupling and the vibrational motion. These detailed insights were facilitated through developments of both experimental and data-analysis methods. Progress in the latter was especially important in this work. This thesis therefore also presents a novel method for the filtering of correlated noise based on singular value decomposition.