UC Berkeley

In this thesis, I present attosecond transient absorption studies on two topics related to ultrafast molecular dynamics.

The first topic is coherent electronic dynamics in atoms and molecules triggered by ultrafast laser excitations. Electronic dynamics constitute the fastest steps of photochemical reactions, and recent advances in the attosecond spectroscopy has enabled their real-time observation. In an experimental study on xenon dications (Xe$^{2+}$), a coherent superposition of electronic states prepared by strong-field double ionization is identified. By comparing the results to an uncorrelated electron-emission model, implications of electron correlation in the preparation mechanism of the electronic coherence are suggested. As a preparatory step to proceed to molecular systems, a theoretical framework is developed to interpret molecular core-level absorption signals by performing electronic-structure calculations on a prototype diatomic molecule, hydrogen bromide (HBr) and its ion (HBr$^+$). The results reveal that the bond characters as well as ligand-field splittings are critical to interpret molecular transient absorption signals. A first molecular experiment is performed on deuterium bromide (DBr), and the results demonstrate a capability of attosecond transient absorption spectroscopy to probe coherent electronic and vibrational dynamics simultaneously. Effects of rotational motion in causing a slow decrease in the electronic coherence are further discussed. A second molecular experiment is performed on bromine molecules (Br$_2$), where a dramatic time dependence caused by vibrational anharmonicity is found in the electronic quantum beats. An extensive quantum-mechanical analysis shows that vibronic-structure information is imprinted as discrete frequency signals on the electronic quantum beats.

The second topic is photodissociation dynamics in molecules, especially those involving potential crossings. Potential degeneracies induce nonadiabatic interactions between neighboring electronic states, and those singular regions are known as avoided crossings and conical intersections. The implications of potential crossings in photochemical reactions are now recognized as ubiquitous phenomena; however, their experimental probing remains elusive. The challenges lie in the fact that one needs experimental methods that have femtosecond time resolutions as well as capabilities to resolve closely-lying electronic states that are energetically indistinguishable. In this thesis, attosecond transient absorption spectroscopy is employed to tackle these challenges. In a theoretical study on sodium iodide (NaI), charge-state switching on UV-excited potentials is predicted to be captured as rapid appearance and disappearance of the ionic absorption signals, enabling direct observation of an avoided crossing. In an experimental study on iodine monobromide (IBr), it is demonstrated that curve-crossing dynamics can be directly mapped in the attosecond transient absorption spectra. Diabatic and adiabatic pathways at an avoided crossing are unambiguously identified by comparing the experimental results to simulated spectra. A further analysis on orbital information shows that the core-level absorption is able to encode orbital-resolved information, thus successfully resolving state-character switching at the avoided crossing.