UC Berkeley

Time-dependent motion (dynamics) of nuclei in molecules often occurs on ultrafast (picosecond (10-12 second) or femtosecond (10-15 second)) timescales. Experimentally, it is a question how to launch and detect these ultrafast nuclear dynamics in molecules in real time? In this dissertation, I studied two representative systems with the technique Extreme Ultraviolet (XUV)-Attosecond Transient Absorption Spectroscopy (ATAS). One is the coherent vibrational dynamics in CBr4 in the electronic ground state of the fully symmetric stretch mode, with a period of 125 fs. The results on CBr4 show that XUV-ATAS can resolve tiny bond-length displacements down to the order of 10-4 Å with 26 fs temporal resolution, with the knowledge of calculated core-excited potential energy surfaces. Such spatial and temporal resolution of XUV-ATAS is exceptional. It provides roughly 1,000 times better spatial resolution and 5 times better temporal resolution compared to the current MeV-UED at SLAC. The other is the photodissociation dynamics of Br2 in the electronically excited C 1Πu 1u state. The presented measurement has the shortest temporal resolution (26 ± 1 fs) among all other reported time-resolved studies, to the best of our knowledge. In this work, it is found that the measured rise time varies based on the probing transition. For the atomic Br 3d 2P3/2 → 4p 2D5/2 and 2D3/2 absorptions at 64.31 and 65.34 eV, the rise times are different, 38 ± 1 fs and 20 ± 5 fs, respectively. With simulations, these results show that measured probe signals record the dissociative wavepacket dynamics and how the different probing transitions affect the apparent progress toward dissociation. These probe-dependent effects should be considered when interpreting measured signals and their timescales. In addition, the presented broadband XUV absorption spectra cover both Br 3d core-to-valence and core-to-Rydberg transitions simultaneously, and our time-resolved measurements trace out the whole transformation from Br2 molecules to Br atoms. This work unifies the molecular and atomic static absorption spectra scattered in the literature, highlighting the distinct advantages of time-resolved absorption spectroscopy with broadband XUV light. In particular, this dissertation is a combined study of experiments, simulations, and analytical theories. The simulated time-resolved XUV absorption spectra are compared to measured ones, which help to assign and interpret the measured spectra. In addition, analytical theories are developed or adapted from literature to describe the measured physical phenomenon. Under proper approximations, closed-form relations between key physical quantities are obtained, and the essential physical picture is revealed. This dissertation demonstrates how ATAS can be used to successfully capture and resolve ultrafast molecular dynamics, and highlights how such experiment-theory combined study effectively pinpoints and deepens our understanding of the underlying physical picture.