Photochemical Reaction Dynamics Studied by Femtosecond Soft X-ray Transient Absorption Spectroscopy
The ability to discern and understand the time evolution of both molecular and electronic structure during a chemical reaction in real-time, and with atomic-scale sensitivity, is a primary goal of modern chemical dynamics research. In this dissertation, femtosecond extreme ultraviolet/soft x-ray transient absorption spectroscopy is developed on a table-top apparatus and applied to investigate the evolution of both vibrational and electronic degrees of freedom in molecules following photoexcitation or ionization. Several molecular systems are considered in detail. In the first study, the strong-field ionization of a heteronuclear diatomic molecule, IBr, and the ensuing vibrational wavepacket dynamics in the electronic ground state of the neutral molecule are characterized with atomic-site sensitivity. The element-specificity of the core-level transient absorption technique is further extended to investigate the multichannel ultraviolet photodissociation reaction of bromoiodomethane, CH2IBr. The core-to-valence absorption spectrum of the bromomethyl radical, CH2Br, which is formed via the major C-I photodissociation coordinate at 266 nm, is characterized for the first time. The measured C-I dissociation time (~50 fs) differs significantly from the C-Br dissociation time (~110 fs), which indicates that more than one excited-state surface is involved. In the third study, the evolution of the transient valence electronic structures in the transition state regions of both the methyl and allyl iodide photodissociation reactions are directly observed and characterized. As the C-I bond is breaking, new 4d(I) core-to-valence resonances localized on the I atom are observed and assigned to repulsive valence-excited transition-state regions. The transition-state resonances directly probe the evolving valence electronic structure during these fundamental bond-breaking reactions. Finally, a full-scale apparatus upgrade was undertaken to extend the femtosecond light source into the soft x-ray energy domain up to and beyond 300 eV. With this new setup, the first femtosecond soft x-ray transient absorption experiments at the carbon K-edge (~284 eV) are performed. In particular, the ultrafast electrocyclic ring-opening reaction of 1,3-cyclohexadiene is investigated and the evolution of the valence electronic structure during the ~180 fs ring-opening process is directly captured. The core-to-valence spectroscopic signatures of the elusive transient intermediate excited states, which lead to ring-opening, are characterized, in combination with time-dependent density functional theory calculations, to reveal overlap and mixing of the frontier valence orbital energy levels.