Resolving Non-Adiabatic Dynamics in Small Molecules with Few-Femtosecond Sensitivity

The molecules that make up the world around us display large responses to the ultraviolet (UV), vacuum ultraviolet (VUV), and extreme ultraviolet (XUV) region of the electromagnetic spectrum, and the absorption of this light can serve as a catalyst for chemistry./nIn these ultrafast photochemical reactions, the motion of the nuclei following excitation often enables electronic relaxation that facilitates further rearrangements such as dissociation or isomerization./nIn some small molecular systems, including ethylene and methanol, such effects become important on the sub-10 femtosecond time scale./nAlthough several time-resolved spectroscopic tools, including transient absorption, Raman spectroscopy, and photoion or photoelectron imaging, have enabled the study of many photochemical reactions, resolving the evolution of an initially excited vibrational wavepacket on these short time scales has proved difficult./nThis thesis discusses an approach to studying sub-angstrom nuclear motion and non-adiabatic dynamics that follow photoexcitation with few-femtosecond sensitivity by quantifying dynamical spectral shifts in time-resolved photoelectron spectra (TRPES) obtained with ultrafast pump and probe light pulses in the VUV/XUV./nThe laser system, beamline, and experimental end station are described in detail./nUsing a state-of-the-art 1 TW peak power, 30 W average power infrared laser, we implement a bright source of VUV/XUV light, based on high harmonic generation and needed for pump-probe spectroscopy./nThis source can deliver high energy ultrafast pulses in the VUV region allowing for efficient excitation of gas phase molecular targets at a 1 kHz repetition rate./nThe evolution of this prepared excited population can be followed through subsequent perturbative photoionization with another high energy photon./nSeparation of the light spectrally and temporally into pump and probe pulses with filters and a split-mirror interferometer is discussed./nTime-resolved photoelectron and photoion imaging experiments are conducted using a velocity map imaging spectrometer optimized for VUV/XUV pump-probe measurements./nUltrafast dynamics following excitation with the 5-th harmonic at 156 nm in ethylene (C$_2$H$_4$) to the $\pi\pi^*$ state and in methanol (CH$_3$OH) to the 2 ${}^1$A'' state are studied. In ethylene, photoexcitation has been predicted to lead to torsion of the molecule from its initial planar geometry to a fully twisted configuration in $\sim$10 fs./nBy examining small changes in the onset time of signal at different photoelectron kinetic energies in the TRPES, this torsional motion is resolved experimentally with few-femtosecond sensitivity./nThe transient population of the low-lying $\pi$3s Rydberg state, predicted to be enabled by this torsional motion, is also observed./nIn methanol, the ultrafast relaxation and dissociation dynamics following excitation are studied through combination of the experimental TRPES with theoretical calculations of the energies of the first few excited states along two identified reaction pathways./nWe resolve sub-angstrom C--O bond stretching leading to a conical intersection within 15 fs of excitation.

Advisor: Falcone, Roger