PHYSICS DISSERTATION DEFENSE: Jordan O'Neal
Ph.D. Candidate: Jordan O'Neal
Research Advisor: Philip Bucksbaum
Date: September 16, 2022
Time: 12:00 pm
Location: PAB 102/103
Zoom Link: https://stanford.zoom.us/j/98203820889
Zoom Password: email nickswan [at] stanford.edu (nickswan[at]stanford[dot]edu) for password.
Title: Tracking Charge Migration with Attosecond X-ray Pulses from a Free-Electron Laser
Abstract: Photochemistry begins with electrons reacting to an incident field on an attosecond timescale. At longer time scales, the electron dynamics couples to the nuclear dynamics, driving chemical changes. I report an observation of the purely electronic attosecond response, dubbed charge migration, in the time domain, and find the coherence lifetime is at least a few femtoseconds. We use attosecond X-ray pump/attosecond X-ray probe pulses developed recently by our group at the Linac Coherent Light Source~(LCLS) X-ray free-electron laser in an impulsive ionization scheme. The short pulse enables high temporal resolution, while the X-ray wavelength allows measurements of the charge density around specific atomic sites. This measurement agrees with simulations including the effect of zero-point geometry spread, showing how the nuclear wavepacket affects charge migration.
From a separate experiment, I report an observation of impulsive stimulated X-ray Raman scattering~(SXRS) using a single attosecond X-ray pulse. Impulsive SXRS is a building block for nonlinear X-ray spectroscopic techniques that can pump and probe charge migration of valence electrons in a controllable manner. I will also discuss planned future experiments to implement impulsive SXRS in a Raman interferometry scheme to measure and manipulate charge migration more systematically.