Tracking Charge Migration with Attosecond X-ray Pulses from a Free-electron Laser PDF Download

Author: Jordan Tyler O'Neal
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

Get Book Here

Book Description
The motion of electrons in molecular superposition states happens on the attosecond timescale, roughly three orders of magnitude faster than nuclear motion. To study the first steps in a photochemical reaction, then, we must study the electron dynamics in early times before the nuclei have a chance to move. These purely electronic dynamics, uncoupled from nuclear motion, are dubbed charge migration (CM). CM can be described as a coherent superposition of delocalized electronic states evolving in time, which appears as a time-dependent oscillation in charge density across the molecule. On longer timescales, thought to be tens of femtoseconds, nuclear motion couples to the pure electronic CM. This tends to average out the CM, and there is evidence that this process can influence the nuclear reaction pathways. One can further envision using sequences of laser pulses to prepare CM states as a method of coherent control directing photochemical reaction pathways. However, understanding the loss of electronic coherence as nuclear motion occurs is difficult. Theoretical modeling of CM is computationally challenging, and there are open questions about the effects of nuclear motion. Experimental measurements can guide us towards better, more efficient approximations to help answer these outstanding questions. To this end, I present two experiments towards developing two separate methods for measuring CM. Both methods rely on the recently developed attosecond X-ray free-electron laser source at the LCLS free-electron laser facility. These two methods for measuring CM serve as early implementations of this new attosecond X-ray source. The first method to study CM, impulsive ionization, prepares a CM state by ionizing with an attosecond X-ray pulse which has coherent bandwidth large enough to populate multiple cationic states. The final state is then measured with X-ray absorption spectroscopy. I present preliminary results showing signatures of CM and nuclear motion damping the time-dependent charge density. The second method, nonlinear X-ray spectroscopy, is a sophisticated toolset using sequences of X-ray pulses to prepare and measure CM states. I present a measurement of a basic building block of nonlinear X-ray spectroscopy, impulsive stimulated X-ray Raman scattering, and use this result to show the feasibility in the near future of measuring valence CM states in a neutral molecule.