The core question under investigation in SFA is the nature of the short wavelength strong field interaction with bound and free electrons.
We study core-hole wavepacket dynamics (1-10 fs) and driven electron dynamics (20-50 fs) in molecules utilizing intense laser and x-ray fields and momentum and energy resolved detection. The key questions involve the interplay between correlated electron motion and relaxation, nuclear motion, and photabsorption when these systems are subjected to strong infrared, optical or x-ray fields. These are critical issues for strong-field imaging strong-field molecular dynamics, and light source development.
New knowledge in this area will aid some of the novel science protocols at LCLS and other 4th generation sources. Science programs at LCLS that are influenced by strong field electronic effects include the ultrafast pump probe studies of x-ray induced processes in the focused LCLS beam. It includes the coulomb explosion physics that occurs in non-periodic imaging experiments; saturation effects in x-ray absorption studies; and the x-ray-generated plasma and concomitant reflectivity changes currently used to detect the time jitter at LCLS. Strong field effects are even more evident in electron beam induced processes such as THz creation and laser-induced electron beam commissioning.
Research capabilities: We concentrate on elemental aspects common to all of these strong field regimes. Our recent work pioneered studies of multiple core hole formation and all forms of multiple ionization. Previous to this we have performed THz studies, as well as studies of coherent acoustic phonons in x-ray beams. We have made use of our extensive knowledge of nonlinear quantum processes to produce and perfect rotational wave packet targets for ultrafast x-rays. We have devised new ways to utilize both impulsive and adiabatic interactions to achieve a high degree of control over the quantum evolution of systems in strong fields.
Strong field x-ray physics:
The LCLS x-ray free electron laser at SLAC can deliver 2 x1011 photons in a 5 fs pulse, with a focus capable of producing double core vacancies through rapid sequential ionization. This enables double core vacancy Auger electron spectroscopy, an entirely new way to study femtosecond chemical dynamics with Auger electrons that probe the local valence structure of molecules near a specific atomic core.
Using 1.1 keV photons for sequential x-ray ionization of impulsively aligned molecular nitrogen, we observed a rich single-site double core vacancy Auger electron spectrum near 413 eV, in good agreement with ab initio calculations, and we measured the corresponding Auger electron angle dependence in the molecular frame (Cryan et al, 2010).