Ryan Coffee, LCLS
The advent of ultrashort "femtosecond" laser pulses brought an optical revolution to the field of physical chemistry. Now that two-pulse [Ding2012] and two-color [Lutman2013,Marinelli2013] operation has become available at the Linac Coherent Light Source (LCLS), we are free to dream of how x-ray Raman and other nonlinear techniques will address electronic mode coupling and energy exchange. In this presentation, I will give a brief introduction to some of the hopes for nonlinear spectroscopy using multiple pulse and multiple color x-ray fields and then give a brief review of the current methods for producing such fields. We will then focus on our recent one-color two-pulse results from the LCLS which show a dynamically shifting soft x-ray absorption resonance in the oxygen molecule. This resonance follows a strong initial 1s->pi* core excitation to an intermediate state. The subsequent resonance shifts by up to 5 eV on the 10 fs time scale and shows a distinct symmetry change relative to the initial excitation. We compare the energy shift and molecular symmetry to theoretical calculations to build a picture of the ultrafast evolution of the core excited O2 molecule.
We will quickly see how this interpretation reveals a need for a two color field. We therefore will discuss a subsequent experiment that employed two methods for two-color operation [Lutman2013,Marinelli2013]. Although very preliminary, the results from the two-color campaign seem to confirm our interpretation and will surely help to build a deeper understanding of the interaction of these very intense x-ray pulses with simple molecules, in particular when multiple resonances may be involved.
