Using Attosecond X-ray Free Electron Lasers to Probe Ultrafast Electron Motion

The ultrafast motion of electrons is a frontier problem for photochemical processes, as electron motion is a key ingredient of all chemical reactions. Electronic rearrangement is also the means by which light energy is harnessed in photochemistry. The timescale for coherent electron dynamics is set by the energetic splitting of the electronic states, which in small molecular systems, is on the scale of an electron volt (eV). This sets the natural timescale for electronic motion to be few-to-sub femtosecond (fs).To approach these extreme timescales, we can use short pulses of light to excite small quantum systems. For instance, the impulsive interactions between a light field and a quantum system can induce time-dependent oscillations in the charge density. Such electronic wavepacket motion (in the absence of nuclear motion) has come to be referred to as charge migration [1]. While the initial charge dynamics following impulsive excitation (or ionization) begins as purely electronic motion, this wavepacket will couple to other degrees of freedom in the system (i.e. nuclear motion or chemical dynamics) and lead to localization of the charge.  The transfer of electronic charge across molecular bonds is fundamental to an understanding of charge transfer phenomena. The study of these fundamental phenomena requires state-of-the-art light sources, such as the Linac Coherent Light Source (LCLS), an X-ray free electron laser (XFEL) facility which produces high-brightness, ultrashort pulses, with wavelength continuously tunable across the x-ray regime. Schemes to provide isolated, sub-femtosecond pulses from an FEL are being explored at facilities world-wide, and recently we have demonstrated such pulses at the LCLS [2]; opening the door for time-resolved measurements of ultrafast electron dynamics on their natural timescale. In my talk I will highlight our recent developments in probing electronic motion in small molecular systems. We have employed sub-femtosecond pulses from the XFEL to study ultrafast charge dynamics in both core-excited [3,4,5] and low-lying cationic systems [6]. I will also provide an overview of the early science efforts using the high repetition rates available with the new LCLS-II superconducting accelerator to study problems in both chemical and material science. 

Speaker: James Cryan, Stanford University