Probing Ultrafast Electron Motion with Attosecond X-ray Free Electron Lasers

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]. We are also developing nonlinear x-ray spectroscopies [7,8] to initiate and control electron dynamics. The control of coherent electron motion represents a significant step towards achieving charge-directed reactivity [9], a grand challenge for the field of attosecond science.

[1] Cederbaum and Zobeley 1999 Chemical Physics Letters 307 205–210
[2] Duris and Li et al. 2020 Nature Photonics 14 30-36
[3] Li and Driver et al. 2022 Science 375 285-290
[4] Driver et al. 2024 Nature 632 762-767 (2024)
[5] Wang and Driver et al. Phys. Rev. X 15 011008 (2005)
[6] Driver et al. ArXiv:2411.01700
[7] O’Neal et al. 2020 Phys. Rev. Lett. 125 073203
[8] Biggs et al. 2023 Proc. Nat. Acad. Sci. 110 15597-15601
[9] F. Remacle, R. D. Levine, and M. A. Ratner 1998 Chem. Phys. Lett. 285, 25