Controlled and Stochastic Attosecond Spectroscopies at XFELs*

Probing dynamics in valence-excited electronic states is an enduring challenge where ultrafast x-ray spectroscopies provide a unique means to disentangle electronic and nuclear motions and thus capture decisive moments in chemical reactions.  Within the context of experimental x-ray methods, two general strategies exist: the use of controlled pairs of attosecond pulses, or, the use of noisy SASE pulses comprised of an envelope of attosecond spikes.  The former has been realized [1] and the latter has been the subject of theoretically proposed schemes [2].  In this talk I will present experimental realizations of both strategies:  all-x-ray attosecond transient absorption spectroscopy of liquid water [1] and stochastic stimulated x-ray Raman spectroscopy (s-SXRS) [3].  In s-SXRS, propagation-based amplification of spontaneous x-ray Raman scattering enables us to achieve near Fourier-limited joint energy-time resolution.  This is achieved with a super-resolution variant of standard covariance analysis that allows us to beat instrumental energy resolution and that can be improved statistically, analogous to super-resolution microscopy.

References

[1] S. Li,  et al., Attosecond-pump/attosecond probe x-ray spectroscopy of liquid water, Science 383, 1118-1122 (2024).

[2] S. M. Cavaletto, D. Keefer and S. Mukamel, High Temporal and Spectral Resolution of Stimulated X-Ray Raman Signals with Stochastic Free-Electron-Laser Pulses, Phys. Rev. X 11, 011029 (2021).

[3] K. Li, et al., Super-resolution Stimulated x-ray Raman Spectroscopy - submitted.