Probing molecular coherences by correlation Raman signals with stochastic FEL pulses *

Hard-x-ray free-electron lasers (FELs) relying on the self-amplified spontaneous emission (SASE) mechanism generate stochastic x-ray pulses lacking phase control. This has represented a major bottleneck, since most time-resolved multidimensional nonlinear x-ray spectroscopy schemes are based on sequences of coherent, phase-controlled pulses. We show that suitable correlation signals averaged over independent realizations of stochastic FEL pulses can retrieve the same joint temporal and spectral resolutions of signals with phase-controlled pulses [1]. This is demonstrated for TRUECARS, an off-resonant x-ray Raman technique that monitors conical-intersection dynamics by directly accessing molecular coherences in a background-free manner. We apply our protocol to the RNA base uracil, showing that correlation signals at FELs can offer the same physical insight as TRUECARS with phase-controlled pulses.

[1] S. M. Cavaletto, D. Keefer, and S. Mukamel, Phys. Rev. X 11, 011029 (2021).