The advent of X-ray free-electron lasers (XFELs) has enabled the generation of intense, ultrafast x-ray pulses, unlocking new possibilities for studying nonlinear light-matter interactions in the x-ray regime. The sub-femtosecond duration of XFEL pulses allows tracking ultrafast molecular dynamics with atomic resolution via pump-probe techniques, capturing events on their natural timescales. Among these advanced techniques, stimulated x-ray Raman spectroscopy (SXRS) stands out for its ability to achieve both subfemtosecond time resolution and sub-lifetime energy resolution, making it a key tool in nonlinear x-ray spectroscopy. In this talk, we present both theoretical and experimental investigations into the resonant propagation of XFEL pulses through dense media. We report the observation of intriguing nonlinear x-ray phenomena, including the exponential growth of SXRS and x-ray lasing signals. By combining covariance analysis of single-shot spectra with super-resolution techniques, we achieve an unprecedented energy resolution of approximately 0.1 eV. This high-resolution SXRS is expected to have significant applications, such as probing long-range charge transfer and electronic coherences through conical intersections.
a joint institute of 
and 
and