Description
We present two recent experimental advances that constitute independent but essential steps towards realizing multidimensional spectroscopy in the X‑ray domain.
In the first experiment, we demonstrate coherent all‑X‑ray four‑wave mixing (XFWM) in neon near the K‑edge (870 eV) using single broadband SASE pulses from the Swiss Free Electron Laser. A square spatial mask splits the incoming beam into three cut‑out beams arranged in a ‘folded BOX’ geometry, enabling background‑free signal detection. The beams intersect in a neon gas cell at a pressure of several hundred millibars, generating multi‑color resonant nonlinear signals involving Raman transitions, including X‑ray coherent anti‑Stokes processes, which are characterized with an inline photon spectrometer. A rate‑equation model supports the interpretation of the measured XFWM signals.
In the second experiment, performed at the SQS instrument of EuXFEL, we use attosecond X‑ray interferometry to generate and probe site‑specific electronic wavepackets in acetic acid. By selectively exciting the two dominant pre‑edge resonances below the oxygen K‑edge with attosecond pump–probe pulses in a collinear transient‑absorption geometry, we demonstrate site‑selective bond excitation. Interference of the pulses on a 1D detector located after a grating‑based refocusing optics, enables retrieval of the real pump–probe delay with tens‑of‑attoseconds precision via Fourier analysis, providing spectrally and temporally resolved insights into the induced dynamics. The observations are compared with a few‑level time‑dependent Schrödinger equation model.
As an outlook, we discuss how these two methodologies can be combined to establish a pathway towards 2D X‑ray spectroscopy, and outline the next experimental developments required to achieve this goal.
