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X-ray free electron lasers (XFELs) have revolutionized the study of matter, yet traditional optical pump - X-ray probe spectroscopy remains limited by arrival time jitter and probe pulse duration, often restricting temporal resolution to tens of femtoseconds. While current methods rely mainly on spectral changes and momentum transfer the ongoing development of time tools, for instance THz streaking setups [1], may soon enable new methodologies. This work elaborates on the recently conceptualized novel methodology, X-ray chronoscopy, that has potential to bypass the known constraints of ultrafast X-ray methods through precise analysis of the temporal profiles of incident and transmitted X-ray pulses [2,3].
In this work we developed numerical simulations employing a four-level atomic model and analyzed how optical pulses affect the X-ray chronoscopic signal, here defined as $-\ln\left( I_{1}\left( t \right) / I_{0}\left( t \right) \right)$ with time envelopes of the incident probe pulse $I_{0}\left( t \right)$ and the transmitted one $I_{1}\left( t \right)$ [4]. We studied the ZnSe system pumped with 5 fs-short 3.72 μJ pulses of 3.1 eV photons and probed with 0.1 μJ pulses of 9661.5 eV photons (equal to Zn$^{2+}$ K-edge binding energy). Different pump-probe delays were tested, probe pulse duration was tested in the range 5 - 100 fs and both pump and probe pulses were assumed monochromatic. The pump pulse time distribution was described with a Gaussian function and the probe one with a SASE-like function. We showed that the X-ray chronoscopic signal may be used to precisely determine relaxation rates in the material despite lifetimes being shorter than the probe pulse duration and even in the presence of significant pump–probe timing jitter.
Unlike conventional ultrafast methods requiring hundreds of pump-probe cycles, X-ray chronoscopy can capture the trace of electron dynamics within a single sequence of overlapping pump and probe pulses, provided the X-ray pulse duration exceeds the timing jitter and the dynamics in question. Our results indicate that by utilizing precise shot-to-shot temporal diagnostics, this approach can resolve electron processes in the single-femtosecond regime, offering a powerful complement to existing spectroscopy for investigating ultrafast phenomena like surface plasmon resonance and fundamental electron-phonon interactions.
[1] M. Wieland, P. Juranić et al., Optics Express 29, 32739 (2021).
[2] D. J. Bradley, Opt. Commun. 15, 231 (1975).
[3] W. Błachucki, C. Arrell, P. Juranić et al., Applied Sciences (Switzerland) 12, 1721 (2022).
[4] R. Fanselow, W. Błachucki, J. Szlachetko, X-Ray Spectrometry 55, 512 (2026).
| Scientific Topics | AMO Physics |
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