Hetero-site-specific X-ray pump-probe spectroscopy for femtosecond intramolecular dynamics

Abstract

New capabilities at X-ray free-electron laser facilities allow the generation of two-colour femtosecond X-ray pulses, opening the possibility of performing ultrafast studies of X-ray-induced phenomena. Particularly, the experimental realization of hetero-site-specific X-ray-pump/X-ray-probe spectroscopy is of special interest, in which an X-ray pump pulse is absorbed at one site within a molecule and an X-ray probe pulse follows the X-ray-induced dynamics at another site within the same molecule. Here we show experimental evidence of a hetero-site pump-probe signal. By using two-colour 10-fs X-ray pulses, we are able to observe the femtosecond time dependence for the formation of F ions during the fragmentation of XeF2 molecules following X-ray absorption at the Xe site.

Figure 1. Hetero-site X-ray pump-probe scheme.

Sketch of a two-colour X-ray-pump/X-ray-probe experiment. The X-ray absorbed by B induces inner-shell electronic excitation, ionization, fluorescence, Auger-electron emission and charge redistribution to site A. Nuclear dynamics is also induced—dissociation, isomerization and vibrational excitation. The site-to-site intramolecular dynamics is explored by absorbing a second X-ray at site A after a controlled delay. With this approach, it is possible to characterize the response at one site of the molecule when an X-ray photon has been absorbed in another site of the same molecule.

Figure 2. Hetero-site X-ray pump-probe scheme for xenon difluoride.

(a) Sketch of the two-colour X-ray-pump/X-ray-probe scheme used for studying the molecular fragmentation of XeF₂. A 690-eV pump pulse excites the Xe 3d→ɛ_F shape resonance and triggers core-hole decay. In the core-hole decay, several nonradiative electronic processes such as Auger decay participate, emitting valence electrons and leaving a highly charged molecule. The induced dynamics leads to fragmentation of the molecule into three separated ions, . In our experiment, after time delays of 4, 29 and 54 fs, a 683-eV probe pulse excites the 1s→2p resonances of F⁺ or F²⁺ as the molecule dissociates. (b) Photoabsorption cross-sections for the pump (purple arrow) and probe (red arrow) pulses. The cross-sections are arbitrarily scaled. Neutral XeF₂ cross-sections (purple line) are taken from ref. , xenon ions Xe^q+ cross-section (black line) are taken from ref. and averaged over q⁺ using the ion-branching ratios of ref. , and atomic F 1s→2p resonances of F⁺ or F²⁺ (red lines) are calculated with Cowan's code. Cross-sections have been broadened to account for the ∼5-eV widths of the ∼10-fs pulses. The two-colour pulses excite the molecule at the Xe site and probe the F ions formed during dissociation.

Figure 3. KER distributions.

X-ray pump/X-ray probe KERs of the F²⁺–Xe^q+–F³⁺ and F⁺–Xe^q+–F³⁺ break-up channels, averaging over all Xe charge states (q) observed in the experiment, . The time delays are 4 fs (black line), 29 fs (green line) and 54 fs (red line). A time dependence is observed for a pump pulse that excites the Xe site at 690 eV and a probe pulse that excites strong 1s→2p resonances near 683 eV of the transient states leading to F⁺ and F²⁺ ions. (a,d) Experimental results: the time dependence reveals hetero-site pump-probe events. Pump and probe pulses have ∼10-fs FWHM with ∼5-eV energy widths. (b,c,e,f) Theoretical results based on a classical break-up model for the three time delays, following pathways (1), (2), (3) and (4), respectively. The theoretical results only account for pump-probe events and are consistent with the experimental observations in the KER ranges left from the vertical lines. The error bars represent the standard deviation of the mean.

Figure 4. Time-dependent ion momentum distribution.

Dalitz plots of the F²⁺–Xe^q+–F³⁺ break-up channel using 690-eV pump and 683-eV probe pulses. (a) The full Dalitz plot is shown for the pump-only data. (b,c,d) Pump-probe data for 4-, 29- and 54-fs time delays, respectively, are shown on an expanded scale. Only events with KER below the vertical line near 90 eV in Fig. 3a are plotted. The magenta lines are markers of the pump-only data. The colour label has been normalized to the maximum of counts. Movement of the distributions towards the F²⁺ ion with increasing time delay is consistent with the identified pathways (1) and (2). (e) Newton diagram for the break-up channel F²⁺–Xe^q+–F³⁺: the linear momenta distribution is shown for the Xe^q+ and F²⁺ ions, always rotating the reference framework in order to fix the direction of the linear momentum of F³⁺ in the (positive) p_x-direction. The distribution indicates linear break-up of the molecular ion.