Low-temperature optical pump, soft x-ray probe measurements were performed at the Soft X-Ray (SXR) instrument of the Linac Coherent Light Source (LCLS) X-ray FEL at SLAC National Laboratory, Menlo Park, USA (55). The measurements reported in this work were carried out at an RSXS endstation (31) in a vacuum of 3 × 10−9 torr. Low temperatures down to 12 K were achieved with a manipulator equipped with a Helium flow cryostat. Ultrafast probe x-rays at a repetition rate of 120 Hz were obtained by tuning the FEL to the Cu L3/2 edge (931.5 eV) and with a bandwidth of 0.3 eV after passing through a grating monochromator. The p-polarized x-ray pulses had a typical pulse duration of 60 fs and a pulse energy of 1.5 μJ and were focused down to an elliptical spot of 1.5 × 0.03 mm2. The optical pump pulses of 1.55 eV, also p-polarized, were generated with a Ti:sapphire amplifier run at 120 Hz and propagated collinearly with the x-rays into the RSXS endstation. The 50-fs pump was focused down to a spot of 2.0 × 1.0 mm2 to probe a homogeneously excited sample volume. The beams were spatially overlapped onto a frosted Ce:YAG crystal and synchronized by monitoring the reflectivity changes of a Si3N4 thin film. The shot-to-shot temporal jitter between pump and probe pulses was measured by means of a timing tool (56, 57) and corrected by time sorting during the data analysis. The overall time resolution of approximately 130 fs was checked by measuring the cross-correlation signal on a polished Ce:YAG crystal (see fig. S1). Shot-to-shot intensity fluctuations from the FEL were corrected in the photodiode data through a reference intensity readout before the monochromator. The scattered x-rays were measured with an energy-integrating APD located on a rotating arm at 17.3 cm from the sample, while tr-RIXS measurements were performed with a modular qRIXS grating spectrometer (32) mounted on a port at 135° with respect to the incident beam and provided an energy resolution of ~0.7 eV (full width at half maximum) when using the second order of the grating. The spectrometer was equipped with an ANDOR charge-coupled device camera operated at a readout rate of 120 Hz in 1D binning mode along the nondispersive direction. The pump-probe time delay was controlled both electronically and through a mechanical translation stage. All the time-dependent rocking curves presented in this work were referenced to their equilibrium values by selectively varying the pump-probe time delay to negative values during the data acquisition to minimize errors due to motor backlash and step accuracy.

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