Probing the non-equilibrium transient state in magnetite by a jitter-free two-color X-ray pump and X-ray probe experiment

Abstract

We present a general experimental concept for jitter-free pump and probe experiments at free electron lasers. By generating pump and probe pulse from one and the same X-ray pulse using an optical split-and-delay unit, we obtain a temporal resolution that is limited only by the X-ray pulse lengths. In a two-color X-ray pump and X-ray probe experiment with sub 70 fs temporal resolution, we selectively probe the response of orbital and charge degree of freedom in the prototypical functional oxide magnetite after photoexcitation. We find electronic order to be quenched on a time scale of (30 ± 30) fs and hence most likely faster than what is to be expected for any lattice dynamics. Our experimental result hints to the formation of a short lived transient state with decoupled electronic and lattice degree of freedom in magnetite. The excitation and relaxation mechanism for X-ray pumping is discussed within a simple model leading to the conclusion that within the first 10 fs the original photoexcitation decays into low-energy electronic excitations comparable to what is achieved by optical pump pulse excitation. Our findings show on which time scales dynamical decoupling of degrees of freedom in functional oxides can be expected and how to probe this selectively with soft X-ray pulses. Results can be expected to provide crucial information for theories for ultrafast behavior of materials and help to develop concepts for novel switching devices.

The soft X-ray and XUV split-and-delay unit at beamlines FL23/24 at FLASH2

“…Results on time-resolved imaging (Gu ¨nther et al, 2011;Rath et al, 2014), spectro-temporal characterization of XUV pulses (Ding et al, 2021) and stimulated X-ray emission (Kroll et al, 2020) have also been reported. In recent years the technique of two-color pump/ probe experiments was utilized to reveal the non-equilibrium dynamics in diamond (Inoue et al, 2016) and magnetite (Pontius et al, 2018). The investigation of the temporal dynamic of systems is key to the fundamental understanding of its underlying physics.…”

Enabling time-resolved experiments with ultrashort FEL pulses via the split-and-delay unit for FLASH2

“…A prominent principle to explore the temporal dynamic of a system are pump-probe experiments which at FELs are mainly carried out in the ps-and sub ps-region. [1][2][3][4][5][6] . As pump-probe experiments include two pulses with a well-defined time delay between both, the temporal resolution of the experiment is given by the pulse duration of the two pulses and the achieved accuracy of the temporal delay between them.…”

Demonstration of two-color x-ray free-electron laser pulses with a sextupole magnet

“…There is special scientific interest in two-color FEL pulses with time delay and central photon energy separation both adjustable. Among the use cases are pump-probe [7][8][9], coherent stimulated X-ray Raman spectroscopy [10][11][12], and multi-wavelength anomalous diffraction [13] experiments.…”