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Abstract
X-ray absorption spectroscopy (XAS) and the directly linked X-ray reflectivity near absorption edges yield a wealth of specific information on the electronic structure around the resonantly addressed element. Observing the dynamic response of complex materials to optical excitations in pump–probe experiments requires high sensitivity to small changes in the spectra which in turn necessitates the brilliance of free electron laser (FEL) pulses. However, due to the fluctuating spectral content of pulses generated by self-amplified spontaneous emission (SASE), FEL experiments often struggle to reach the full sensitivity and time-resolution that FELs can in principle enable. Here, we implement a setup which solves two common challenges in this type of spectroscopy using FELs: First, we achieve a high spectral resolution by using a spectrometer downstream of the sample instead of a monochromator upstream of the sample. Thus, the full FEL bandwidth contributes to the measurement at the same time, and the FEL pulse duration is not elongated by a monochromator. Second, the FEL beam is divided into identical copies by a transmission grating beam splitter so that two spectra from separate spots on the sample (or from the sample and known reference) can be recorded in-parallel with the same spectrometer, enabling a spectrally resolved intensity normalization of pulse fluctuations in pump–probe scenarios. We analyze the capabilities of this setup around the oxygen K- and nickel L-edges recorded with third harmonic radiation of the free electron laser in Hamburg (FLASH), demonstrating the capability for pump–probe measurements with sensitivity to reflectivity changes on the per mill level.
Highlights
X-ray absorption spectroscopy (XAS) is a common method for characterizing materials in a variety of fields due to its fundamental simplicity, flexibility and element-specificity
To understand the benefits of acquiring two spectra in-parallel, we consider the measured spectral intensity on the detector S(ω), which depends on the photon frequency ω for each beam, to be proportional to the spectral reflectivity R(ω) of the sample, the diffraction efficiency O of the beam-splitting grating in the zeroth or first order and the spectral intensity I (ω) of the free electron laser (FEL): S1 (ω) ∝ R1 (ω)O1 I (ω)S2 (ω) ∝ R2 (ω)O0 I (ω)
It is apparent that the measured spectrum S(ω) of a single beam can be used to measure the reflected spectrum R(ω), if the FEL intensity I (ω) is either known or constant
Summary
X-ray absorption spectroscopy (XAS) is a common method for characterizing materials in a variety of fields due to its fundamental simplicity, flexibility and element-specificity. Tuning to absorption edges allows probing the unoccupied electronic states localized around specific elements. This information content is especially high at the absorption edges in the soft X-ray regime [1], since core hole lifetimes are longer and spectral features sharper than at higher photon energies. XAS measures the spectral absorption coefficient, i.e., the imaginary part of the index of refraction. Measurements in specular reflectivity measure the real part of the refractive index, which is rigorously connected with the imaginary part, through the Kramers–Kronig transform [5] In a pump–probe experiment with sufficiently short pulses, e.g., in free electron lasers (FELs), transient XAS can be used to track changes of the electronic structure during chemical reactions and phase transitions on the femtosecond (fs) to picosecond (ps) scale [2,3,4].
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