Time-resolved Resonant Inelastic X-ray Scattering (ChemRIXS)

(See LCLS Instruments & Standard Configurations for soft X-ray spectroscopy: chemRIXS)

Overview

Resonant inelastic X-ray scattering (RIXS) provides enhanced electronic structural information by measuring the energy of photons scattered/emitted by the sample with high spectral resolution, while the incident photon energy is tuned across an absorption edge.

The corresponding 2D spectrum shows the incident photon energy versus photon energy loss (i.e. the difference in incident and emitted/scattered photon energies). 

This energy transfer is in the range of meV to eV for soft X-ray RIXS, corresponding to vibrational and valence-electronic d–d transitions. RIXS is the X-ray analogue of resonance Raman scattering and probes the occupancy and interactions of both occupied and unoccupied orbitals in valence-excited and core-excited states.

RIXS provides element-specificity and chemical-site-specificity by exploiting the core-level binding energies. In 2p3d RIXS (characteristic of the soft X-ray range), the incident soft X-ray energy is scanned across the L-edge, exciting 2p electrons to unoccupied 3d-derived valence orbitals, where 2p → 3d transitions lead to 2p⁵3dⁿ⁺¹ intermediate states and 3dⁿ* final states. 

2p3d RIXS offers several additional advantages over UV/Vis absorption spectroscopy: it reveals d–d or ligand-field (LF) excitations over much wider energy ranges often hard to access with UV/Vis spectroscopy (down to below 1 eV and up to several 10 eV). LF or charge-transfer excitations can be selectively enhanced by resonant excitation and distinguished from overlapping charge-transfer transitions and solvent absorption features.

Finally, due to spin–orbit interaction in the 2p-excited intermediate states, 2p3d RIXS allows probing transitions that are formally spin-forbidden in UV/Vis spectroscopy. For a review of soft X-ray RIXS and applications to molecular systems see ref. [1].

ChemRIXS instrument

The ChemRIXS instrument is a new endstation designed to exploit the unique capabilities of LCLS-II, and purposefully optimized for time-resolved RIXS studies of samples in solution using monochromatic soft X-ray pulses and a tunable optical pump laser. It is designed with an emphasis on soft X-ray spectroscopy experiments on liquid samples, measured with a Varied Line Spacing (VLS) portable spectrometer. 

For rapid XAS measurements and direct beam detection (in the case of transmissive samples), a suite of in-vacuum detectors will be implemented on a rotating arm. ChemRIXS will be optimized for studying systems with C, N, O and 3d transition metal elements L-edges using various spectroscopy methods. Spectroscopic studies of rare earth elements will also be relevant so the beamline reach will extend to 1,600 eV.

 ChemRIXS via spectroscopy and RIXS methods will offer in combination with the high repetition rate capabilities of LCLS-II new opportunities in studying ultrafast dynamics in chemical and biological samples.

1. U. Bergmann, J. Kern, R. W. Schoenlein, et al., "Using X-ray free-electron lasers for spectroscopy of molecular catalysts and metalloenzymes", Nature Reviews Physics 3, 264 (2021).