The Liquid Jet Endstation for Hard X-ray Scattering and Spectroscopy at the Linac Coherent Light Source

Abstract

The ability to study chemical dynamics on ultrafast timescales has greatly advanced with the introduction of X-ray free electron lasers (XFELs) providing short pulses of intense X-rays tailored to probe atomic structure and electronic configuration. Fully exploiting the full potential of XFELs requires specialized experimental endstations along with the development of techniques and methods to successfully carry out experiments. The liquid jet endstation (LJE) at the Linac Coherent Light Source (LCLS) has been developed to study photochemistry and biochemistry in solution systems using a combination of X-ray solution scattering (XSS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES). The pump-probe setup utilizes an optical laser to excite the sample, which is subsequently probed by a hard X-ray pulse to resolve structural and electronic dynamics at their intrinsic femtosecond timescales. The LJE ensures reliable sample delivery to the X-ray interaction point via various liquid jets, enabling rapid replenishment of thin samples with millimolar concentrations and low sample volumes at the 120 Hz repetition rate of the LCLS beam. This paper provides a detailed description of the LJE design and of the techniques it enables, with an emphasis on the diagnostics required for real-time monitoring of the liquid jet and on the spatiotemporal overlap methods used to optimize the signal. Additionally, various scientific examples are discussed, highlighting the versatility of the LJE.

Keywords: EXAFS; X-ray absorption spectroscopy; X-ray diffraction; X-ray emission spectroscopy; X-ray scattering; XANES; XFEL; biochemistry; photochemistry; time-resolved X-ray studies.

Figure 1

Essential LJE and XCS beamline components. (Right) Upstream LCLS components, including a gas detector (GDET) for measuring the beam intensity, the scannable channel-cut monochromator (CCM), YAG screens for beam position monitoring, X-ray intensity monitors (IPM4), slits (S4), and prefocusing compound refractive X-ray lenses (CRL1). (Middle) XCS beamline components, including a pulse picker/selector (PP), another set of focusing lenses (CRL2), attenuators (ATT), slits (S5), a YAG screen, an X-ray intensity monitor (IPM5) as well as the optical laser and time-tool (TT). The laser system is also depicted in the diagram. (Left) The LJE setup, including a recirculating liquid jet, sample viewing cameras (lj1–3), and clean-up sample slits (SS), as well as the XSS scattering detector (shown here) and/or a flavor of the XAS/XES setup (not shown here).

Figure 2

Schematics of the LJE for the 3 different techniques: XSS, XES, and XAS. (a) The XSS and XES setup seen from upstream. The X-ray beam (light purple) passes through the focusing lenses then the holey mirror before entering the sample chamber quasi-collinearly to the laser beam (lime). Both beams pass through clean-up slits (bronze) before the interaction point (red dot) with the sample from the liquid jet (blue). The liquid jet nozzle is placed in the positive x-direction, while the catcher tube and sample reservoir are placed in the negative x-direction of the chamber. The ePix10k2M detector (SLAC National Laboratory, Palo Alto, CA, USA) is downstream from the interaction point. (b) The XSS setup seen from above. The ePix10k2M detector is located downstream from the interaction point. (c) The XES seen from downstream. A spectrometer is placed above the sample chamber, focusing the emitted X-rays from the sample into the ePix100 detector (SLAC National Laboratory, Palo Alto, CA, USA). The detector is placed to the left (negative x-direction) of the sample chamber. (d) The XAS setup seen from downstream. The ePix100 detector is placed above the sample chamber to directly measure the total fluorescence yield. XSS can be measured simultaneously with either XES or XAS.

Figure 3

(a) XANES steady-state spectrum of aqueous ferricyanide, 30 mM (shown in black), reveals a pre-edge assigned to the 1s–3d (t_2g) transition at 7.1113 keV and the 1s–3d (eg) transition at 7.1143 keV. The transient X-ray absorption spectra of ferricyanide, recorded 0.15–0.25 ps after photoexcitation at 266 nm (shown in blue). (b) The kinetic trace for a single energy at 7.126 keV, along with the corresponding fit (shown in black). These transient absorptions were measured at the Fe K-edge using the liquid jet pump–probe method at XCS.

Figure 4

Pt L₃-edge XANES steady-state spectrum of aqueous hexachloroplatinate (1 mM K₂PtCl₆) (black line), the transient X-ray absorption spectrum after 10 ps photoexcitation at 266 nm (gold line), and the associated 10 ps difference spectrum (blue line).

Figure 5

XANES intensity at the Fe K-edge (7125 eV) after 532 nm photolysis of MbCO. The time trace is binned at 50 fs intervals from –1 to 4 ps delay times, then fit to a single exponential function convolved with a Gaussian instrument response function. The fit provides a mono-exponential rate constant of κ = 3.45 (τ = 290 fs). The residuals between the experimental data and fit are shown below the kinetic trace.

Figure 6

(a) The time-dependent difference spectra (top) corresponding to the changes in the Kß main line and VtC region emission (bottom) following photoexcitation. (b) A comparison between the LMCT population kinetics determined from XES (blue) and the scale low-Q range kinetics from XSS (red). A delayed onset of the low-Q range can clearly be seen. (c) Proposed relaxation scheme of ²[Fe^III(CN)₆]^3– following photoexcitation at 336 nm. Figure adapted from Reinhard et al. (2023) [26] with permission.