The chemRIXS Instrument for the LCLS-II X-Ray Free Electron Laser

This paper presents an overview of the chemRIXS instrument at LCLS-II, highlighting how its high-repetition-rate superconducting accelerator and advanced supporting systems enable unprecedented time-resolved soft X-ray spectroscopy studies on dilute solution-phase samples compared to the previous LCLS-I facility.

The Gist

Imagine you want to watch a movie of a chemical reaction happening in a drop of water. The problem is, the actors (the atoms and electrons) are moving so fast that a normal camera would just see a blur. You need a camera that can take a picture in a fraction of a second, and you need a lot of light to see them clearly.

This paper introduces a brand-new, super-powered scientific instrument called chemRIXS at the SLAC National Accelerator Laboratory. Think of it as the ultimate "slow-motion camera" for the microscopic world of chemistry.

Here is a breakdown of how it works and why it's a big deal, using some everyday analogies:

1. The Problem: The "Dim Flashlight" vs. The "Strobe Light"

In the past, scientists used a machine called LCLS-I to take these pictures. It was like trying to film a race car with a dim flashlight that only flashed 120 times a second. Because the light was so dim and the flashes were so far apart, scientists could only study very thick, concentrated soups of chemicals. If the chemical was dilute (like a drop of dye in a bucket of water), the signal was too weak to see.

The Solution: The new machine, LCLS-II, is like a super-bright strobe light that flashes nearly a million times a second. This massive increase in brightness (flux) means scientists can finally see the "dye in the bucket." They can now study very dilute, realistic chemical solutions that were previously invisible.

2. The Camera: The "Chemical X-Ray Vision"

The chemRIXS instrument doesn't just take a picture; it takes a picture that reveals the energy of the atoms.

• X-ray Absorption (XAS): Imagine shining a specific color of light on a lock. If the light matches the key, the lock opens. This instrument shines soft X-rays to see which "keys" (electrons) fit into which "locks" (atoms). This tells us exactly which elements are present and how they are arranged.
• Resonant Inelastic X-ray Scattering (RIXS): This is the fancy part. When the X-ray hits an atom, it knocks an electron up, and then the electron falls back down, releasing a new photon (a flash of light). By measuring the energy of that new flash, scientists can see how the electrons are dancing and talking to each other. It's like listening to the echo in a canyon to figure out the shape of the canyon walls.

3. The Stage: The "Liquid Jet"

Chemistry usually happens in liquids (like water or alcohol). But X-rays need a vacuum (empty space) to travel, and liquids usually boil away or freeze in a vacuum.

• The Analogy: Imagine trying to film a waterfall in a vacuum chamber. You can't just put a cup of water there; it would evaporate instantly.
• The Fix: The instrument uses a liquid jet. It shoots a super-thin sheet of liquid (thinner than a human hair) through the vacuum. The X-ray camera snaps a picture of the liquid, and then the liquid is caught and recycled. It's like a high-speed conveyor belt of fresh water, ensuring every X-ray pulse hits a brand-new, undamaged sample.

4. The Timing: The "Stopwatch"

To see a reaction, you need to hit the sample with a laser (the "pump") to start the reaction, and then hit it with an X-ray (the "probe") a tiny fraction of a second later.

• The Jitter Problem: Sometimes the laser and the X-ray don't arrive at the exact same time; they might be off by a tiny bit (like a runner starting a race a millisecond late).
• The Fix: The instrument has an Arrival Time Monitor (ATM). Think of this as a super-precise stopwatch that measures exactly when the laser and X-ray arrived for every single shot. If they were slightly off, the computer can correct the data later, stitching the movie together perfectly.

5. The Results: What Did They See?

The team tested this new machine by looking at water being hit by a powerful laser.

• The Old Way: It took a long time to get a blurry, low-quality image of the water molecules breaking apart.
• The New Way: With the new machine, they got a crystal-clear, high-definition movie of the water splitting into ions and radicals in just 5 minutes. They could see the "valence hole" (a missing electron) and how the surrounding water molecules reacted to it almost instantly.

Why Does This Matter?

This instrument opens the door to studying chemistry in a way that mimics real life.

• Real-world applications: We can now study how drugs interact with proteins in the body, how batteries charge and discharge at the atomic level, or how plants turn sunlight into energy (photosynthesis).
• The "Dilute" Revolution: Because the machine is so sensitive, we can finally study chemicals at the concentrations they actually exist in nature, rather than being forced to use thick, artificial mixtures.

In summary: The chemRIXS instrument is a high-speed, high-sensitivity camera that uses a super-bright X-ray strobe light and a liquid conveyor belt to film the fastest, most delicate chemical reactions in the universe, finally letting us see the "invisible" world of molecules in action.

Technical Summary

1. Problem Statement

The field of time-resolved soft X-ray spectroscopy for solution-phase systems faces significant technical barriers that have historically limited its application to high-concentration samples. Key challenges include:

• Low Signal-to-Noise Ratio (SNR): Soft X-ray transitions in light elements and transition metals often suffer from poor contrast between solute and solvent.
• Low Fluorescence Yields: High probabilities of Auger-Meitner decay (electron emission) rather than photon emission reduce the detectable signal.
• Detector Inefficiency: Soft X-ray grazing-incidence grating spectrometers typically have low efficiency.
• Photon Hunger: Techniques like Resonant Inelastic X-ray Scattering (RIXS) require massive photon fluxes to overcome these losses, making them difficult to perform on dilute systems or with previous-generation X-ray Free Electron Lasers (XFELs) like LCLS-I, which operated at low repetition rates (120 Hz).
• Sample Constraints: Measuring liquid samples in high vacuum requires complex jet systems, and previous setups struggled with sample consumption rates and clogging.

2. Methodology and Instrument Design

The paper presents the chemRIXS instrument, a successor to the liquid jet endstation at the Soft X-ray (SXR) instrument, designed specifically for the high-repetition-rate LCLS-II accelerator. The methodology relies on a synergistic integration of four key subsystems:

A. X-ray Source and Beamline

• Source: Utilizes the LCLS-II superconducting accelerator, offering a repetition rate of up to 33.2 kHz (subharmonics of 1 MHz), a massive increase over LCLS-I's 120 Hz.
• Beamline: Features a variable-line-spacing (VLS) grating monochromator (resolving power ~2000) covering 200–1600 eV.
• Focusing: Uses Kirkpatrick-Baez (KB) mirrors to focus the beam to ~10 µm at the interaction point (IP).
• Flux: Delivers up to $2 \times 10^{15}$ photons/second at the IP, enabling studies on dilute systems.

B. Sample Delivery System

• Liquid Jets: Employs etched glass microfluidic nozzles to generate free-standing liquid sheets (0.2–10 µm thick). These sheets flow at >10 m/s, ensuring a fresh sample for every XFEL shot.
• Recirculation: A sophisticated closed-loop recirculation system collects the liquid jet, filters debris, and returns it to the reservoir. This allows the use of small sample volumes (~25 mL) for extended periods, mitigating the high flow rates (1.5–6 mL/min) required for sheet formation.
• Vacuum Management: Uses differential pumping and cryogenic cold traps to maintain vacuum pressures of 0.1–1 mTorr in the interaction chamber while keeping the rest of the beamline below $10^{-6}$ Torr.

C. Optical Laser System

• Pump Source: Initially uses an Optical Parametric Chirped Pulse Amplifier (OPCPA) generating 800 nm pulses (500 µJ, <25 fs) synchronized to the XFEL. Future runs will utilize Ytterbium-based systems (1030 nm).
• Timing: An Arrival Time Monitor (ATM) corrects for timing jitter between the X-ray and laser pulses. The ATM uses a semiconductor target to spatially encode the arrival time, achieving a timing resolution of ~2 fs and correcting for drifts up to several picoseconds.

D. Detection System

• Multi-Channel Detection: The endstation supports parallel detection:
  • Total Fluorescence Yield (TFY): Measured by Avalanche Photodiodes (APDs) for shot-by-shot readout and high time resolution.
  • Partial Fluorescence Yield (PFY) & RIXS: Measured by a Variable Line Spacing (VLS) grating emission spectrometer coupled with a CCD camera.
  • Transmission XAS: Measured by a downstream CCD detector.
• Bursting Mode: To accommodate the readout limitations of 2D detectors (which cannot handle 33 kHz continuously), the XFEL operates in "bursting" mode. X-ray pulses are delivered in bursts synchronized with detector readout, reducing the effective rate by ~33–50% but preventing data artifacts.

3. Key Contributions

• Instrument Commissioning: The paper provides the first comprehensive overview of the chemRIXS instrument, detailing its optical, mechanical, and control systems.
• High-Throughput Liquid Spectroscopy: Demonstrates the successful integration of high-repetition-rate XFELs with liquid jet recirculation, enabling time-resolved studies on dilute solution-phase systems previously inaccessible.
• Advanced Timing Correction: Implementation of the ATM allows for shot-by-shot jitter correction, pushing time resolution toward the instrument response limit (~50–70 fs).
• Versatile Detection: The design supports simultaneous TFY, PFY, and RIXS measurements, offering complementary views of electronic structure evolution.

4. Results

The paper presents initial commissioning results validating the instrument's capabilities:

• Strong-Field Ionization of Water:
  • Experiment: A 2 µm water sheet was ionized by an 800 nm laser, and the dynamics were probed at the Oxygen K-edge.
  • Performance: A full RIXS map (showing valence hole and solvated electron features) was acquired in just 5 minutes at a 5.5 kHz effective repetition rate. This dataset contained >1 million events, representing 3 times the statistics of previous LCLS-I experiments in 1/20th the time.
  • Time Resolution: A time-delay scan of the water valence hole feature yielded an instrument response function of 97 ± 25 fs FWHM after ATM correction.
  • Observations: Clear detection of the OH radical transient absorption at 525 eV and a ~200 fs decay process attributed to vibrational cooling.
• Previous LCLS-I Results: The paper also summarizes prior work on metal-ligand charge transfer in Ruthenium complexes and nonlinear X-ray spectroscopies (attosecond transient absorption, ISXRS, SXSHG) performed on the instrument using the lower-repetition-rate LCLS-I beam.

5. Significance

The chemRIXS instrument represents a paradigm shift in soft X-ray spectroscopy of chemical systems:

• Access to Dilute Systems: The combination of LCLS-II flux and efficient liquid recirculation enables the study of biologically relevant concentrations and dilute catalytic systems that were previously impossible to probe with time-resolved RIXS.
• New Scientific Frontiers: It opens the door to tracking the evolution of occupied and unoccupied orbitals in photochemical reactions with element-specificity and femtosecond resolution.
• Technological Benchmark: The successful integration of high-repetition-rate XFELs with complex liquid jet and timing correction systems sets a new standard for future soft X-ray endstations.
• Broad Applicability: The instrument is designed to serve a wide community, from chemistry and biology to materials science, facilitating the discovery of new reaction mechanisms and electronic couplings in solution.