Time- resolved x-ray diffraction

Time-resolved X-ray diffraction (XRD) using X-ray Free Electron Lasers (XFELs) is an advanced technique that allows us to study the ultrafast dynamics of materials by capturing their structural changes in real time on femtosecond timescales (10⁻¹⁵ seconds). This method is particularly useful for observing structural transitions, phase changes, and other fast processes in materials, molecules, and biological systems, which would be difficult to observe using traditional X-ray sources due to the speed of these processes.

Key Concepts:

1. X-ray Diffraction (XRD):

   • X-ray diffraction is a technique used to determine the atomic and molecular structure of a material by observing how X-rays scatter when they interact with the material. The diffraction pattern provides information about the distances between atoms, the arrangement of atoms in a crystal, and other structural properties.

   • Conventional X-ray diffraction methods rely on steady-state diffraction patterns from a static sample, which reveals the material's long-term structural characteristics.

2. Time-Resolved X-ray Diffraction:

   • In time-resolved X-ray diffraction, the goal is to observe how the diffraction pattern changes over time after a sample is excited or perturbed. This allows for studying dynamic processes such as phase transitions, chemical reactions, or changes in crystal structures as they happen.

   • The challenge here is capturing these fast changes, which require very short X-ray pulses (on the order of femtoseconds or even attoseconds), and XFELs are designed to provide this capability.

3. X-ray Free Electron Lasers (XFELs):

   • XFELs are a new class of X-ray sources that generate extremely short and intense X-ray pulses. Unlike traditional X-ray sources (like synchrotrons), XFELs produce coherent X-rays with incredibly high brightness and ultra-short pulse durations, making them ideal for time-resolved experiments.

   • XFELs work by accelerating electrons through a linear accelerator, which then pass through a magnetic field (undulator) that causes the electrons to emit X-rays. These X-rays are extremely intense and can be as short as femtoseconds or even attoseconds in duration.

   • XFELs can produce single-shot X-ray diffraction patterns, allowing for the study of individual snapshots of a sample's structure over time.

How Time-Resolved X-ray Diffraction Using XFEL Works:

1. Excitation of the Sample:

   • The material or system of interest is typically excited using a femtosecond laser pulse or another form of stimulation (e.g., a pump pulse), inducing rapid changes in the material’s structure or properties (like phase transitions, atomic motion, or chemical reactions).

2. X-ray Probe Pulse:

   • After a time delay (from femtoseconds to picoseconds), an XFEL probe pulse is used to diffract off the sample. The timing of this probe pulse is controlled so that it captures the structural changes of the material at different points in time after the excitation.

3. Data Collection:

   • The scattered X-rays are detected using specialized detectors, and a diffraction pattern is recorded. By changing the time delay between the pump and probe pulses, a time series of diffraction patterns can be collected, showing how the sample’s structure evolves over time.

4. Analysis:

   • The diffraction patterns are analyzed to extract information about changes in the atomic or molecular structure of the material. These changes could involve bond lengths, angles, symmetry, or even phase changes in crystalline materials.

   • By combining these time-resolved diffraction patterns, we can reconstruct the sequence of events that occurs during the ultrafast dynamics, such as how molecules rearrange during a chemical reaction or how a material transitions between different phases.