Ultrafast X-ray sources for time-resolved measurements

Extremely short pulse X-ray sources have attracted considerable attention for unique use in time-resolved X-ray scattering, absorption, and diffraction measurements to investigate the molecular dynamics of chemical reaction in real time. The characteristics of X-ray sources based on accelerators and femtosecond lasers are reviewed from the viewpoints of the X-ray photon energy, pulse duration, intensity and stability of the pulses, and usability of the source. The X-ray generation mechanism of each of the source is discussed from both the experimental and theoretical bases. We also demonstrate the application of ultrafast X-ray diffraction measurements in the phase transition of vanadium dioxide using the laser-induced plasma X-ray source. The series of real-time structural observations with pulsed X-ray sources contributes toward understanding the mechanism of photo-induced nonequilibrium process.