Ablation of liquids with X-ray lasers: from enabling MHz-rate measurements to uncovering new physics

by Claudiu Stan (Rutgers University Newark, USA)

ELI-NP Big Meeting Room

ELI-NP Big Meeting Room


Liquid microjets with diameters smaller than a single strand of hair have become an extraordinary tool for the delivery of samples in experiments using X-ray free-electron lasers (XFELs). Despite the violent destruction of the samples and the jet itself by high intensity X-ray pulses, liquid jets easily recover at the first-generation XFELs with pulse repetition rates up to 120 Hz. Also, their small diameter and flow rate allows one to record tens of thousands of measurements using less than one gram of precious samples such as crystals of complex proteins.

The second-generation XFELs have repetition rates in the MHz regime and place much tougher requirements for sample delivery methods. To determine if liquid jets can handle such repetition rates, we investigated the dynamics of XFEL-induced liquid jet explosions, using time-resolved optical microscopy. This study resulted in the prediction of the jet parameters needed for MHz-rate measurements, which enabled the first successful experiments at the European XFEL. But equally important, it uncovered the spectacular shock and fluid dynamics of X-ray laser ablation in liquids.

Compared with ablation by optical lasers, the tighter focusing and the linear absorption of X-rays lead to simpler dynamics that can be controlled at smaller length scales. In particular, XFEL pulses generate short-lived and highly symmetrical shock waves that can be used to access new physical regimes. We found that shock reflections in liquid microdrops can be used to reach previously inaccessible metastable states of liquid water at negative pressures. We also generated in liquid water jets ultrasounds with unprecedently high intensities, close to the limit where the concept of sound waves cannot be used anymore because a single wave oscillation is enough to cavitate the liquid