CHAPTER 16:Sample Delivery Methods: Liquids and Gases at FELs

Liquids and gases are a convenient method for delivering samples at X-ray free-electron lasers (XFELs). The high speed and positional accuracy with which they can be delivered allows for rapid replenishment of sample in the X-ray focus between the arrival of X-ray pulses. This is often desirable to avoid the effects of radiation damage—in the case of high intensity “diffract before destroy” experiments, it is a necessity. This chapter provides a brief overview of liquid and gas sample delivery in use at the Linac Coherent Light Source (LCLS) along with some recent developments. Liquid injection in the form of cylindrical jets, drops and sheets have been used at the LCLS and elsewhere with thickness down to 100 nm. In the case of cylindrical jets, the thinnest jets are used only for aerosol generation: the short breakup length makes them an unattractive direct target. Larger cylindrical jets of at least 20 µm diameter, liquid sheets of any thickness and gas jets can often, depending on their composition, be recirculated to reduce sample consumption. Pulsed liquid and pulsed gas sources have also been employed for a similar reason.