  X-ray Free Electron Lasers (XFELs) can probe chemical and biological
reactions as they unfold with unprecedented spatial and temporal resolution. A
principal challenge in this pursuit is the delivery of samples to the X-ray
interaction point in a way that produces data of the highest possible quality
and efficiency. This is hampered by constraints posed by the light source and
operation within a beamline environment. For liquid samples, the solution
typically involves a high-speed liquid jet, capable of keeping up with the rate
of X-ray pulses. However, conventional jets are not ideal because of
radiation-induced explosions of the jet, as well as their cylindrical geometry
combined with the X-ray pointing instability of many beamlines causes the
interaction volume to differ for every pulse. This complicates data analysis
and contributes to measurement errors. An alternative geometry is a liquid
sheet jet which, with its constant thickness over large areas, eliminates the
X-ray pointing related problems. Since liquid sheets can be made very thin, the
radiation-induced explosion is reduced, boosting their stability. They are
especially attractive for experiments which benefit from small interaction
volumes such as fluctuation X-ray scattering and several types of spectroscopy.
Although they have seen increasing use for soft X-ray applications in recent
years, there has not yet been wide-scale adoption at XFELs. Here, we
demonstrate liquid sheet jet sample injection at the European XFEL. We evaluate
several aspects of its performance relative to a conventional liquid jet
including thickness profile, stability, and radiation-induced explosion
dynamics at high repetition rates. The sheet jet exhibits superior performance
across these critical experimental parameters. Its minute thickness also
suggests ultrafast single-particle solution scattering is a possibility.
