Supplementary Figure 5: Picture of the Roadrunner goniometer installed at the XPP instrument at LCLS for measurements at cryogenic temperatures.

The beam first passes through a drill hole in the inline sample viewing microscope used to visualize and align the sample. A molybdenum collimator tube inserted into the drill hole in the objective lens absorbs radiation caused by air scattering in order to reduce background scattering. The silicon chip sample holder is mounted from the top on the high-precision goniometer stage (not shown). The sample is cooled to temperatures down to 80 K by a cold nitrogen gas flow. Behind the sample the direct beam enters a molybdenum beam pipe absorbing X-rays scattered by air. Background scattering is further reduced by generating a helium gas sheet along the primary beam path between the sample and the beam pipe enclosing the direct beam behind the sample. For room-temperature measurements the cold nitrogen gas flow is replaced by a stream of humidified gas to prevent dehydration of the crystals (not shown here).