Fig. 1

Schematic sketch of the synchrotron and argon beamline. The red circles denote probe packets of ammonia molecules which are targeted by the argon beam (shown in green). The blue circles denote reference packets of ammonia, which do not encounter argon atoms and provide a simultaneous background measurement. The grey circles denote reference packets of ammonia that encounter the leading or trailing end of the argon beams, and are therefore discarded