  A permanently available molecular-beam injection setup for controlled
molecules (COMO) was installed and commissioned at the small quantum systems
(SQS) instrument at the European x-ray free-electron laser (EuXFEL). A $b$-type
electrostatic deflector allows for pure state-, size-, and isomer-selected
samples of polar molecules and clusters. The source provides a rotationally
cold ($T\approx1$~K) and dense ($\rho\approx10^8$~cm$^{-3}$) molecular beam
with pulse durations up to 100~\us generated by a new version of the Even-Lavie
valve. Here, a performance overview of the COMO setup is presented along with
characterization experiments performed both, with an optical laser at the
Center for Free-Electron-Laser Science and with x-rays at EuXFEL under
burst-mode operation. COMO was designed to be attached to different instruments
at the EuXFEL, in particular at the small quantum systems (SQS) and single
particles, clusters, and biomolecules (SPB) instruments. This advanced
controlled-molecules injection setup enables XFEL studies using highly defined
samples with soft and hard x-ray FEL radiation for applications ranging from
atomic, molecular, and cluster physics to elementary processes in chemistry and
biology.
