First observation of the rotational spectrum of the HD–CO weakly bound complex

Abstract In this Note we present the first observation of the rotational spectrum of the HD–CO van der Waals complex. Three b -type transitions, namely R (1), R (2) and R (3) corresponding to the rotation of the CO subunit within the complex, were measured in the millimeter-wave region using the orotron jet spectrometer. An approximate low energy level scheme for the HD–CO complex was built on the basis of the previous investigations of the para H 2 –CO and ortho D 2 –CO complexes and the presented measurements.