Rotational rest frequencies for CrO (X ⁵Π_r) and CrN (X ⁴∑^-)

The pure rotational spectra of CrO (X ⁵Π_r) and CrN (X ⁴∑^-) have been recorded using millimeter/sub-millimeter-wave spectroscopy in the frequency range 248-636 GHz. These radicals were created by the reaction of chromium vapor, produced in a Broida-type oven, with N₂O or NH₃ under DC discharge conditions. For CrO, 12 rotational transitions were recorded, in which up to five spin-orbit components were observed - the first measurement of the pure rotational spectrum of a molecule in a ⁵Π electronic state. Nine rotational transitions were recorded for CrN; here all four spin components were measured in every transition. The data obtained for both radicals were analyzed using an appropriate effective Hamiltonian. The resulting spectro-scopic parameters of these species were determined to high precision, including those related to fine structure and lambda doubling. This work will enable radioastronomical searches for CrN and CrO to be carried out. CrO has already been observed in the optical spectra of stellar atmospheres.