Pure rotational spectrum of FeCO

The rotational spectrum of the FeCO radical has been observed by using a Fabry–Perot type Fourier-transform microwave spectrometer combined with a pulsed discharge nozzle. The radicals have been generated by a discharge of Fe(CO)5 diluted in Ar, and subsequently cooled down to a few kelvins in a supersonic free jet. Four spectral lines observed in the 8.5–35.5 GHz region have been assigned to the transitions in the Ω=0 spin component of 56FeCO, the most abundant isotopic species, in the 3Σ− electronic ground state, and analyzed to obtain the spectroscopic constants. Transitions in the Ω=±1 spin components have not been detected due to the small population in these levels. Spectra of other four isotopic species, 54FeCO, 57FeCO, Fe13CO, and Fe13C18O, have also been observed to confirm the identification of the species, yielding the substituted coordinates of the molecule. The determined Fe–C bond length [rs(Fe–C)=1.727 Å] is about 0.1 Å shorter than that of Fe(CO)5, but the C–O bond length [rs(C–O)=1.160 Å] is almost the same as that of Fe(CO)5, which indicates stronger Fe–CO bonding than that of Fe(CO)5. We have searched for the 34 GHz transition of the radical toward several molecular clouds on various evolutionary stages by using the Nobeyama 45 m radio telescope. The signal of this radical was not detected with rms noise level of 10 mK, which gives an upper limit for the molecular column density of (2–4)×1012 cm−2.