The millimeter and submillimeter spectrum of the FeCO radical in the 3&~ground electronic state has been observed in the laboratory. The radical was generated by a DC glow discharge in a mixture of and argon. A total of 33 rotational transitions in the three spin sublevels with ) \ 1, 0 were Fe(CO) 5 observed in the frequency region between 223 and 372 GHz. The observed frequencies were analyzed using a Hamiltonian for the 3&~state, and the rotational, centrifugal distortion, spin-spin

more »

... and spin-rotation coupling constants were determined. Based on these constants, we obtained the rest frequencies in the 2 and 3 mm regions of FeCO and searched for this radical toward the circumstellar envelope of IRC ]10216 and the star-forming regions W3(OH), DR 21, and Sgr B2 with the Instituto de Radioastronomia Milimetrica 30 m radio telescope. The FeCO lines were not detected with an rms noise of 3È7 mK in four sources. The upper limit on the FeCO column density was estimated to be 3 ] 1013 cm~2 in IRC ]10216.