Laboratory Detection of FeCO + ( X 4 Σ - ) by Millimeter/Submillimeter Velocity Modulation Spectroscopy

Abstract

The millimeter/submillimeter spectrum of the molecular ion FeCO (X 4 S) has been recorded using velocity modulation spectroscopy. The molecular ion was created in an AC discharge of Fe(CO)5 and argon. Twentyseven rotational transitions, each consisting of four fine-structure components, were measured in the range 198– 418 GHz. The data were fit with a case b Hamiltonian, and rotational, spin-rotation, and spin-spin constants were determined. Because of the presence of higher order spin-orbit interactions, probably caused in part by a nearby 4 P excited state, numerous centrifugal distortion terms were needed for the spectral analysis. The value of g s, the third-order spin-rotation constant, was also remarkably large at 72.4 MHz. Rest frequencies for FeCO are now available for interstellar and circumstellar searches. This species may be present in molecular clouds, where CO is abundant and gas-phase iron should be in the form of Fe. Molecular ions such as FeCO could be the hidden carriers of metallic elements in such clouds. Subject headings: astrochemistry — ISM: molecules — line: identification — methods: laboratory — molecular data — stars: AGB and post-AGB