Carbon monoxide ices – a semicentennial review and update for crystalline CO along with the first IR spectrum and band strength for amorphous CO

Abstract

The intrinsic intensity (band strength) of the fundamental vibrational band of crystalline carbon monoxide (CO) was first determined 50 yr ago, but those pioneering measurements have seldom been revisited despite CO’s importance in interstellar and Solar System chemistry and CO’s abundance and distribution in extraterrestrial environments. On the semicentennial of those first measurements, which remain in wide use among observational and laboratory astrochemists, we have reexamined those infrared (IR) band-strength determinations, including the density and refractive index data on which they rest, and find that the published results rest on a mere two data points. Here we use new laboratory data to identify and address errors and concerns in that original work from a half-century ago, plus a result from nearly 100 yr ago. We report new IR intensities for crystalline CO’s fundamental IR band over a range of temperatures and resolutions for the first time, finding that there is not a single unique band strength that applies to all conditions. Optical constants have been calculated, including a set at a higher resolution than in the literature. We also present the first direct IR intensity measurements of the fundamentals of solid13C16O,12C17O, and12C18O. Finally, the first IR transmission spectrum of amorphous CO is presented along with a band-strength estimate. All results are for temperatures below 30 K, and so are relevant to studies of solid CO in the outer Solar System and the interstellar medium.