Isotope effect on the sublimation curves and binding energies of 12CO and 13CO interstellar ice analogues

Abstract

Aims. Understanding the desorption properties and sublimation temperatures of CO is key toward constraining the astrophysical regimes within which CO exists in the gas and ice phases. Previous experimental studies using temperature programmed desorption (TPD) determined the binding energies of 12CO and 13CO without the precision that is necessary to determine the effect of isotopes on these properties, which is required when analyzing astronomical data of CO isotopologues. The purpose of this work is to precisely determine the binding energies of 12CO and 13CO. Methods. We conducted experiments using temperature interval desorption (TID), which ensures that thermal equilibrium is reached at each temperature, as well as TPD experiments on interstellar analogues of 12CO and 13CO ices. Results. Sublimation curves show a small but distinct separation between 12CO and 13CO ices. We found that complete sublimation of pure 12CO occurs at 28.9 ± 0.2 K and pure 13CO at 29.0 ± 0.2 K. A systematic difference of 0.1 K was found for 13CO ice compared to 12CO ice under similar desorption conditions, implying that the binding energy in the ice phase for 13CO ice is higher than that of 12CO. Our experimentally derived binding energies were determined through TID to be (12CO–12CO)Eb = (833 ± 5 K) and (13CO–13CO)Eb = (848 ± 6 K). Our results quantitatively show that 13CO is more tightly bound than 12CO in the ice phase, which could have a significant effect on CO isotopic enrichment in astrophysical settings.