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

¹ Appalachian State University, Department of Physics & Astronomy, 231 Garwood Hall, 525 Rivers Street, Boone, NC 28608-2106, USA
² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
e-mail: murthy.gudipati@jpl.nasa.gov
³ North Carolina Museum of Natural Sciences, 121 West Jones Street, 3802B, Raleigh, NC 27603, USA
⁴ University of North Carolina at Chapel Hill, Department of Physics & Astronomy, 120 East Cameron Avenue, Phillips Hall CB3255, Chapel Hill, NC 27599, USA

Received: 11 June 2021
Accepted: 6 September 2021

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)E_b = (833 ± 5 K) and (¹³CO–¹³CO)E_b = (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.