Competition between CO and N2 desorption from interstellar ices

Millimeter observations of pre- and protostellar cores show that the abundances of the gas-phase tracer molecules, (CO)-O-18 and N2H+, anticorrelate with each other and often exhibit holes where the density is greatest. These results are reasonably reproduced by astrochemical models, provided that the ratio between the binding energies of N-2 and CO, R-BE, is taken to be between 0.5 and 0.75. This Letter is the first experimental report of R BE the desorption of CO and N-2 from layered and mixed ices at temperatures relevant to dense cores, studied under ultrahigh vacuum laboratory conditions using temperature programmed desorption. From control experiments with pure ices, R-BE, = 0.923 +/- 0.003 given E-b(N-2-N-2) = 790 +/- 25 K and E-b(CO-CO) = 855 +/- 25 K. In mixed (CO:N-2 = 1: 1) and layered (CO above or below N-2) ice systems, both molecules become mobile within the ice matrix at temperatures as low as 20 K and appear miscible. Consequently, although a fraction of the deposited N-2 desorbs at lower temperatures than CO, up to 50% of the N-2 molecules leave the surface as the CO itself desorbs, a process not included in existing gas-grain models. This codesorption suggests that for a fraction of the frozen-out molecules, R-BE is unity. The relative difference between the CO and N-2 binding energies as derived from these experiments is therefore significantly less than that currently adopted in astrochemical models.