Ice Sublimation During Star and Planet Formation: A Laboratory Characterization of CO and N2 Binding Energies

Millimeter observations show that abundances of gaseous molecular tracers CO and N2H+ anti-correlate with each other, implying a process of relative chemical differentiation. These molecules have been applied to probe a wide range of astrophysical contexts, but their utility depends on understanding their chemical behavior. In turn, the distribution and evolution of molecules critically hinges on their sublimation processes. The anti-correlation can be reproduced by current astrochemical models only by assuming that the ratio of the binding energies, the kinetic parameter that constrains relative desorption behavior, between N2 and CO is approximately RB = EB;N2(N2 — N2)=EB;CO(CO — CO) = 0:60. However, previous laboratory studies have shown that the difference between the CO and N2 binding energies is small, with a binding energy ratio lower bound of 0:936 ± 0:03, contradicting modeling assumptions. A proposed explanation for this discrepancy is that the H2O present in interstellar ices affects the relative desorption of CO and N2H+, contributing to a lower binding energy ratio in astrophysical contexts. To assess this possibility, we present Temperature Programed Desorption (TPD) experiments of CO and N2 ices in pure, layered, and mixed morphologies over a range of surface coverages and H2O substrate structures. To understand the experimental data, we construct a four-phase kinetic model consisting of zeroth-order multilayer desorption and firstorder monolayer desorption. The values of the binding energies are then extracted from the generative model via Markov Chain Monte Carlo (MCMC) fitting algorithms. We find that the the binding energy ratio, defined as RB = EB;N2(N2 — H2O)=EB;CO(CO — H2O) ranges from 0:758±0:027 to 0:964±0:035. The lowest binding energy ratio was observed in a sub-monolayer mixed ice environment deposited on a highly porous amorphous H2O substrate. This implies that the sublimation processes of CO and N2 are different in astrophysically relevant conditions; importantly, in star and planet forming regions.