Stability of CH 3 NCO in Astronomical Ices under Energetic Processing: A Laboratory Study

Abstract

Methyl isocyanate (CH 3 NCO) was recently found in hot cores and suggested to exist on comet 67P/CG. The incorporation of this molecule into astrochemical networks requires data on its formation and destruction. In this work, ices of pure CH 3 NCO and of CH 3 NCO(4%–5%)/H 2 O mixtures deposited at 20 K were irradiated with a UV D 2 lamp (120–400 nm) and bombarded by 5 keV electrons to mimic the secondary electrons produced by cosmic rays (CRs). The destruction of CH 3 NCO was studied using IR spectroscopy. After processing, the ν a –NCO band of CH 3 NCO disappeared and IR bands corresponding to CO, CO 2 , OCN − , and HCN/CN − appeared instead. The products of photon and electron processing were very similar. Destruction cross sections and half-life doses were derived from the measurements. Water ice provides a good shield against UV irradiation (half-life dose of ∼64 eV molecule −1 for CH 3 NCO in water ice), but is not so good against high-energy electrons (half-life dose ∼18 eV molecule −1 ). It was also found that CH 3 NCO does not react with H 2 O over the temperature range 20–200 K. These results indicate that hypothetical CH 3 NCO in the ices of dense clouds should be stable against UV photons and relatively stable against CRs over the lifetime of a cloud (∼10 7 yr), and could sublime in the hot core phase. On the surface of a Kuiper Belt object (the original location of comet 67P/CG) the molecule would be swiftly destroyed, by both photons and CRs, but embedded below just 10 μ m of water ice, the molecule could survive for ∼10 9 yr.