The desorption of H₂CO from interstellar grains analogues

Abstract

Context. Much of the formaldehyde (H 2 CO) is formed from the hydrogenation of CO on interstellar dust grains, and is released in the gas phase in hot core regions. Radio-astronomical observations in these regions are directly related to its desorption from grains. Aims. We study experimentally the thermal desorption of H 2 CO from bare silicate surfaces, from water ice surfaces and from bulk water ice in order to model its desorption from interstellar grains. Methods. Temperature-programmed desorption experiments, monitored by mass spectrometry, and Fourier transform infrared spectroscopy are performed in the laboratory to determine the thermal desorption energies in: (i.) the multilayer regime where H 2 CO is bound to other H 2 CO molecules; (ii.) the submonolayer regime where H 2 CO is bound on top of a water ice surface; (iii.) the mixed submonolayer regime where H 2 CO is bound to a silicate surface; and (iv.) the multilayer regime in water ice, where H 2 CO is embedded within a H 2 O matrix. Results. In the submonolayer regime, we find the zeroth-order desorption kinetic parameters 0 = 10 28 mol cm -2 s -1 and E = 31.0 +/-0.9 kJ mol -1 for desorption from an olivine surface. The zeroth-order desorption kinetic parameters are 0 = 10 28 mol cm -2 s -1 and E = 27.1 +/-0.5 kJ mol -1 for desorption from a water ice surface in the submonolayer regime. In a H 2 CO:H 2 O mixture, the desorption is in competition with the H 2 CO + H 2 O reaction, which produces polyoxymethylene, the polymer of H 2 CO. This polymerization reaction prevents the volcano desorption and co-desorption from happening. Conclusions. H 2 CO is only desorbed from interstellar ices via a dominant sub-monolayer desorption process (E = 27.1 +/-0.5 kJ mol -1 ). The H 2 CO which has not desorbed during this sub-monolayer desorption polymerises upon reaction with H 2 O, and does not desorb as H 2 CO at higher temperature.