The desorption of H2CO from interstellar grains analogues

Context. Much of the formaldehyde (H2CO) 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 H2CO 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 H2CO is bound to other H2CO molecules; (ii.) the submonolayer regime where H2CO is bound on top of a water ice surface; (iii.) the mixed submonolayer regime where H2CO is bound to a silicate surface; and (iv.) the multilayer regime in water ice, where H2CO is embedded within a H2O matrix. Results. In the submonolayer regime, we find the zeroth-order desorption kinetic parameters nu(0) = 10(28) mol cm(-2) s(-1) and E = 31.0 +/- 0.9 kJmol(-1) for desorption from an olivine surface. The zeroth-order desorption kinetic parameters are nu(0) = 10(28) mol cm(-2) s(-1) and E = 27.1 +/- 0.5 kJmol(-1) for desorption from a water ice surface in the submonolayer regime. In a H2CO:H2O mixture, the desorption is in competition with the H2CO + H2O reaction, which produces polyoxymethylene, the polymer of H2CO. This polymerization reaction prevents the volcano desorption and co-desorption from happening. Conclusions. H2CO is only desorbed from interstellar ices via a dominant sub-monolayer desorption process (E = 27.1 +/- 0.5 kJmol-1). The H2CO which has not desorbed during this sub-monolayer desorption polymerises upon reaction with H2O, and does not desorb as H2CO at higher temperature.