The desorption of H₂CO from interstellar grains analogues

¹ Aix-Marseille Univ, PIIM UMR 7345, 13397 Marseille, France CNRS, PIIM UMR 7345, 13397 Marseille, France
e-mail: patrice.theule@univ-amu.fr
² LERMA-LAMAp, UMR 8112 du CNRS, Observatoire de Paris et Université de Cergy-Pontoise, 5 Mail Gay-Lussac, 95000 Cergy Pontoise, France

Received: 18 April 2012
Accepted: 18 May 2012

Abstract

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 ν₀ = 10²⁸ 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 ν₀ = 10²⁸ 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 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 kJ mol^-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.