Accretion and photodesorption of CO ice as a function of the incident angle of deposition

Non-thermal desorption processes of molecules present in inter- and circum-stellar ice mantles on dust grains, in particular ultraviolet (UV) photon-induced desorption, have gained impor- tance in recent years. These processes may account for the observed gas phase abundances of certain molecules like CO toward cold interstellar clouds. Ice mantle growth results from gas molecules impinging on the dust surface from all directions and incidence angles. Nev- ertheless, the effect of the incident angle for deposition on the photo-desorption rate has not been studied. This work explores the impact on the accretion and photodesorption rates of the incidence angle of CO gas molecules with the cold surface during deposition of a CO ice layer, under mimicked interstellar conditions. Infrared spectroscopy was used to follow the evolution of the ice upon deposition at different angles, UV-irradiation, and subsequent warm-up. Vacuum-UV spectroscopy and a Ni-mesh served to monitor the emission of the UV lamp. The molecules ejected from the ice to the gas phase during irradiation or warm- up were characterized by a quadrupole mass spectrometer. The photodesorption rate of CO ice deposited near 11 K at different incident angles was rather stable between 0 and 45o. A maximum in the photodesorption rate appeared around 70o. A 70o-incidence angle for the deposition leads to maxima in the photodesorption rate of CO ice and the surface area of water ice. Although this study of the surface area could not be performed for CO ice, the similar angle dependence in the photodesorption and the ice surface area suggests that they are closely related. This is supported by literature works reporting the effect of deposition angle in the formation of metallic films. Further evidence for a dependance of CO ice morphology on the deposition angle is provided by the thermal desorption curves of CO ice in our experiments. Astrophysical models that incorporate photo-evaporation of ice mantles can be fine-tuned by taking into consideration the effect of incidence angle during ice build-up in the accretion and photodesorption rates.

MNRAS, 486,5519 (2019)