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

## TL;DR

This study investigates how the angle at which CO molecules deposit onto cold surfaces affects the growth and photodesorption rates of CO ice, revealing a maximum photodesorption at around 70 degrees incidence angle.

## Contribution

It is the first to analyze the effect of incident angle on CO ice photodesorption and surface morphology, providing new insights into interstellar ice processes.

## Key findings

- Photodesorption rate peaks at ~70° incident angle.
- Deposition angle influences ice surface area and morphology.
- Photodesorption stability observed between 0° and 45°.

## Abstract

Non-thermal desorption of inter- and circum-stellar ice mantles on dust grains, in particular ultraviolet photon-induced desorption, has gained importance in recent years. These processes may account for the observed gas phase abundances of molecules like CO toward cold interstellar clouds. Ice mantle growth results from gas molecules impinging on the dust from all directions and incidence angles. Nevertheless, the effect of the incident angle for deposition on ice 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. Infrared spectroscopy monitored CO ice upon deposition at different angles, ultraviolet-irradiation, and subsequent warm-up. Vacuum-ultraviolet spectroscopy and a Ni-mesh measured the emission of the ultraviolet lamp. Molecules ejected from the ice to the gas during irradiation or warm-up were characterized by a quadrupole mass spectrometer. The photodesorption rate of CO ice deposited at 11 K and different incident angles was rather stable between 0 and 45$^{\circ}$. A maximum in the CO photodesorption rate appeared around 70$^{\circ}$-incidence deposition angle. The same deposition angle leads to the maximum 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. Further evidence for a dependence of CO ice morphology on deposition angle is provided by thermal desorption of CO ice experiments.