Disentangling UV photodesorption and photoconversion rates of H2O ice at 20 K

Abstract

Context. The nondissociative ultraviolet photodesorption of water ice is a nonthermal desorption mechanism required to account for detected abundances of gas-phase water toward cold regions within the interstellar medium. Previous experimental and theoretical studies provide a range of photodesorption rates for H2O ice and point to a convoluted competition with other molecular processes following the absorption of a UV photon in the ice. Ultraviolet irradiation also induces photodissociation, resulting in the formation of radicals that may directly desorb triggering gas-phase reactions or recombine in surface reactions. Aims. In this work, we aim to quantify the effects of photodesorption and investigate photoconversion upon UV photolysis of an H2O ice. Methods. We irradiated a porous amorphous H2O ice at 20 K with UV photons in the 7–10.2 eV range and compared the measurements to a nearly identical experiment that included a layer of argon coating on top of the water ice. The purpose of the argon coating is to quench any type of photon-triggered desorption. To trace ice composition and thickness, laser desorption post ionization time-of-flight mass spectrometry was utilized. This method is independent of the (non)dissociative character of a process and provides a diagnostic tool different from earlier studies that allows an independent check. Results. The total photodesorption rate for a porous amorphous H2O ice at 20 K we derive is (1.0 ± 0.2) × 10−3 per incident UV photon (7–10.2 eV), which is in agreement with the available literature. This rate is based on the elemental balance of oxygen-bearing species. As a result, we placed an upper limit on the intact (H2O) and dissociative (OH) desorption rates equal to 1.0 × 10−3 per incident UV photon, while for the reactive desorption (O2), this limit is equal to 0.5 × 10−3 per incident UV photon. Photoconversion depletes the H2O ice at a rate of (2.3 ± 0.2) × 10−3 per incident UV photon. At low UV fluence (9.0 × 1017 photons cm−2), the loss of H2O is balanced by photoproduct formation (O2 and H2O2). At high UV fluence (4.5 × 1018 photons cm−2), about 50% of the initial H2O molecules are depleted. This amount is not matched by the registered O-bearing products, which points to an additional, unaccounted loss channel of H2O.