Disentangling UV photodesorption and photoconversion rates of H 2 O ice at 20 K: Measured with laser desorption post ionization mass spectrometry

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 H 2 O 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 H 2 O ice. Methods. We irradiated a porous amorphous H 2 O 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 H 2 O 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 (H 2 O) and dissociative (OH) desorption rates equal to 1.0 × 10 −3 per incident UV photon, while for the reactive desorption (O 2 ), this limit is equal to 0.5 × 10 −3 per incident UV photon. Photoconversion depletes the H 2 O ice at a rate of (2.3 ± 0.2) × 10 −3 per incident UV photon. At low UV fluence (9.0 × 10 17 photons cm −2 ), the loss of H 2 O is balanced by photoproduct formation (O 2 and H 2 O 2 ). At high UV fluence (4.5 × 10 18 photons cm −2 ), about 50% of the initial H 2 O molecules are depleted. This amount is not matched by the registered O-bearing products, which points to an additional, unaccounted loss channel of H 2 O.