Photodesorption from low-temperature water ice in interstellar and circumsolar grains

Abstract

Dust grains in the interstellar medium and the outer Solar System commonly have a coating of water ice, which affects their optical properties and surface chemistry. The thickness of these icy mantles may be determined in part by the extent of photodesorption (photosputtering) by background ultraviolet radiation. But this process is poorly understood, with theoretical estimates of the photodesorption rate spanning several orders of magnitude. Here we report measurements of the absolute ultraviolet photodesorption yield of low-temperature water ice. Our results indicate that the rate of photodesorption is appreciable. In particular, it can account for the absence of icy mantles on grains in diffuse interstellar clouds, it exceeds solar-wind ion erosion and sublimation in the outer Solar System, and it is important in determining the lifetimes of icy mantles in dense molecular clouds.

CO$\mathsf{_{2}}$ synthesis in solid CO by Lyman-α photons and 200 keV protons

“…A microwave-discharge hydrogen lamp produced Lyman-α radiation at 10.2 eV, which enters the vacuum chamber through an MgF 2 window 1 . To increase the light flux on the sample, we used a coaxial aluminum cylinder to reflect light onto the sample (Westley et al 1995a). With this setup, the photon incidence is within a few degrees of the normal to the surface.…”

“…First, the large band-gap of solid CO makes the ionization channel energetically unavailable for Lyman-α photolysis, and previous experiments ) gave an extremely small cross section for CO 2 photosynthesis in CO ∼ 10 −20 cm 2 . Second, previous studies revealed that erosion yields from water ice by fast protons (Brown et al 1984) are much larger than by Lyman-α (Westley et al 1995a) due to a dominant double collision channel only available to ions, which is manifested in a quadratic dependence of the sputtering yield on electronic stopping power (S e ; Baragiola et al 2003). The erosion of CO is expected to be analogous since in this case the sputtering yield is also proportional to S 2 e (Brown et al 1984).…”

SPIRE spectroscopy of the prototypical Orion Bar photodissociation region

“…H 2 O is extremely sensitive to the local physical conditions in molecular clouds: close to the surface, molecules are photodissociated, while deeper into the cloud molecules freeze onto grain surfaces (i.e., Hollenbach et al 2009). Desorption of ices (Westley et al 1995;Seperuelo Duarte et al 2009) could supply gas-phase species. The high abundances of sulphur species remain an interesting puzzle for interstellar chemistry (i.e., Goicoechea et al 2006).…”

Controlling the emission profile of an H2discharge lamp to simulate interstellar radiation fields

“…For the last two decades, microwave discharge hydrogen-flow lamps (MDHLs;Warneck 1962;Davis & Braun 1968) have been widely used as vacuum-ultraviolet (VUV) light sources in laboratory experiments to simulate interstellar radiation fields in astrochemical studies (Westley et al 1995;Gerakines et al 1996;Baratta et al 2002;Muñoz Caro et al 2002, 2010Cottin et al 2003;Leto & Baratta 2003;Loeffler et al 2005;Watanabe et al 2007;Öberg et al 2007;Islam et al 2014;Cook et al 2014;Henderson & Gudipati 2015). These experiments aim to simulate and ultimately unravel the complex photoprocesses (photodesorption, photodissociation, and photochemistry) taking place in interstellar ice analogues.…”