Desorption of Hot Molecules from Photon Irradiated Interstellar Ices

Abstract

We present experimental measurements of photodesorption from ices of astrophysical relevance. Layers of benzene and water ice were irradiated with a laser tuned to an electronic transition in the benzene molecule. The translational energy of desorbed molecules was measured by time-of-flight (ToF) mass spectrometry. Three distinct photodesorption processes were identified -a direct adsorbate-mediated desorption producing benzene molecules with a translational temperature of around 1200 K, an indirect adsorbate-mediated desorption resulting in water molecules with a translational temperature of around 450 K, and a substrate-mediated desorption of both benzene and water producing molecules with translational temperatures of around 530 K and 450 K respectively. The translational temperature of each population of desorbed molecules is well above the temperature of the ice matrix. The implications for gas-phase chemistry in the interstellar medium are discussed.Subject Headings: astrochemistry ---ultraviolet (ISM) ---ISM: molecules ---comets: general --methods: laboratory

Computational Study of the Interactions between Benzene and Crystalline Ice Ih: Ground and Excited States

“…Experimental studies on photo processing of model interstellar ices have revealed three distinct photo-desorption mechanisms in such systems; namely,( 1) direct adsorbate-mediated desorption of benzene, (2) indirect adsorbate-mediated desorption of water,a nd (3) substrate-mediated desorptiono fb oth benzene and water. [23] The translational temperature of both desorbed species, that is, the benzene and the water molecules, is found to be higher than the ambient temperature of the complex system.I th as been observed experimentally that photon absorptionb yb enzene can make H 2 Od esorption possible at wavelengths where the photon-absorption cross-section for H 2 Oi sn egligible. [22] Ar ecent experimental study on the photodesorption of benzene from H 2 Oi ce has shown as trong dependence of desorption of both Bz and water on the morphology of the ice.…”

“…[18,19] Recently there has also been increased interest in the UV processing of laboratory models of interstellar ices. [20][21][22][23][24][25][26][27][28][29] Benzene (Bz) has been detected in the proto-planetary nebula CRL6 18, [30] and is chosen as ar epresentative of complex PAHm olecules within ice mantles, whereas crystalline water ice (I h )s tructures are chosen to model water-dominated interstellar icy mantles.U nder ambient conditions, hexagonal crystal ice ist he dominant solid form of H 2 O. Experimental and computational studies have been performed to understand the effect of temperature on the crystalline ice surfaces that leads to proton ordering-disordering at the surface, which also affects the interactions between the ice crystal surface and different adsorbates.…”

Laboratory investigations of the interaction between benzene and bare silicate grain surfaces

“…Furthermore to aid in interpreting the results of irradiation experiments, it is necessary to understand the nature of the pre‐irradiated ice. For example, in our recently reported experimental measurements of the photodesorption of C 6 H 6 from H 2 O ice (Thrower et al 2008), the desorption of both C 6 H 6 and H 2 O was found to depend strongly on the morphology of the ices. It is clear that a more detailed understanding of the interactions between both C 6 H 6 and H 2 O and the underlying substrate is required.…”

Laboratory studies of desorption from model interstellar ices using surface science methodologies

“…Photon absorption by molecules present in the ice (adsorbate-or absorbate-mediated, whether at the surface or in the bulk) and by the H 2 O solid itself (substrate-mediated) can result in desorption. Recent work in which layered C 6 H 6 -H 2 O systems were irradiated with photons at around 250 nm demonstrates this through observation of the desorption of C 6 H 6 and H 2 O molecules, and (H 2 O) x clusters (Thrower et al 2008a(Thrower et al ,b, 2010. This wavelength corresponds to the lowest lying electronic transitions of C 6 H 6 .…”