Martin R. S. McCoustra

Evaporation of ices near massive stars: models based on laboratory temperature programmed desorption data

Hot cores and their precursors contain an integrated record of the physics of the collapse process in the chemistry of the ices deposited during that collapse. In this paper, we present results from a new model of the chemistry near high-mass stars in which the desorption of each species in the ice mixture is described as indicated by new experimental results obtained under conditions similar to those in hot cores. Our models show that provided there is a monotonic increase in the temperature of the gas and dust surrounding the protostar, the changes in the chemical evolution of each species due to differential desorption are important. The species H2S, SO, SO2, OCS, H2CS, CS, NS, CH3OH, HCOOCH3 ,C H 2CO, C2H5OH show a strong time dependence that may be a useful signature of time evolution in the warm-up phase as the star moves on to the main sequence. This preliminary study demonstrates the consequences of incorporating reliable temperature programmed desorption data into chemical models. Ke yw ords: stars: formation ‐ ISM: abundances ‐ ISM: clouds ‐ ISM: molecules.

Thermal desorption of water ice in the interstellar medium

Water (H2O) ice is an important solid constituent of many astrophysical environments. To comprehend the role of such ices in the chemistry and evolution of dense molecular clouds and comets, it is necessary to understand the freeze-out, potential surface reactivity, and desorption mechanisms of such molecular systems. Consequently, there is a real need from within the astronomical modelling community for accurate empirical molecular data pertaining to these processes. Here we give the first results of a laboratory programme to provide such data. Measurements of the thermal desorption of H2O ice, under interstellar conditions, are presented. For ice deposited under conditions that realistically mimic those in a dense molecular cloud, the thermal desorption of thin films (�50 molecular layers) is found to occur with zero order kinetics characterised by a surface binding energy, Edes, of 5773 ±60 K, and a pre-exponential factor, A, of 10 30±2 molecules cm 2 s 1 . These results imply that, in the dense interstellar medium, thermal desorption of H2O ice will occur at significantly higher temperatures than has previously been assumed.

Carbon Monoxide Entrapment in Interstellar Ice Analogs

The adsorption and desorption of CO on and from amorphous H2O ice at astrophysically relevant temperatures has been studied using temperature programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS). Solid CO is able to diffuse into the porous structure of H2O at temperatures as low as 15 K. When heated, a phase transition between two forms of amorphous H2O ice occurs over the 30-70 K temperature range, causing the partial collapse of pores and the entrapment of CO. Trapped CO is released during crystallization and desorption of the H2O film. This behavior may have a significant impact on both gas-phase and solid-phase chemistry in a variety of interstellar environments.

Liquid water in the domain of cubic crystalline ice Ic

Vapor-deposited amorphous water ice when warmed above the glass transition temperature (120-140 K), is a viscous liquid which exhibits a viscosity vs temperature relationship different from that of liquid water at room temperature. New studies of thin water ice films now demonstrate that viscous liquid water persists in the temperature range 140-210 K. where it coexists with cubic crystalline ice. The liquid character of amorphous water above the glass transition is demonstrated by (1) changes in the morphology of water ice films on a nonwetting surface observed in transmission electron microscopy (TEM) at around 175 K during slow warming, (2) changes in the binding energy of water molecules measured in temperature programmed desorption (TPD) studies, and (3) changes in the shape of the 3.07 micrometers absorption band observed in grazing angle reflection-absorption infrared spectroscopy (RAIRS) during annealing at high temperature. whereby the decreased roughness of the water surface is thought to cause changes in the selection rules for the excitation of O-H stretch vibrations. Because it is present over such a wide range of temperatures, we propose that this form of liquid water is a common material in nature. where it is expected to exist in the subsurface layers of comets and on the surfaces of some planets and satellites.

Laboratory studies of the interaction of carbon monoxide with water ice

The interaction of carbon monoxide (CO) with vapour-deposited water(H2O) ices has been studied using temperature programmed desorption (TPD) and Fourier transform reflection-absorption infrared spectroscopy (FT-RAIRS) over a range of astrophysically relevant temperatures. Such measurements have shown that CO desorption from amorphous H2Oices is a much more complex process than current astrochemical models suggest. Re-visiting previously reported laboratory experiments (Collings et al., 2003), a rate model has been constructed to explain, in a phenomenological manner, the desorption of CO over astronomically relevant time scales. The model presented here can be widely applied to a range of astronomical environments where depletion of CO from the gas phase is relevant. The model accounts for the two competing processes of CO desorption and migration, and also enables the entrapment of some of the CO in the ice matrix and its subsequent release as the water ice crystallises and then desorbs. The astronomical implications of this model are discussed.

Adsorption of stratospherically important molecules on thin D2O ice films using reflection absorption infrared spectroscopy

The adsorption of DCl, CCl4, CFCl3, CF2Cl2 and CF3Cl on thin D2O ice films at 110 K has been studied using reflection–absorption infrared spectroscopy (RAIRS). These compounds interact to varying degrees with the ice surface via the OD bonds dangling into the vacuum. DCI was found to dissociate ionically, evidenced by a strong band attributable to D3O+ and the absence of any absorption band for molecular DCl.

Dissociative adsorption of methane on Pt(111) induced by hyperthermal collisions

Measurements have been made of the dissociative adsorption of methane on a Pt(111) surface at surface temperatures of 150 and 550 K using hyperthermal beam techniques. Sticking coefficient measurements made at 550 K are in close agreement with existing literature data. Measurements made at 150 K extend our understanding of the dissociative sticking of methane into a new, low-temperature regime. Reflection–absorption infrared (RAIR) spectral measurements made at 150 K definitively identify the primary product of the dissociative adsorption as the methyl (CH3) moiety. The thermal evolution of this moiety has subsequently been followed by RAIRS and temperature-programmed desorption (TPD). The observation of the C2 ethylidyne moiety at temperatures above 300 K provides clear evidence for the formation of C—C bonds during the thermal chemistry of the methyl moiety.

Laboratory surface astrophysics experiment

In this article we describe the design and construction of a laboratory astrophysics experiment that recreates the harsh conditions of the Interstellar Medium (ISM) and is used to study the heterogeneous chemistry that occurs there. The Nottingham Surface Astrophysics Experiment is used to determine, empirically, accurately, and usually for the first time, key physical and chemical constants that are vital for modeling and understanding the ISM. It has been designed specifically to investigate gas–solid interactions under interstellar conditions. The pressure regime is ideally matched to molecular densities in dusty disks in protostellar or protoplanetary regions. The ultrahigh vacuum system is routinely capable of obtaining pressures that are only three orders of magnitude above those in the ISM, with similar relative concentrations of the two most abundant gases in such regions, H2 and CO, and an absence of any other major gas components. A short introduction describes the astronomical motivation behind this experiment. In Sec. II we then give details of the design, construction, and calibration of each component of the experiment. The cryostat system has far exceeded design expectations, and reaches temperatures between 7 and 500 K. This is comparable with the ISM, where dust temperatures from 10 K have been observed. Line-of-sight mass spectrometry, reflection absorption infrared spectroscopy, and quartz crystal microbalance mass measurements were combined into a single instrument for the first time. The instrument was carefully calibrated, and its control and data acquisition system was developed to ensure that experimental parameters are recorded as accurately as possible. In Sec. III we present some of the experimental results from this system that have not been published elsewhere. The results presented here demonstrate that the system can be used to determine desorption enthalpies, ΔdesH, bonding systems, and sticking probabilities between a variety of gases and ices common to the ISM. This instrument will greatly facilitate our understanding of surface processes that occur in the ISM, and allow us to investigate “mimic” ISM systems in a controlled environment. In this article we illustrate that laboratory surface astrophysics is an exciting and emerging area of research, and this instrument in particular will have a major impact through its contributions to both surface science and astronomy.

Adsorption and ionization of HCl on an ice surface

In this article, we describe a series of experiments investigating the interaction of the important stratospheric reservoir species HCl with the surface of a thin ice film. Reflection absorption infrared spectroscopy and thermal desorption spectroscopy have been used to identify the nature of the ionic hydrates formed under a variety of pressure, temperature, and exposure regimes.

DESORPTION OF HOT MOLECULES FROM PHOTON IRRADIATED INTERSTELLAR ICES

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 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.