Ice Desorption Research Articles

Photodesorption of ices – Releasing organic precursors into the gas phase

AbstractA long-standing problem in interstellar chemistry is how molecules can be maintained in the gas phase at the extremely low temperatures in space. Photodesorption has been suggested to explain the observed cold gas in cloud cores and disk mid-planes. We are studying the UV photodesorption of ices experimentally under ultra high vacuum and at astrochemically relevant temperatures (15 – 27 K) using a hydrogen discharge lamp (7-10.5 eV). The ice desorption during irradiation is monitored using reflection absorption infrared spectroscopy and the desorbed species using mass spectrometry. We find that both the UV photodesorption rates and mechanisms are highly molecule specific. CO photodesorbs without dissocation from the surface layer of the ice. N2, which lacks dipole allowed electronic transitions in the range of the lamp, does not photodesorb. CO2 desorbs through dissociation and subsequent recombination from the top few layers of the ice. At low temperatures (15 – 18 K) the derived photodesorption rates are ~ 10−3 for CO and CO2 and < 2 × 10−4 for N2 ice per incident photon.

Fundamental data on the desorption of pure interstellar ices

The desorption of molecular ices from grain surfaces is important in a number of astrophysical environments including dense molecular clouds, cometary nuclei and the surfaces and atmospheres of some planets. With this in mind, we have performed a detailed investigation of the desorption of pure water, pure methanol and pure ammonia ices from a model dust-grain surface. We have used these results to determine the desorption energy, order of desorption and the pre-exponential factor for the desorption of these molecular ices from our model surface. We find good agreement between our desorption energies and those determined previously; however, our values for the desorption orders, and hence also the pre-exponential factors, are different to those reported previously. The kinetic parameters derived from our data have been used to model desorption on time-scales relevant to astrophysical processes and to calculate molecular residence times, given in terms of population half-life as a function of temperature. These results show the importance of laboratory data for the understanding of astronomical situations whereby icy mantles are warmed by nearby stars and by other dynamical events.

Chemistry as a Probe of the Structure and Processes in Massive YSO Envelopes

The distribution and composition of the dust and gas surrounding Young Stellar Objects (YSOs) is of continuing interest. Fortunately, rapid advances in observational capa- bilities have led to data of high spatial and spectral resolution, as well as the opportunity to observe in previously unavailable windows using satellites. Such high-quality data have moti- vated enhancements in theoretical models. Key to these models is the chemical evolution of the gas. Since the chemical evolution depends upon temperature, density, and time, the state and history of the source is encoded in the spatial distribution of the chemical abundances. It is possible, using both parametric and detailed physical-chemical modeling, to constrain many source properties, and identify potential reactions of further laboratory interest. Using specific examples, I discuss some successes toward constraining the source properties, as well as challenges posed by current problems. Finally, I discuss the potential effect of infall dynamics and recent laboratory measurements of temperature programmed desorption of ices from grains on inferring source properties.

Laboratory investigations of the role of the grain surface in astrochemical models

The rich chemistry often detected in star forming regions is now recognized to be a consequence of solid-state astrochemistry and the thermal desorption of its products. In recent experimental studies, desorption of a range of ices from a gold surface was investigated using temperature programmed desorption (TPD). These data were then used in astrochemical models. In this paper we investigate the sensitivity of these models to the inclusion of TPD data obtained from different surfaces (simulating different dust grains) and different thicknesses of the icy mantles. Detailed laboratory TPD studies of the desorption of ices from a highly oriented pyrolytic graphite (HOPG) surface have been performed. Desorption temperatures and kinetic parameters have been determined directly from the TPD data and have been used to determine the expected desorption temperature for the ices from grain surfaces. The results of these experiments have been incorporated into astrochemical models of high mass star forming regions and have then been compared with the results of previous experiments. From this comparison, we are able to determine whether the nature and composition of the grain surface is important in dictating the chemistry that occurs in star forming regions.

Formation of large water clusters by IR laser resonant desorption of ice

Large H 3O +(H 2O) n clusters, up to n ≈ 100, were produced by IR laser resonant desorption of an ice matrix using an optical parametric oscillator (OPO) at 3.1 μm. The velocity distribution has been analyzed to characterize the laser–sample interaction and the plume dynamics desorption. The velocity distribution curves of the clusters show two distinct components corresponding to a phase explosion regime followed by vaporization. We discuss the shift of these curves with mass and the formation mechanism. A distinction in the desorption process is made between small and large clusters.

Resonant desorption of ice with a tunable LiNbO 3 optical parametric oscillator

We report on the use of a pulsed tunable mid-infrared optical parametric oscillator (OPO) for the IR resonant desorption of ice in the 3 μm range. Desorbed neutrals are detected by UV multi-photon ionization and time-of-flight mass spectrometry. Desorption yield is found to increase linearly with the OPO pulse energy in the 0–3 mJ range. The resonant character of the desorption process is very clearly evidenced by the study of the wavelength dependency of the desorption yield. The desorption spectrum peaks at 3.1 μm and is in remarkable agreement with the absorption spectrum of crystalline ice at T=100 K. This result differs from previous experiments using NIR macro-pulses of a free electron laser. The velocity distribution of the desorption products reveals the presence of at least two components. The fast component is consistent with our previous results obtained on doped ices and indicates a phase explosion process. The observation of high- n (H 3O) +(H 2O) n clusters is also reported.

The water ice distribution in Taurus determined by gas–grain chemistry

The detection of water ice in interstellar grain mantles by observation of absorbed radiation from field stars through the Taurus molecular cloud complex has revealed that the column density of water ice increases linearly,or nearly linearly with visual extinction except at the lowest values (below A(V) approximate to 3). Previous explanations of the small or negligible abundance of ice at low extinction have focused on the destruction and desorption of ice by radiation, and have ignored the efficiency of formation of ice. To understand the dependence of the ice column density on A(V) in more detail and to understand what it tells us about the Taurus dark cloud, we have run gas-grain chemical models under a variety of physical conditions. For higher values of extinction, our models predict that much of the elemental abundance of available oxygen is in the form of ice for evolutionary times greater than about 10(5) yr, and this leads to the observed near-linear relationship between ice column density and A(V). For lower extinction, our models show that if any significant amount of ice is still present, its existence can be attributed to great cloud age, to our poor understanding of dust-grain surfaces, or to cloud clumpiness. The first and third explanations pertain only if photodesorption of ice is inefficient.

Silver nanoparticles synthesized by vapor deposition onto an ice matrix

As a strategy for synthesizing metal nanoparticles, thermally evaporated Ag was deposited onto a thin (∼1.2 nm) crystalline ice layer on hafnia (HfO2) at 100 K. The Ag atoms penetrate into the ice matrix but do not reach the underlying HfO2 substrate. After controlled thermal desorption of water by heating to 300 K, atomic force microscopy reveals Ag particle formation. Their lateral dimensions are between 5 and 20 nm and, in many cases, their heights exceed the thickness of the original water layer. Fewer, higher and more regular Ag particles are formed in the presence, as compared to the absence, of ice. This is discussed in terms of two factors – Ag atoms reaching HfO2 are thermally colder when they arrive from the ice matrix and desorption of water involves formation of liquid droplets, a process that concentrates Ag into the volumes occupied by the water droplets.

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, E_{des}, 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.

On the Abundance Gradients of Organic Molecules along the TMC‐1 Ridge

Gradients in molecular abundances along the TMC-1 ridge have been observed by several authors, most recently in a comprehensive study by Pratap et al. These can be explained by there being a difference in density, C/O ratio, or chemical evolutionary state along the ridge. The presence at the carbon-rich cyanopolyyne peak (CP) of specific oxygen-bearing molecules, believed to be products of grain surface reactions, leads us to investigate the possibility that the gradients are produced following the removal of icy grain mantles. We identify the mantle removal mechanism as the explosive desorption of photolyzed ices, induced by MHD waves as they propagate within the cloud. By identifying the protostellar object IRAS 04381+2540 as the source of these waves, we have constructed a dynamical-chemical model for the evolution of the TMC-1 ridge. The results of detailed chemical kinetic calculations are presented. We find that injection into the gas phase of the carbon-bearing species C2H2, C2H4 and CH4 can transiently enhance the production of many organic molecules, particularly the cyanopolyynes and polyacetylenes, and hence that the observed gradients can indeed be produced in this model. We suggest that other mantle-driven reaction pathways, involving formation of methylated chains (e.g., CH3C3N) by gas phase methyl cation transfer, as well as formation of carbenes and suboxides by fragmentation of surface-formed cumulenone compounds, might explain the presence of many other organic molecules in TMC-1. In our view the current chemical state of TMC-1 reflects a transient phase that accompanies the earliest epochs of low-mass formation and that regions rich in organic molecules should be observable around known Class 0 protostellar sources and be offset from them by about an ion-neutral damping length. Molecular clumps in dense cores should also show structure on this scale-length.

Impact of nitric acid on ice evaporation rates

Recent studies have suggested that nitric acid uptake by ice clouds may decrease ice evaporation rates and thereby prolong the cloud lifetimes. To test this suggestion, ice desorption rates were studied as a function of HNO3 partial pressure (10−6–10−5 Torr), relative humidity (28–92%), and temperature (192–204 K) using optical interference of a helium neon laser. Ice evaporation rates in the presence of 1 × 10−6 Torr HNO3 were indistinguishable from those of pure ice. In contrast, ice evaporation in the presence of 8 × 10−6 Torr HNO3 resulted in lower evaporation rates by 33% relative to pure ice. Higher partial pressures of HNO3 result in a supercooled H2O/HNO3 liquid layer over ice, which may freeze to form a sealed NAT coating. This causes a lowering of the ice evaporation rate and prolongs the lifetime of ice. Ice exposed to lower partial pressures of HNO3 will not form a liquid layer and will thus evaporate at the same rate as pure ice.

Water on Ni(110): Resolution of monolayer, bilayer, and ice layers by thermal desorption, Δφ, and associated absolute coverages

The various adsorbed states of D2O on Ni(110) have been resolved by thermal desorption and simultaneous Δφ measurements, permitting determination of the effective dipole moment in each state. The states detected by Madey and co-workers are all confirmed but an additional state at the end of the ice-desorption region is clearly resolved by Δφ methods. The three higher temperature desorption states (T>ice layer desorption temperature) can be selectively saturated by water exposure at 180 K. The absolute coverage of water in these states was then determined by nuclear reaction analysis to be 5.3×1014 water molecules cm−2. This coverage is significantly lower than the coverage proposed by Madey and co-workers for the bilayer structure (1.1×1015 mol cm −2). The state at the end of the ice desorption region can also accommodate 5–6×1014 mol cm−2 and may result from water bonded mostly to other water molecules, with weak residual bonding to Ni.