Assigning a structural motif using spontaneous molecular dipole orientation in thin films.

Spontaneous orientation of molecular dipoles has been observed to produce bulk electric fields, termed 'spontelectric' fields, in a broad variety of molecular solid thin films formed by condensation from the gas phase. Such spontelectric fields are found in cis-methyl formate (cis-MF) and the present work combines observation of these fields with high quality ab initio studies of cis-MF monomers and dimers. This enables a prediction of the structural motif within the unit cell of the crystalline phase of solid cis-MF, showing it to be a non-polar dimer. Dimer formation at deposition temperatures of >90 K is therefore cited to contribute to the observed collapse of the spontelectric field at these temperatures. This is the first time that such a structural prediction has been made using observations of spontelectric behaviour as a key indicator.

A fibre-coupled UHV-compatible variable angle reflection-absorption UV/visible spectrometer.

We present a novel UV/visible reflection-absorption spectrometer for determining the refractive index, n, and thicknesses, d, of ice films. Knowledge of the refractive index of these films is of particular relevance to the astrochemical community, where they can be used to model radiative transfer and spectra of various regions of space. In order to make these models more accurate, values of n need to be recorded under astronomically relevant conditions, that is, under ultra-high vacuum (UHV) and cryogenic cooling. Several design considerations were taken into account to allow UHV compatibility combined with ease of use. The key design feature is a stainless steel rhombus coupled to an external linear drive (z-shift) allowing a variable reflection geometry to be achieved, which is necessary for our analysis. Test data for amorphous benzene ice are presented as a proof of concept, the film thickness, d, was found to vary linearly with surface exposure, and a value for n of 1.43 ± 0.07 was determined.

Non-linear and non-local behaviour in spontaneously electrical solids.

Using reflection-absorption infrared spectroscopy (RAIRS), we show that solids displaying spontaneous dipole orientation possess quite general non-local and non-linear characteristics, exemplified through their internal electric fields. The most graphic illustration of this, uncovered originally through electron beam studies, may be found in films of cis-methyl formate (cis-MF), for which data demonstrated the counter-intuitive property that the degree of dipole order in the film does not monotonically decrease as the temperature of deposition rises, but rather increases sharply above ∼77 K. Here we show how RAIRS provides independent evidence to support this conclusion. These new data confirm (i) that the behaviour of spontelectrics is governed by an expression for the degree of dipole orientation, which is continuous in temperature, but with a discontinuity in the derivative, and (ii) that the temperature of deposition associated with this discontinuity matches the temperature above which dipole order switches from the expected reduction with temperature to an increase with temperature.

Impact of oxygen chemistry on model interstellar grain surfaces.

Temperature-programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS) are used to probe the effect of atomic and molecular oxygen (O and O2) beams on amorphous silica (aSiO2) and water (H2O) surfaces (porous-amorphous solid water; p-ASW, compact amorphous solid water; c-ASW, and crystalline solid water; CSW). Altering the deposition method of O2 is shown to result in different desorption energies of O2 due to differences in O2 film morphology when deposited on the aSiO2 surface. O2 enthalpy of formation is dissipated into the aSiO2 substrate without changes in the silica network. However, on the H2O surfaces, O2 formation enthalpy release is dissipated into the H-bonded matrix leading to morphological changes, possibly compacting p-ASW into c-ASW while CSW appears to undergo amorphisation. The enthalpy release from O2 formation is, however, not enough to result in reactive desorption of O2 or H2O under the current experimental circumstances. Further to this, O2 formation on sub-monolayer quantities of H2O leads to enhanced de-wetting and a greater degree of H-bond reconnection in H2O agglomerates. Lastly, O3 is observed from the O + O2 reaction on all surfaces studied.

Wannier-Mott Excitons in Nanoscale Molecular Ices.

The absorption of light to create Wannier-Mott excitons is a fundamental feature dictating the optical and photovoltaic properties of low band gap, high permittivity semiconductors. Such excitons, with an electron-hole separation an order of magnitude greater than lattice dimensions, are largely limited to these semiconductors but here we find evidence of Wannier-Mott exciton formation in solid carbon monoxide (CO) with a band gap of >8 eV and a low electrical permittivity. This is established through the observation that a change of a few degrees K in deposition temperature can shift the electronic absorption spectra of solid CO by several hundred wave numbers, coupled with the recent discovery that deposition of CO leads to the spontaneous formation of electric fields within the film. These so-called spontelectric fields, here approaching 4×10^{7} V m^{-1}, are strongly temperature dependent. We find that a simple electrostatic model reproduces the observed temperature dependent spectral shifts based on the Stark effect on a hole and electron residing several nm apart, identifying the presence of Wannier-Mott excitons. The spontelectric effect in CO simultaneously explains the long-standing enigma of the sensitivity of vacuum ultraviolet spectra to the deposition temperature.

Laboratory surface astrochemistry experiments.

Although several research groups have studied the formation of H2 on interstellar dust grains using surface science techniques, few have explored the formation of more complex molecules. A small number of these reactions produce molecules that remain on the surface of interstellar dust grains and, over time, lead to the formation of icy mantles. The most abundant of these species within the ice is H2O and is of particular interest as the observed molecular abundance cannot be accounted for using gas-phase chemistry alone. This article provides a brief introduction to the astronomical implications and motivations behind this research and the requirement for a new dual atomic beam ultrahigh vacuum (UHV) system. Further details of the apparatus design, characterisation, and calibration of the system are provided along with preliminary data from atomic O and O2 beam dosing on bare silica substrate and subsequent temperature programmed desorption measurements. The results obtained in this ongoing research may enable more chemically accurate surface formation mechanisms to be deduced for this and other species before simulating the kinetic data under interstellar conditions.

Laboratory studies of electron and ion irradiation of solid acetonitrile (CH3CN).

The structure and bonding of solid acetonitrile (CH3CN) films on amorphous silica are studied, and chemical and physical processes under irradiation with 200 keV protons and 250-400 eV electrons are quantified using transmission infrared spectroscopy, reflection-absorption infrared spectroscopy and temperature-programmed desorption, with the assistance of basic computational chemistry and nuclear materials calculations. The thermal desorption profiles are found to depend strongly on the balance between CH3CN-surface and CH3CN-CH3CN interactions, passing from a sub-monolayer regime (binding energy: 35-50 kJ mol⁻¹) to a multilayer regime (binding energy: 38.2±1.0 kJ mol⁻¹) via a fractional order desorption regime characteristic of islanding as the coverage increases. Calculations using the SRIM code reveal that the effects of the ion irradiation are dominated by electronic stopping of incident protons, and the subsequent generation of secondary electrons. Therefore, ion irradiation and electron irradiation experiments can be quantitatively compared. During ion irradiation of thicker CH3CN films, a cross section for secondary electron-promoted chemical destruction of CH3CN of 4 (±1) × 10⁻¹8 cm² was measured, while electron-promoted desorption was not detected. A significantly higher cross section for electron-promoted desorption of 0.82-3.2 × 10⁻¹5 cm² was measured during electron irradiation of thinner CH3CN films, while no chemical products were detected. The differences between the experimental results can be rationalized by recognizing that chemical reaction is a bulk effect in the CH3CN film, whereas desorption is a surface sensitive process. In thicker films, electron-promoted desorption is expected to occur a rate that is independent of the film thickness; i.e. show zeroth-order kinetics with respect to the surface concentration.

Thermal desorption of C6H6 from surfaces of astrophysical relevance.

The thermal desorption of C(6)H(6) from two astrophysically relevant surfaces has been studied using temperature programmed desorption. Desorption from an amorphous SiO(2) substrate was used as a mimic for bare interstellar grains, while multilayer films of amorphous solid water (ASW) were used to study the adsorption of C(6)H(6) on grains surrounded by H(2)O dominated icy mantles. Kinetic parameters were obtained through a combination of kinetic modeling, leading edge analysis, and by considering a distribution of binding sites on the substrate. The latter is shown to have a significant impact on the desorption of small exposures of C(6)H(6) from the amorphous SiO(2) substrate. In the case of adsorption on ASW, dewetting behavior and fractional order desorption at low coverage strongly suggest the formation of islands of C(6)H(6) on the H(2)O surface. The astrophysical implications of these observations are briefly outlined.

Meteorite nanoparticles as models for interstellar grains: synthesis and preliminary characterisation.

Dust particles and their interaction with gases play important roles in star formation and in solar nebulae. Appropriate model dust grains are needed for the laboratory simulation of gas-grain interactions. Nanoparticles formed from carbonaceous meteorites may be particularly suitable, as these particles are formed from materials that were formed originally from interstellar/nebula dust. Extending our previous studies with grounded meteorite powders, we demonstrate here the production of nanoparticles formed from meteorites using the laser desorption/controlled condensation method developed in our laboratory. The product nanoparticle aggregates have porous, web-like morphologies similar to interstellar dust grains, indicating that they can present large specific surface areas for gas/grain interactions. In this paper, we present polarisation modulation reflection-absorption infrared spectra (PM-RAIRS) of supported thin films and compare these spectra with the known silicate bands in the spectra of interstellar dust recorded during the ISO mission. We also report an ultrahigh vacuum (UHV) temperature programmed desorption (TPD) study of the adsorption of CO on the supported nanoparticle films. The latter allow us to estimate the CO binding energy on the meteorite nanoparticles as 13.5 +/- 3.0 kJ mol(-1), cf. a value of 9.8 +/- 0.2 kJ mol(-1) for CO binding to a water ice substrate. Such thermochemical data can be useful for computational modelling of gas-grain interactions under the diverse conditions in interstellar clouds and solar nebulae.