Victoria L. Frankland

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.

The uptake of HO2 on meteoric smoke analogues

The kinetics of heterogeneous HO2 uptake onto meteoric smoke particles (MSPs) has been studied in the laboratory using analogues of MSP aerosol entrained into a flow tube. The uptake coefficient, γ, was determined on synthetic amorphous olivine (MgFeSiO4) to be (6.9 1.2) × 10 2 at a relative humidity (RH) of 10%. On forsterite (Mg2SiO4), γ= (4.3 0.4) × 10 3 at RH= 11.6% and (7.3 0.4) × 10 2 at RH= 9.9% on fayalite (Fe2SiO4). These results indicate that Fe plays a more important mechanistic role than Mg in the removal ofHO2 from thegasphase. Electronic structure calculations show that Featomsexposedat theparticle surface provide a catalytic sitewhereHO2 is converted toH2O2 via an Eley-Ridealmechanism, but this does not occur on exposed surfaceMg atoms. The impact of this heterogeneous process in themiddle atmosphere was then investigated using a whole atmosphere chemistry-climate model which incorporates a microphysical treatment of MSPs. Using a global MSP production rate from meteoric ablation of 44 t/day, heterogeneous uptake (with γ=0.2) on MSPs significantly alters the HOx budget in the nighttime polar vortex. This impact is highly latitudedependent and thus couldnotbe confirmedusing currently available satellitemeasurements of HO2, which are largely unavailable at latitudes greater than 70°.