Positron Beam Spectroscopy of Ice Films

Project: Research council

Description

The abundance and many unusual properties of water have made it the subject of a large number of experimental investigations over many decades. While on Earth most solid water is crystalline, amorphous solid water is the most abundant phase of water elsewhere in the galaxy. Its physical properties are strongly influenced by growth conditions - for example, water vapour deposited on a very cold surface forms low-density amorphous ice, believed to be a major component of comets, planetary rings, and interstellar clouds. It is also a model system for studying deeply supercooled liquids. Above -138 degrees C amorphous ice transforms to stable crystalline ice, and the reactivity of the ice surface is sensitive to its structure. Consequently, its physical and chemical properties are of considerable interest to physical chemists, astrophysicists, planetary scientists, and cryobiologists. There is still controversy about the fundamental properties of ice - for example, temperature at which it forms a glass, whether crystallization begins in the bulk or at the surface, and the nature of the porosity of deposited ice films. On this last point, pores smaller than 2nm may not be detected if they are isolated rather than interconnected. We plan to provide new insights into as many of these issues as possible. The application of Variable-Energy Positron Annihilation Spectroscopy (VEPAS) to these profoundly interesting systems has been encouraged by pilot measurements which indicated that there may be many new phenomena to be uncovered by this technique.A positron - the anti-particle of the electron - is implanted into a sample with a depth distribution determined by its incident energy and is eventually annihilated by an electron. Doppler broadening of the annihilation radiation line at mc-squared, is caused by the motion of the electrons at the various annihilation sites and is thus associated with each structural feature of the material. Positrons are highly sensitive to open volume point defects in a material, ranging from missing single atoms to small clusters of up to ~20 missing atoms. Further, in ice films the positron-electron bound state positronium can be formed. If there are pores or cavities in the sample of diameters of a few nm or more, positronium atoms can reside in them for periods long enough to allow annihilation into three gamma rays to occur - events which we can detect. The larger or more interconnected the pores, the greater the fraction of Ps decaying into three gammas, providing a sensitive probe of these relatively large open volumes.While other techniques have provided some insights into surface changes and pore properties, it is expected that VEPAS - with its mixture of positron and positronium spectroscopies - will provide a more direct method for the depth-sensitive characterization of atomic structure and of pore evolution for a range of ices grown under a variety of conditions. The technique, although appearing to be perfectly suited to these studies, has not been used to date to probe the structure of ice films, making the proposed measurements entirely novel.
StatusFinished
Effective start/end date1/08/1131/01/14

Funding

  • Engineering and Physical Sciences Research Council

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positrons
ice
spectroscopy
porosity
positronium
positron annihilation
water
electrons
physical properties
planetary rings
cold surfaces
atoms
probes
comets
atomic structure
chemical properties
point defects
water vapor
energy
reactivity