POSITRON BEAM SPECTROSCOPY OF ICE FILMS

水的丰富和许多不寻常的性质使它成为几十年来大量实验研究的主题。在地球上，大多数固体水是结晶的，而在银河系的其他地方，无定形固体水是水的最丰富的阶段。它的物理性质受到生长条件的强烈影响——例如，水汽沉积在非常寒冷的表面上，形成低密度的无定形冰，被认为是彗星、行星环和星际云的主要组成部分。它也是研究深度过冷液体的模型系统。在-138℃以上，非晶冰转变为稳定的结晶冰，冰表面的反应性对其结构很敏感。因此，它的物理和化学性质引起了物理化学家、天体物理学家、行星科学家和低温生物学家的极大兴趣。关于冰的基本性质仍然存在争议，例如，它形成玻璃的温度，结晶是在整体还是在表面开始，以及沉积冰膜的多孔性的性质。在最后一点上，小于2nm的孔隙如果是孤立的而不是相互连接的，可能无法被检测到。我们计划对尽可能多的这些问题提供新的见解。可变能量正电子湮没光谱（VEPAS）在这些非常有趣的系统中的应用受到了先导测量的鼓舞，这表明该技术可能会发现许多新的现象。一个正电子——电子的反粒子——被植入一个深度分布由其入射能量决定的样品中，并最终被电子湮灭。mc²处湮灭辐射线的多普勒展宽是由电子在各种湮灭位点的运动引起的，因此与材料的每个结构特征有关。正电子对材料中的开放体积点缺陷高度敏感，范围从缺失单个原子到多达20个缺失原子的小簇。此外，在冰膜中可以形成正电子-电子结合态正电子。如果在直径为几纳米或更大的样品中存在孔隙或空洞，正电子原子可以在其中停留足够长的时间，从而湮灭成三种伽马射线——这是我们可以探测到的事件。孔隙越大或连通程度越高，Ps衰变成三伽马的比例就越大，这就为这些相对较大的开放体积提供了一个敏感的探针。虽然其他技术已经提供了一些关于表面变化和孔隙性质的见解，但预计VEPAS -其正电子和正电子离子光谱的混合物-将为在各种条件下生长的一系列冰的原子结构和孔隙演化的深度敏感表征提供更直接的方法。这项技术，虽然看起来非常适合这些研究，但迄今为止还没有被用来探测冰膜的结构，这使得提出的测量方法完全新颖。

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.