POSITRON BEAM SPECTROSCOPY OF ICE FILMS

冰膜的正电子束光谱

• 批准号: EP/I016767/1
• 负责人: Paul Coleman
• 金额: $ 30.47万
• 依托单位: University of Bath
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2011
• 资助国家: 英国
• 起止时间: 2011 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FI016767%2F1
• 关键词: POSITRON; BEAM; SPECTROSCOPY; ICE; FILMS

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.

水的丰富和许多不同寻常的性质使它成为几十年来大量实验研究的主题。虽然在地球上大多数固体水是结晶的，但无定形固体水是银河系其他地方最丰富的水相。它的物理性质受到生长条件的强烈影响--例如，沉积在非常冷的表面上的水蒸气形成低密度的无定形冰，据信是彗星、行星环和星际云的主要成分。它也是一个研究深过冷液体的模型系统。在-138摄氏度以上，无定形冰转变为稳定的结晶冰，冰表面的反应性对其结构敏感。因此，物理化学家、天体物理学家、行星科学家和低温生物学家对它的物理和化学性质非常感兴趣。关于冰的基本性质仍然存在争议--例如，它形成玻璃的温度，结晶是从整体开始还是从表面开始，以及沉积冰膜的孔隙率的性质。在最后一点上，如果小于2 nm的气孔是孤立的而不是相互连接的，则可能无法检测到它们。我们计划对尽可能多的这些问题提供新的见解。变能正电子湮没谱(VEPAS)在这些非常有趣的系统中的应用受到了试验性测量的鼓舞，这些测量表明，这项技术可能会发现许多新的现象。正电子-电子的反粒子-被注入到样品中，其深度分布由其入射能量决定，并最终被电子湮灭。湮没辐射线在MC平方处的多普勒展宽是由不同湮没位置的电子运动引起的，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，从丢失的单个原子到丢失多达20个原子的小团簇。此外，在冰膜中可以形成正电子结合态的正电子。如果在直径几纳米或更大的样品中存在气孔或空洞，正电子原子可以在其中停留足够长的时间，从而允许湮没成三条伽马射线--这是我们可以检测到的事件。虽然其他技术已经提供了对表面变化和孔洞性质的一些见解，但预计VEPAS及其正电子和正电子能谱的混合将为在各种条件下生长的一系列冰块的原子结构和孔演化的深度敏感表征提供更直接的方法。尽管这项技术似乎非常适合这些研究，但到目前为止还没有被用来探测冰膜的结构，这使得拟议的测量完全是新奇的。

项目成果

Structure and sublimation of water ice films grown in vacuo at 120-190 K studied by positron and positronium annihilation.

通过正电子和正电子湮灭研究 120-190 K 真空中生长的水冰膜的结构和升华。

• DOI: 10.1088/0953-8984/26/12/125402
• 发表时间: 2014
• 期刊: an Institute of Physics journal
• 作者: Townrow S