Sputtering, Radiolysis, and Photolysis: Physical Mechanisms and Astrophysical Applications

高能带电粒子、紫外线光子和x射线改变了许多天体物理环境中的物质。这种辐射可以改变冰和其他低温分子固体的组成，并引起原子和分子的喷射。因此，由光子在材料中引起的化学反应称为光解，而由带电粒子产生的化学反应称为辐射分解，这两种化学反应都与气相物质的产生有关，通常称为解吸或溅射。紫外光子、x射线和宇宙射线（CR）离子维持了在星际介质（ISM）区域、年轻恒星物体（YSOs）周围的圆盘以及最近发现的土星环上的氧气大气中观察到的气相物质。一个巨大的环面大气是由土星内部磁层的颗粒溅射产生的。木卫二和木卫三表面的化学成分发生了变化，并且有由高能离子和电子产生的重力束缚的氧气大气层。木卫二上可能存在地下海洋，这对于描述其相对年轻的表面上氧化剂的辐射分解作用具有重要意义。为了理解这些令人兴奋的观测结果，有必要开发出可供天文学家使用的溅射、辐射分解和光分解模型。由于表面和环境气体通过辐射处理耦合，因此将使用材料科学中最新的计算工具作为指导构建实用模型。有了这样的模型，天体物理环境中的气体或等离子体观测可以用来确定嵌入表面材料的组成。相反，对表面材料组成的观察可以用来预测周围的中性和等离子体环境。虽然在许多空间环境中发生的物理和化学过程的共性是本研究的重点，但这些模型将应用于土星磁层中的表面-等离子体耦合，木卫二表面的辐射，以及YSOs盘中x射线诱导的气体生产。分子固体辐射处理的基本物理和化学将被描述。由于广泛的材料和辐射类型（x射线、CR离子、太阳等离子体和磁流等离子体）需要大量的数据，因此需要一个研究项目来总结现有的数据，然后使用材料科学中的现代计算工具来开发适用的模型，最后将新模型应用于一些局部问题。这项工作把天文学和材料科学的问题结合在一起。由于强调基础物理和化学，所开发的模型引起了广泛科学界的兴趣。事实上，这项研究是由理解天体物理观测的需要所驱动的，它已经对分子材料辐射修饰的物理和化学产生了影响，并改变了入门材料课程的教学。由于其跨学科性质，这项研究将由弗吉尼亚大学工程物理专业的学生进行。这是一个典型的跨学科研究生项目，与邻近的本科院校和主要为少数民族服务的高中有很强的互动。＊＊＊

AST 0507030JohnsonEnergetic charged particles, UV photons and x-rays alter materials in a number of astrophysical environments. Such radiations can change the composition of ices and other low temperature molecular solids and cause the ejection of atoms and molecules. Therefore, the chemistry induced in materials by photons, called photolysis, and that produced by charged particles, called radiolysis, are related to the production of gas-phase species, typically called desorption or sputtering. UV-photons, x-rays and cosmic-ray (CR) ions maintain the gas-phase species observed in regions of the interstellar medium (ISM), in discs around young stellar objects (YSOs) and in the recently discovered oxygen atmosphere over Saturn's rings. A giant toroidal atmosphere is produced in Saturn's inner magnetosphere by sputtering of grains. Europa and Ganymede have chemically altered surfaces and gravitationally bound oxygen atmospheres produced by energetic ions and electrons. The possible presence of a sub-surface ocean on Europa has lent importance to obtaining a description of the radiolytic production of oxidants in its relatively young surface. In order to understand these exciting observations, it is necessary to develop models of sputtering, radiolysis, and photolysis that can be used by astronomers. Because the surfaces and ambient gases are coupled by radiation processing, practical models will be constructed using the latest computational tools in materials science as guides. With such models, gas or plasma observations in an astrophysical environment can be used to determine the composition of embedded surface materials. Conversely, observations of the composition of the surface materials can be used to predict the ambient neutral and plasma environments. Although the commonality of the physical and chemical processes occurring in a number of space environments is the focus of this research, the models will be applied to the surface-plasma coupling in Saturn's magnetosphere, to the irradiation of the surface of Europa, and to x-ray-induced production of gas from the discs of YSOs. The fundamental physics and chemistry of radiation processing of molecular solids will be described. Because a massive amount of data is needed for a broad range of materials and radiation types (x-rays, CR ions, solar and magnetopheric plasmas), a research program is needed that summarizes the available data, then uses the modern computational tools in materials science to develop applicable models, and finally applies the new models to a number of topical problems.This work integrates problems in astronomy with those in material science. Because of the emphasis on underlying physics and chemistry, the models that are developed are of interest to a broad scientific community. In fact, this research, which is driven by the need to understand astrophysical observations, has already had an impact on the physics and chemistry of radiation modification of molecular materials and has altered the teaching of introductory materials courses. Because of its interdisciplinary nature, the research will be carried out by students in Engineering Physics at the University of Virginia. This is a model cross-disciplinary graduate program that has strong interactions with neighboring undergraduate institutions and high schools that serve primarily minority populations. ***