Department of Chemistry, University of Wisconsin-Eau Claire, 105 Garfield Avenue, Eau Claire, Wisconsin 54702, USA.
J Chem Phys. 2009 Dec 14;131(22):224706. doi: 10.1063/1.3257628.
Low-temperature, amorphous water ice films grown by vapor deposition under high-vacuum are exposed to microwave-frequency discharge-activated oxygen in order to investigate its effect on the ice surface. Adsorption of methane is used to probe alterations to microscale structures and surface morphology. Films are interrogated throughout the experiment by grazing-angle Fourier-transform infrared reflection-absorption spectroscopy, and after the experiment by temperature-programmed desorption mass spectrometry. Multilayer Fresnel thin-film optics simulations aid in the interpretation of absorbance spectra. Using these techniques, structural alterations are observed over a range of spatial and time scales. At first, spectral absorbance features arising from incompletely coordinated water molecules disappear. The density of high-energy methane adsorption sites is reduced, lowering the equilibrium amount of adsorbed methane. At longer exposure times, this is manifested in a narrowing of the width of the primary methane desorption peak, indicating a narrower range of methane adsorption energies on the ice surface. Together these observations indicate restructuring of micropores resulting in an increase in the structural homogeneity of the film. Enhancement of small, higher-temperature methane desorption features associated with methane encapsulation during thermal annealing indicates alterations to larger pore structures by the same restructuring process. Attribution of these effects to various energetic species in active oxygen is discussed. Based on their abundance, O((3)P) and O(2)(a (1)Delta(g)) are the most likely candidates; other trace atomic and molecular species may also contribute.
低温非晶态水冰膜通过高真空条件下的蒸汽沉积生长，然后暴露于微波频率放电激活氧中，以研究其对冰表面的影响。甲烷的吸附被用来探测微尺度结构和表面形貌的变化。在整个实验过程中，通过掠角傅里叶变换红外反射吸收光谱法对薄膜进行检测，实验后通过程序升温脱附质谱法进行检测。多层菲涅耳薄膜光学模拟有助于吸收光谱的解释。使用这些技术，可以在不同的空间和时间尺度上观察到结构变化。首先，不完全配位的水分子引起的光谱吸收特征消失。高能甲烷吸附位的密度降低，降低了吸附甲烷的平衡量。在更长的暴露时间内，这表现为主要甲烷脱附峰的宽度变窄，表明冰表面上甲烷吸附能的范围变窄。这些观察结果共同表明，微孔的重构导致薄膜结构均匀性的提高。与热退火过程中甲烷包封相关的小、高温甲烷脱附特征的增强表明，同样的重构过程改变了较大的孔结构。讨论了这些效应归因于活性氧中的各种高能物质。基于它们的丰度，O((3)P) 和 O(2)(a (1)Delta(g)) 是最有可能的候选物；其他痕量原子和分子物种也可能有贡献。
