Townrow S, Coleman P G
Department of Physics, University of Bath, Bath BA2 7AY, UK.
J Phys Condens Matter. 2014 Mar 26;26(12):125402. doi: 10.1088/0953-8984/26/12/125402. Epub 2014 Mar 6.
The crystalline structure of ∼ 5-20 μm water ice films grown at 165 and 172 K has been probed by measuring the fraction of positrons forming ortho-positronium (ortho-Ps) and decaying into three gamma photons. It has been established that films grown at slower rates (water vapour pressure ≥ 1 mPa) have lower concentrations of lattice defects and closed pores, which act as Ps traps, than those grown at higher rates (vapour pressure ∼ 100 mPa), evidenced by ortho-Ps diffusion lengths being approximately four times greater in the former. By varying the growth temperature between 162 and 182 K it was found that films become less disordered at temperatures above ∼ 172 K, with the ortho-Ps diffusion length rising by ∼ 60%, in this range. The sublimation energy for water ice films grown on copper has been measured to be 0.462(5) eV using the time dependence of positron annihilation parameters from 165 to 195 K, in agreement with earlier studies and with no measurable dependence on growth rate and thermal history.
通过测量形成正电子素（o-Ps）并衰变成三个伽马光子的正电子分数，对在165K和172K下生长的约5-20μm水冰膜的晶体结构进行了探测。已经确定，以较慢速率（水蒸气压力≥1 mPa）生长的膜比以较高速率（蒸气压约100 mPa）生长的膜具有更低浓度的晶格缺陷和封闭孔隙，而晶格缺陷和封闭孔隙会充当正电子素陷阱，这一点由前者中正电子素扩散长度大约大四倍得到证明。通过在162K至182K之间改变生长温度，发现温度高于约172K时膜的无序程度降低，在此温度范围内正电子素扩散长度增加约60%。利用165K至195K之间正电子湮灭参数的时间依赖性，测得在铜上生长的水冰膜的升华能为0.462(5) eV，这与早期研究一致，且与生长速率和热历史无关。
