在低温实验室等离子体中形成的水冰颗粒的相态与形貌
Phase and morphology of water-ice grains formed in a cryogenic laboratory plasma
AI总结：
该研究发现在低温弱电离等离子体中注入水蒸气可自发形成悬浮冰颗粒，其具有纺锤状分形结构并随时间生长，通过FTIR光谱和Mie散射理论分析其相态组成与尺寸，证实核化由离子催化，且其性质与原行星盘天体物理冰相似。
摘要：
当水蒸气被注入到背景气体已被冷却至与深空相当的低温的弱电离实验室等离子体中时，冰颗粒会自发形成。这些冰颗粒在等离子体中被无限期悬浮，因此可以在自由漂浮条件下观察它们随时间的演化。使用显微镜成像显示，冰颗粒具有纺锤状的分形结构，并且会随时间生长。利用傅里叶变换红外（FTIR）光谱，观察到了冰的结晶相和非晶相。在特定热力学条件下，结晶和非晶冰颗粒混合共存，并在围绕 3.2 微米的冰吸收带上使用线性混合模型来检查冰的相态组成及其时间稳定性。随着颗粒生长，消光光谱也受到非弹性散射的影响，通过 Mie 散射理论获得了特征颗粒半径，并与直接成像的尺寸测量结果进行了比较。这些观察结果用于比较可能的冰核化机制，并得出结论：核化很可能是由离子催化的，因为在没有等离子体的情况下冰不会成核，且未检测到杂质。冰颗粒性质和红外消光光谱与在原行星盘中观察到的一些天体物理冰显示出相似性，这表明冰的分形形貌和观察到的均相冰核化过程也可能发生在具有弱电离条件的此类天体物理环境中。
英文摘要：
Grains of ice are formed spontaneously when water vapor is injected into a weakly-ionized laboratory plasma in which the background gas has been cooled to cryogenic temperatures comparable to those of deep space. These ice grains are levitated indefinitely within the plasma so that their time evolution can be observed under free-floating conditions. Using microscope imaging, ice grains are shown to have a spindle-like fractal structure and grow over time. Both crystalline and amorphous phases of ice are observed using Fourier Transform Infrared (FTIR) spectroscopy. A mix of crystalline and amorphous grains coexist under certain thermal conditions and a linear mixing model is used on the ice absorption band surrounding 3.2 microns to examine the ice phase composition and its temporal stability. The extinction spectrum is also affected by inelastic scattering as grains grow, and characteristic grain radii are obtained from Mie scattering theory and compared to size measurements from direct imaging. Observations are used to compare possible ice nucleation mechanisms, and it is concluded that nucleation is likely catalyzed by ions, as ice does not nucleate in absence of plasma and impurities are not detected. Ice grain properties and infrared extinction spectra show similarity to observations of some astrophysical ices observed in protoplanetary disks, implying that the fractal morphology of the ice and observed processes of homogeneous ice nucleation could occur as well in such astrophysical environments with weakly-ionized conditions.
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