Study on Structural Change of Coulomb Crystals in Plasmas

用2台CCD摄像机和Mie散射椭偏仪分析了碳颗粒在甲烷等离子体中生长形成的库仑晶体的结构变化。在同一时间和同一位置，从顶部和侧面观察了晶体的三维结构。因此，可以理解，对于较小的颗粒(1.2-1.5微米)，该结构是三维的，如以面部为中心或以身体为中心；而对于较大的颗粒(&GT；1.8微米)，则是简单的六角形的二维结构。研究还发现，三维晶体在转变为二维晶体之前，曾一度处于液态，从下层开始向三维结构转变，而粒子排列，即从上层向二维转变。对三维库仑晶体的三维结构进行了精确的分析，证明其为严格的面心正交体。据了解，该结构是由面心立方结构(111)面的滑移和粒子在平面内的最小库仑能量重构而形成的。通过施加激光的辐射压力或光照力以及粒子密度的降低，研究了二维库仑晶体中垂直于电极方向的粒子的排列力。结果发现，粒子取向是由引力和库仑排斥力共同作用的结果。

Structural change of Coulomb crystals, which were formed by growing carbon particles in a methane plasma, have been analyzed with the use of 2 CCD video cameras and Mie-scattering ellipsometry. Three dimensional structures of the crystals were observed from the top and the side at the same time and at the same position. As a result, it has been understood that the structure was three-dimensional like face-centered or body-centered for smaller particles (1.2-1.5 mu m) while two-dimensional of simple hexagonal one for larger particles (>1.8 mu m). It has bee also found that the three-dimensional crystal once became in the liquid phase before changing to two-dimensional one and that transition to three-dimensional structure occurred from the lower region while the particle alignment , 1. e., transition to two-dimensional one from the upper.The three-dimensional structure of a three-dimensional Coulomb crystal was precisely analyzed and proved to be strictly face-centered orthorohmbic. It was understood that the structure was formed from the slip of (111) plane of a face-centered cubic structure and the reconstruction of particles for minimum Coulomb energy in the plane.The force of aligning particles in the direction perpendicular to an electrode in the two-dimensional Coulomb crystal was investigated by the application of radiation pressure or photophoretic force of laser light and the decrease of particle density. Consequently it has been found that the particle alignment is caused by the resultant of an attractive and the Coulomb repulsive forces.