共聚焦拉曼测量中激光焦点在球形液滴中的穿透深度

Penetrating Depth of Laser Focus in a Spherical Droplet during Confocal Raman Measurements

针对在共聚焦拉曼测量中激光在球形液滴中的聚焦问题建立理论模型,推导出激光焦点在球形液滴中的穿透深度公式,将激光焦点在球形液滴中的实际穿透深度和拉曼平台的垂直移动距离关联起来.研究表明,平台移动距离和激光焦点移动距离是非线性的,激光焦点分别聚焦于液滴上表面和球心处时,激光焦点的移动距离等于平台移动距离,而当激光焦点在液滴上表面和液滴球心之间时,激光焦点的移动距离大于平台移动距离.并利用此结论初步获得MgSO4球形液滴胶态结构的空间分布信息.发现MgSO4液滴在低湿度下形成的胶态结构是具有一定厚度的壳状结构,且其厚度与液滴所处环境的相对湿度有关.

In this paper we attempt to establish a theoretical model to deal with the laser focusing problem during confocal Raman measurements within a spherical droplet. A depth penetration formula was deduced and the actual penetrating depth of the laser focus in a spherical droplet was related to the vertical moving distance of a Raman spectrograph′s platform. The results indicate that a nonlinear correlation exists between the platform′s moving distance and the moving distance of the laser focus. The moving distance of the laser spot is equal to the moving distance of the platform when the laser focus is located at the surface or at the center of the droplet. However, the moving distance of the laser spot is found to be longer than the moving distance of the platform when the laser is focused at the zone between the surface and the center. Therefore, spatial distribution information for the MgSO4 droplet gel structure can be obtained according to the above-mentioned conclusion. We found that the gelation of an MgSO4 droplet under low relative humidity （RH） was a shell structure with a certain thickness and this thickness was closely related to the relative humidity of the local environment.