Penetrating depth of laser focus in a spherical droplet during confocal Raman measurements

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.