Department of Atmospheric and Oceanic Sciences and Department of Chemistry, McGill University, 805 Sherbrooke Street West, Montreal, QC, CanadaH3A 0B9.
Phys Chem Chem Phys. 2018 Jun 27;20(25):17038-17047. doi: 10.1039/c8cp02966k.
A quantitative understanding of light scattering by small homogeneous particles requires accurate knowledge of particle geometry and complex refractive index, m = n + ik. In weakly absorbing particles, k can be on the order of 10-9, which is well below the detection limit of almost all light scattering based instruments. Here, we describe a dual-beam optical trap that can simultaneously determine n, k, and the radius, s, of weakly absorbing aerosol particles. We utilize cavity-enhanced Raman scattering to determine n and s and electromagnetic heating from the trapping laser itself to determine k. The relationship between particle size, the trapping cell conditions, the parameters of the trapping laser, and electromagnetic heating is thoroughly discussed and it is shown that the proper choice of a light scattering model is necessary to retrieve accurate values of k when fitting measurements. The phenomenon of optical multistability and its connection to thermal locking and thermal jumping is investigated through both modeling and measurements as understanding this behavior is essential when interpreting results from electromagnetic heating experiments. Measurements are made on three different atmospheric aerosol model systems and k as low as 5.91 × 10-9 are found.
定量理解小均一粒子的光散射需要准确了解粒子的几何形状和复折射率 m=n+ik。在弱吸收粒子中，k 可以在 10-9 的量级，这远低于几乎所有基于光散射的仪器的检测极限。在这里，我们描述了一种双光束光阱，它可以同时确定弱吸收气溶胶粒子的 n、k 和半径 s。我们利用腔增强拉曼散射来确定 n 和 s，利用光阱激光本身的电磁加热来确定 k。详细讨论了粒子尺寸、阱室条件、光阱激光参数与电磁加热之间的关系，并表明在拟合测量值时，选择合适的光散射模型对于获得准确的 k 值是必要的。通过建模和测量研究了光的多稳定性现象及其与热锁定和热跃变的关系，因为在解释电磁加热实验结果时理解这种行为是至关重要的。在三个不同的大气气溶胶模型系统上进行了测量，发现 k 低至 5.91×10-9。
