Measurement of bimodal size distribution of nanoparticles by using the spatial distribution of laser-induced plasma

Laser-induced breakdown detection was performed to monitor the nanoparticles in an aqueous solution by means of a two-dimensional optical imaging method. To verify the relationship between the particle size and the optical image of a laser-induced plasma, we investigated the characteristics of its spatial distribution corresponding to the number of breakdown events plotted on the laser beam propagating axis. It was found that, for particles smaller than 50 nm in diameter, the spatial distribution follows a single Gaussian curve. For particles in the diameter range from 100 to 1000 nm, however, the spatial distribution follows a sum of the multiple Gaussian curves with different peak positions and peak heights. We demonstrated that particles smaller than 20 nm in trace concentrations, which are mixed with larger particles in the diameter range of a few hundred nm, can be measured by a peak deconvolution of the spatial distribution of a laser-induced plasma.