Angular ion species distribution in droplet-based laser-produced plasmas

The angular distribution of the ion species generated from a laser irradiated droplet target is measured. The employed instrument was an electrostatic energy analyzer with differential pumping. Singly and doubly charged ions were detected at an argon ambient gas pressure of 2 × 10⁻² mbar. The amount of Sn⁺ and Sn²⁺ and their kinetic energy is measured from 45° to 120° from the laser axis. Sn⁺ expands approximately isotropically, and Sn²⁺ expansion is peaked towards the incoming laser radiation. The singly charged ion kinetic energy is close to constant over the measurement range, while it decreases by around 30% for Sn²⁺. A calibrated model of the ion expansion that includes recombinations correctly predicts the mean ion charge distribution. The model is able to qualitatively estimate the influence of the laser wavelength on the mean ion charge distribution. The results show a more pronounced isotropic distribution for shorter wavelengths, and a more forward-peaked distribution for longer wavelengths. The ion charge distribution expected without the ambient gas is estimated through the measured ion kinetic energy. The presence of the ambient gas results in a decrease of the mean ion charge state and a decrease in angular anisotropy