Study of atoms and multiply charged ions features in the nanosecond laser produced Mo plasma in vacuum using optical emission spectroscopy and time-of-flight electrostatic energy analyzer

Abstract

The species including atoms and multiply charged ions in the laser produced molybdenum (Mo) plasma are investigated in this work using optical emission spectroscopy and time-of-flight electrostatic energy analyzer. Nanosecond laser (5 ns, 1064 nm,) pulses were focused on the Mo target surface with a spot size of 0.4 mm2, energy of ~150mJ/pulse (corresponding to a power density of ~7.5 GW/cm2) to generate the Mo plasma in vacuum environment. Time-resolved spectral analysis was carried out to investigate the temporal evolution of continuous background, atomic, and monovalent ionic spectral signals. The Saha-Boltzmann method is applied for spectral fitting, providing insight into the temporal evolution of electron temperature (Te) and electron density (ne). Over the time from 40 ns to 500 ns, the Te decreases from 3.6 eV to 0.52 eV, and the ne decreases from 2.5 × 1020 cm⁻³ to 1.05 × 1015 cm-3. Linear fitting extrapolation predicts the Te and ne could be even up to 6.3 eV and 2.5 × 1022 cm-3, respectively, at the early stage of 10 ns. This indicates the generation of multiply charged ions during the laser ablation process. The multiply charged ions up to 6 charge states were observed by the time-of-flight electrostatic energy analyzer and the energy distributions for the different charged ions were also obtained. It was found the ion kinetic energy is positively related to the number of charge state indicates the existence of acceleration electric field. The equivalent accelerating potential is determined as approximately 570 V at the current laser power density. This research provides a significant reference for the establishment of models for laser ablation plasmas and a profound understanding of the underlying physical processes.