Early nitrogen emission and charge-state separation in nanosecond laser-produced tin plasma under varying pressures

Abstract

The spatiotemporal evolution of nanosecond laser-produced Sn plasma was investigated under N 2 pressures from 10 −3 to 10 4 Pa using ICCD imaging and time- and space-resolved optical emission spectroscopy. The plasma evolution exhibits an early “fast” process (<100 ns) and a later “slow” process (>100 ns)(10-10 3 Pa). The “fast” process is characterized by continuum radiation and spatially extended N 2 /N 2 + emissions reaching several centimeters, likely arising from VUV/EUV-induced photoionization of ambient N 2 and subsequent collisional excitation by photoelectrons and secondary electrons. The “slow” process is dominated by sequential emissions from Sn ions and neutral atoms. Spatially resolved spectra reveal clear charge-state-dependent separation(<~50 Pa), with higher-charge-state ions located closer to the plume front and lower-charge-state ions and neutral atoms distributed farther behind. Self-electrostatic acceleration and pressure-dependent collisional and recombination processes are discussed as possible contributors to this ordering. Electron density (n e ) and electron temperature (T e ) were determined by Stark broadening and a spectral matching algorithm(SMA), respectively. Within the measured time delay range, ne decreases from approximately 3.0×10 25 to 4.0×10 23 m −3 , while Te decreases from approximately 3.2 to 1.0 eV. Te shows an apparent transition around 100 ns, corresponding to the fast-to-slow transition, whereas the late-stage decay of ne is strongly regulated by pressure-induced collisional confinement. These results provide a pressure-resolved picture of early background-gas activation and charge-state-dependent transport in laser-produced Sn plasma.