Spectroscopic studies of laser produced Bi-Pb alloy plasma

Abstract

Generating soft X-rays in the water-window (WW) region is of great interest as a light source for cellular microscopy. The spectroscopic studies of laser-driven high-Z target plasma are important for parameters optimization in laser produced plasma (LPP) for efficient soft X-ray sources. A novel high-Z alloy target with very low melting point composed of five elements (Bi, Pb, In, Cd and Sn) was designed and used in this study. Two-dimensional (2D) spectral images were recorded and analyzed between 330 and 510 nm from LPP generated using Nd:YAG laser with 1064 nm wavelength in vacuum. The 2D spectral images showed that the Pb II and Bi II species had a higher velocity than neutral atoms. The emission lines were relatively dense in the short wavelength region of 330–380 nm and most came from Bi and Pb neutral species, which have higher atomic numbers among the five elements in the alloy target. The experimentally measured spectra were in good agreement with the theoretical results. The temporal and the spatial evolution of excitation temperature and electron density were investigated using Boltzmann plot method and Stark broadening of the Pb I spectral lines. The space resolved excitation temperature at 400 ns showed approximately similar values within the error bars regardless of laser energies. The electron density at higher laser energy was higher than that at low-energy laser (especially before 300 ns), but the difference among the three laser energies used was not significant at the later times. The assumption of Local Thermodynamic Equilibrium (LTE) was verified based on the three required criteria and the optically thin for the LPP was justified using the self-absorption analysis.