Investigation of extreme ultraviolet spectra from highly charged holmium ions in 1 μ m laser-produced plasmas

Abstract

Absolute intensity calibrated extreme ultraviolet spectra radiated by highly charged holmium (Ho) ions from 1 μm Nd:YAG laser generated plasmas in the 1–8 nm wavelength spectral range were measured and investigated. The spectral features show a broad structured continuum-like emission band, most prominent in the wavelength range 6–8 nm, which accounts for more than half of the emitted power. Assuming local thermodynamic equilibrium (LTE) in the laser produced plasmas and using the flexible atomic code and Cowan suite of codes, the wavelengths and LTE-gA values of unresolved transition arrays (UTAs) from E 1-contributing transition arrays 4p–4d, 4d–4f and super transition arrays such as 4p64d k−14f + 4p54d k+1–4p64d k−24f2 + 4p44d k+2 + 4p54d k 4f and 4d10(5s5p) k 4f m−k –4d9(5s5p) k 4f m−k+1 were calculated and shown to be responsible for the strongest observed spectral structure. Transitions between these excited to excited states are considered, since even if only weakly present, they will not be influenced by opacity effects unlike resonance transitions involving ground configurations. Another two intense continuous emission bands at 3–6 nm and 2–3 nm that dominate the spectra in the shorter wavelength range mainly arise from 4d–5p, 4d5s–4d5s5p, 4f–5g, 4f5s–4f5s5g and 4d–5f, 4d5s–4d5s5f, 4p–5s transitions. Spectral line shape parameters of Gaussian fits to the LTE-gA weighted radiation spectra of each Ho ion were given, enabling direct explanation of recorded spectra. Based on the LTE-gA UTAs formalism parameters, reasonable agreement is obtained between the synthetic spectrum and experimental result enabling some of the observed spectral features to be identified.