Multiply-excited states and their contribution to opacity in CO2 laser-driven tin-plasma conditions

Abstract

A recent study (2020 Nat. Commun. 11 2334) has found that transitions between multiply-excited configurations in open 4d-subshell tin ions are the dominant contributors to intense EUV emission from dense, Nd:YAG-driven (laser wavelength λ = 1.064 μm) tin plasmas. In the present study, we employ the Los Alamos Atomic code to investigate the spectral contribution from these transitions under industrially-relevant, CO2 laser-driven (λ = 10.6 μm) tin plasma conditions. First, we employ Busquet’s ionisation temperature method to match the average charge state (Z) of a non-local-thermodynamic equilibrium (non-LTE) plasma with an LTE one. This is done by varying the temperature of the LTE calculations until a so-called ionisation temperature TZ is established. Importantly, this approach generates LTE-computed configuration populations in excellent agreement with the non-LTE populations. A corollary of this observation is that the non-LTE populations are well-described by Boltzmann-type exponential distributions having effective temperatures Teff ≈ TZ. In the second part of this work, we perform extensive level-resolved LTE opacity calculations at TZ. It is found that 66% of the opacity in the industrially-relevant 2% bandwidth centred at 13.5 nm arises from transitions between multiply-excited states. These results reinforce the need for the consideration of complex, multiply-excited states in modelling the radiative properties of laser-driven plasma sources of EUV light.