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

A recent study [Nat. Commun. 11, 2334 (2020)] 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 = 1.064 micrometres) 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 (laser wavelength = 10.6 micrometres) tin plasma conditions. First, we employ Busquet’s ionisation temperature method to match the average charge state 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 T_Z 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 T_eff approximately equal to T_Z. In the second part of this work, we perform extensive level-resolved LTE opacity calculations at T_Z. It is found that 66% of the opacity in the industrially-relevant 2% bandwidth centered 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.