Effective temperatures in hot plasmas: the case of open-N-shell tin ions

Atomic populations in non-local thermodynamic equilibrium (non-LTE) plasmas often follow Boltzmann distributions at temperatures different from the electron temperature. We study the emergence of these so-called effective temperatures in open- N shell tin ions created in laser-produced plasmas relevant for extreme ultraviolet lithography. Configuration populations are calculated using the Los Alamos suite of atomic structure and population kinetics codes. We identify the collisional-radiative processes that dominate population transfer for hundreds of configurations in Sn 11+- Sn 14+ ions in a range of plasma conditions. Our calculations reveal that the configuration populations are well-described by Boltzmann distributions at a single effective temperature. We attribute this behavior to the spontaneous emission process and the unique atomic structures of tin ions. We also show that a simple two-level model can reliably predict these effective temperatures.

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