Atomic structure considerations for the low-temperature opacity of Xenon*

We have investigated the opacity of Xe at low temperatures (<50 eV). The emissivity and opacity of Xe is a vital factor in determining the utility of Xe in EUV lithography, with numerous industrial applications. To this end, we have explored the accuracy of some approximations used in opacity models for the pertinent ion stages of Xe (6 times ionized through 20 times ionized). Due to the complexity of Xe atomic structure, one needs to use full configuration-interaction to appropriately describe the strong mixing in the various n=4 sub-shells that give rise to the Δn=0 and Δn=1 transitions that dominate the opacity spectrum at low temperatures. Calculations that consist of full configuration-interaction for large numbers of configurations quickly become computationally prohibitive, thus we have explored hybrid calculations, in which full configuration-interaction is retained for the most important transitions, while intermediate coupling is employed for all other transitions. After verifying atomic structure properties, local-thermodynamic-equilibrium (LTE) opacities are generated using the ATOMIC code at selected temperatures and densities and compared to several experimental results.