Relativistic calculations of electron impact excitation cross-sections of neutral tungsten

TL;DR

This paper provides detailed relativistic calculations of electron impact excitation cross-sections for neutral tungsten, crucial for plasma diagnostics, including new data for many transitions and the influence of metastable states.

Contribution

It introduces the first comprehensive set of fine-structure-resolved excitation cross sections for W I using relativistic distorted-wave calculations, with benchmarked energies and radiative probabilities.

Findings

Metastable states significantly influence excitation cross sections.

Most calculated cross sections are reported here for the first time.

Excitation from metastable states yields the largest cross sections.

Abstract

Reliable tungsten spectroscopy is essential for modelling and diagnosing tungsten-containing plasmas, particularly in edge/divertor conditions where neutral tungsten (WI) is produced by sputtering and contributes to visible/near-UV emission. In this work, we present fine-structure-resolved EIE cross sections for W I computed using the relativistic distorted-wave (RDW) method. Target states are described by multi-configurational Dirac-Fock (MCDF)wave functions with an extensive configuration-interaction (CI) expansion including valence-valence and core-valence correlation through hole and deeper-core configurations. The resulting energies are bench marked against available theoretical results and the recommended values from the NIST database. Fine-structure-resolved cross sections are reported for excitations from the ground level 5d4 6s2 (5 D0) and six metastable levels 5d4 6s2 (5D1 , 5D2 , 5D3, 5D4 , 3P0) and 5d5 (6S)6s (7S3 ) into excited levels belonging primarily to the 5d4 6s(6 D)6p and 5d5 (6S)6p configurations, over incident electron energies from threshold to 500 eV. With the exception of a limited set of previously studied transitions, most of the reported fine-structure-resolved cross sections are presented here for the first time. The results show a strong dependence on the initial level and indicate that excitation from the metastable state yields the largest cross sections among the states considered, highlighting the importance of metastable populations in W I modelling. In addition, radiative transition probabilities for selected prominent transitions are calculated and compared with existing data. The present dataset is expected to be valuable for collisional-radiative modelling and spectroscopic diagnostics of tungsten plasmas in the 1-50 eV range.