High-resolution emission spectroscopy of W I lines: Comparing near-threshold sputtering of mono- and polycrystalline tungsten by Ar ions

This work presents the first experimental study on the near-threshold sputtering regime for monocrystalline low-index plane tungsten targets investigated using high-resolution emission spectroscopy. We analyzed the line shape emitted by sputtered atoms, which contains information on the angular and velocity distribution functions via Doppler broadening. Specifically, we report changes in the line profile of the resonant W I 498.4 nm transition during plasma exposure of polycrystalline and monocrystalline (100) and (111) tungsten targets at the linear plasma device PSI-2. Biasing the targets from −60 V to −100 V provided low-energy argon ions for near-threshold sputtering. The line shapes, measured along the angle of observation perpendicular to the normal of the sample, were significantly broader for the monocrystalline (100) and (111) compared to that of the polycrystalline target. In particular, the (111) target demonstrates a pronounced heart-shaped profile. The modeling captures this distribution via a ∝ cos(θ)exp(−bθ) function—θ is the polar angle—combined with a parameterized Thompson velocity distribution. Furthermore, comparing the experimental data to molecular dynamics simulations at 100 eV illustrates a reasonable agreement of the angular distribution function with the measurements.