Measurements of the energy distribution of W atoms sputtered by low energy Ar ions using high-resolution Doppler spectroscopy

Emission spectra of neural tungsten (W) sputtered by impact of argon (Ar) ions in a weakly magnetized (<0.1 T) Ar plasma were measured using a high resolution spectrometer at normal incidence angle to the surface. The measurements were performed for the mono-energetic impact energies between 70 and 150 eV using the neutral tungsten (W I) line at 4982.593 angstrom. The line shape of this line was simulated using a Doppler-shifted emission model to determine the energy distribution. Additional broadening mechanisms were taken into account: instrumental broadening, Zeeman effect and finally the photon or light reflectance at the W surface. The obtained energy distribution was found in a very good agreement with the Thompson distribution, even though deviations for lower impact energies are observed, e.g. the high-energy tail of sputtered particles demonstrates a faster drop compared to 1/E-2 at energies below 100 eV. Moreover, the standard cosine (Knudsen cosine law) distribution provides a rather good description of emission spectra in the energy range of study. Finally, the energy distribution was also compared with simulations carried out with the binary collision approximation (BCA) based Monte-Carlo code SDTrimSP. It shows a marginally worse description at low energies and better description of the high energy tail compared to the Thompson one. Furthermore, the model was used to determine in-situ the degree of light reflection at the W surface. The results are in excellent agreement with the literature data.