Comments on the production of autoionizing states in the scattering of argon ions from a magnesium surface

Abstract

This paper comments on the results of recent studies of Ar + scattering on Mg, as well as Al and Si, surfaces by Blum et al and Nixon et al . The strong excitation of Ar * * 3p 4 4s 2 states is reported in these works, especially for low (1 keV) collision energies. This result contradicts our earlier work and the work of other authors. We re-investigated Ar + scattering but did not find any evidence of Ar * * state production. However, Ar * * state production is observed in Ar ++ collisions and we conclude that a contamination of the Ar + beam with Ar ++ in the work of the above authors is responsible for their observations. We also comment on the identification of structures due to excited sputtered Mg.

Contribution of singly and doubly ionized argon to the autoionization (Auger) spectrum of argon excited by bombardment of magnesium and scandium surfaces

“…If we use the height of the Ar + peak as a measure of the Ar + current this gives an Ar + current of 1.71 µA for the 40 eV IE. The measured current was 1.82 µA suggesting a contribution of 0.11 µA or 6% from Ar 2+ , which is of the order estimated by Guillemot et al [5] as ∼10% with an IE of 150 eV. This means that the normalized peak heights of the Ar 2+ peaks will be a very severe underestimate of the relative contribution of Ar 2+ ions.…”

“…One consequence of this is that the calculated parameters in the Doppler model referred to above will be in error since, under the conditions of the measurements reported in [3], the peaks will have been produced by Ar 2+ ions as suggested by Guillemot et al [5]. This means that the doubly charged incident ion will have energy 2E I and the theoretical value of k will increase by a factor of √ 2 to 0.037.…”

“…In our work on the Ar and Kr spectra [3,4] the ion gun used was of a simple hot filament type, without velocity selection, and with the electron energy in the ionizer box set at a high value to maximize beam current, so that the beam certainly contained multiply ionized Ar. The suggestion of Guillemot et al [5] would seem reasonable in these circumstances. However, Brenten et al [8], using velocity selected Ar + ions, recorded similar spectra to that reported by Nixon et al [4] when scattering Ar + from a thin film of potassium.…”

“…This cut-off energy was estimated by extrapolation of the best fit smooth curves through the data to zero peak height and values of 280 eV for Sc and 2.8 keV for Mg were obtained. Since Guillemot et al [5] used selected Ar + ions above this energy in their measurements on Mg, the question to be asked is why did they not observe an argon spectrum? The answer probably lies in the signal to noise ratio in their measurements.…”

“…Guillemot et al [5] commenting on the work of Blum et al [3] suggested that the electron spectrum of Ar produced by argon ions incident on a magnesium target arises solely because of the presence of Ar 2+ ions in the ion beam used to excite the spectrum. Guillemot et al reported that when they used velocity selected ions containing only Ar + to excite an electron spectrum no structure was observed in the Ar region but that spectra similar to those published were obtained when Ar 2+ ions were used.…”