Optical emission and mass spectrometric characterization of laser ablation process of Ca, Mg, and Ba at 1064nm

Abstract

A systematic experimental study of the laser ablation of three alkaline-earth metals (Ca, Mg, and Ba) was carried out to understand the ablation processes of the metals. In this work the infrared laser ablation of alkaline-earth metals was studied by mass spectrometry together with the optical emission spectroscopy of the species generated in the ablation process. The analysis and modeling of the results were done using a heuristic equation that includes the Arrhenius, the screening, and the lineal regions that describe the three different regimes usually observed as a function of the laser fluence. We intend to prove that this equation depends on a fit surface parameter that includes the roughness surface, and it allows us to determine a critical fluence that can be related to the physical properties of the metal target.

Internal state populations and velocity distributions of monatomic species ejected after the 1064 nm laser irradiation of barium

“…I, recent modeling of the data of ablated monatomic total Ba mass as a function of incident laser fluence provides further support for this suggestion. 13 As before, 18 explosive boiling appears the most probable photothermal mechanism at the prevailing laser irradiances: Within this picture, the IR high-power, 10 ns long ablation laser pulse used here induces a rapid heating of a highly localized surface layer of the Ba target, bringing it near to its critical point and ultimately leading to its breakdown into a mixture of vapor and liquid droplets that are ejected from the target. Free electrons along with cations will likely be present in the plume through thermionic emission from the hottest region of the target surface ͑possibly supplemented by a number of gas phase ionization processes; see below͒ and those ions will be accelerated out of the focal volume by the so-called dipolar diffusion mechanism, as a result of Coulombic fields generated by the fastest moving electrons that escape from the expanding plume at early times.…”

“…Consideration of possible mechanisms for the initial formation of multiply charged Ba ͑z+1͒+ ions can now be resumed. On the basis of the evidence presented here and elsewhere, 13,18 it is apparent that direct ejection of ions from the target surface as a result of a photochemical mechanism 12 should be dismissed, which leads to thermionic emission as the most plausible surface mechanism for the generation of initial free electrons and positive ions in the ablation plume. Such species will further act as very efficient initial absorbers of the IR laser pulse via electron-ion ͑e-i͒ IB.…”

“…The latter would be especially interesting to contrast with the corresponding modeling by an extension of the treatment that was developed in Ref. 13. This will be the subject of a forthcoming investigation.…”

“…This situation should be contrasted with the findings of a recent study of the 1064 nm PLA of alkalineearth metals from this laboratory, where the total ablated monatomic mass of the various metals was measured at a range of incident laser fluences from the vaporization thresh-old to the laser ablation regime. 13 Modeling of the data showed a correlation between the fluence above which laser ablation becomes dominant ͑the so-called critical fluence͒ and the Fermi energy of barium, which would suggest that the primary ablation process at 1064 nm is dominated by thermal contributions.…”

“…There are few reports of investigations of the electronic state populations of neutral atoms in plumes formed by PLA, involving only UV and visible ͑vis͒ laser wavelengths to compare with these thermal predictions. 19,20 The present work was undertaken mainly as an attempt to explore this topic in the case of the infrared ͑IR͒ PLA of a metal target, which is likely to favor thermally rather than electronically driven material ablation, 13,18 and at a range of laser fluences which are appropriate for plasma formation above the target surface in order to get information on the primary process under ablation conditions of practical use.…”