Fundamental Study on Ablation Sampling of Fe-based Binary Alloys in Laser-induced Breakdown Optical Emission Spectrometry

Abstract

This paper describes the fundamental process of laser ablation occurring in a laser-induced plasma. The sampling process in laser-induced breakdown plasma spectrometry is very complicated and thus has not been fully understood. Our study focused on a relationship between the composition of ablation amounts and the bulk composition, when Fe-based binary alloys were employed as test samples. For this purpose, the ablation amounts of constituent elements in the alloys were determined by ICP-OES, through a method in which ablated deposits by laser irradiation were collected on a glass plate and then dissolved in an acid solution. In Fe-Ni binary alloys, the Ni content in the ablated deposits was almost the same as the bulk composition, which implied that Ni and Fe atoms evaporated along with the chemical composition of the samples; however, in Fe-Cr binary alloys, the Cr content in the ablated deposits was half of the bulk composition, probably because Cr atoms were difficult to be released from the sample surface. X-ray photoelectron spectra of ablated Fe-Cr alloy samples indicated that the resultant surface layer after laser irradiation comprised a thin oxide layer, consisting of Cr₂O₃ and FeO, and a relatively thick oxide layer beneath the outermost surface oxide, of which the composition was a complex of Cr₂O₃, Fe and FeO. The reason for this is that the dissociation energy of Cr₂O₃ is obviously higher than that of FeO, and thus Cr₂O₃ decomposed with more difficulty and thus left preferentially in a surface oxide layer of the Fe-Cr alloys. As a result, the Cr₂O₃ layer could suppress the ablation of Cr.