Studies in photoionisation of laser ablated plumes: application to determination of isotope ratio

Interaction of high power laser beam with materials has several interesting applications, which include for example, laser ablation elemental analysis, micromachining, and pulse laser deposition of thin films. While the technique has been exploited for these and other applications in recent years, the understanding of the very process of ablation including the laser-surface interaction and the gas dynamics of the plumes is relatively incomplete. In this paper we present the development of the technique of laser induced ablation mass spectrometry (LIAMS), which involves synchronized photoionization of plumes generated by laser ablation followed by ion detection. We discuss here the technique, its implementation, the studies relating to the physics of ablating plumes and finally application to determination of isotope ratios in the sample under consideration. In these studies, atomic plumes are generated by ablating a target material with the second harmonic of an Nd-Yag laser. Non-resonant photoionization of the ablated material is done by using another Nd-Yag laser synchronized with the ablation laser and resultant photo-ions are detected using a time of flight mass spectrometer (TOFMS) of resolving power of ∼ 250. The critical parameter in these investigations is the time delay between the photoionizing laser and the ablating laser. We have studied the behavior of the TOFMS signal as a function of this time delay to obtain the velocity distribution of the atoms in the plume, and its dependence on the power of the ablating laser. We further use these studies to determine the isotope ratios. As typical case studies, we use copper and iron as target materials and show that the measured ratios 63Cu/65Cu (∼2.24) and 56Fe/54Fe (∼15.7) are consistent with the natural abundance. Finally we present the distinctive advantages of the LLAMS technique, notably, the requirement of small sample size, high spatial resolution and its suitability for a wider class of materials. (author)