Grimaudo Valentine, Tulej Marek, Riedo Andreas, Lukmanov Rustam, Ligterink Niels F W, de Koning Coenraad, Wurz Peter
Physics Institute, Space Research and Planetary Sciences, University of Bern, Switzerland.
Center for Space and Habitability, University of Bern, Switzerland.
Rapid Commun Mass Spectrom. 2020 Jul 30;34(14):e8803. doi: 10.1002/rcm.8803.
Laser ablation combined with mass spectrometry forms a promising tool for chemical depth profiling of solids. At irradiations near the ablation threshold, high depth resolutions are achieved. However, at these conditions, a large fraction of ablated species is neutral and therefore invisible to the instrument. To compensate for this effect, an additional ionization step can be introduced.
Double-pulse laser ablation is frequently used in material sciences to produce shallow craters. We apply double-pulse UV femtosecond (fs) Laser Ablation Ionization Mass Spectrometry to investigate the depth profiling performance. The first pulse energy is set to gentle ablation conditions, whereas the second pulse is applied at a delay and a pulse energy promoting the highest possible ion yield.
The experiments were performed on a Cr/Ni multi-layered standard. For a mean ablation rate of 3 nm/pulse (72 nJ/pulse), a delay of ~73 ps provided optimal results. By further increasing the energy of the second pulse (5-30% higher with respect to the first pulse) an enhancement of up to 15 times the single pulse intensity was achieved. These conditions resulted in mean depth resolutions of ~37 and ~30 nm for the Cr and Ni layers, respectively.
It is demonstrated on the thin-film standard that the double-pulse excitation scheme substantially enhances the chemical depth profiling resolution of LIMS with respect to the single-pulse scheme. The post-ionization allows for extraordinarily low ablation rates and for quantitative and stoichiometric analysis of nm-thick films/coatings.
激光烧蚀与质谱联用是一种很有前景的固体化学深度剖析工具。在接近烧蚀阈值的辐照条件下，可实现高深度分辨率。然而，在这些条件下，很大一部分烧蚀物种是中性的，因此仪器无法检测到。为了补偿这种影响，可以引入额外的电离步骤。
双脉冲激光烧蚀在材料科学中经常用于产生浅坑。我们应用双脉冲紫外飞秒（fs）激光烧蚀电离质谱来研究深度剖析性能。第一个脉冲能量设置为温和的烧蚀条件，而第二个脉冲在一定延迟和脉冲能量下施加，以促进尽可能高的离子产率。
实验在Cr/Ni多层标准样品上进行。对于平均烧蚀速率约为3nm/脉冲（约72nJ/脉冲），约73ps的延迟提供了最佳结果。通过进一步增加第二个脉冲的能量（相对于第一个脉冲高5 - 30%），单脉冲强度提高了15倍。这些条件分别使Cr层和Ni层的平均深度分辨率达到约37nm和30nm。
在薄膜标准样品上证明，双脉冲激发方案相对于单脉冲方案显著提高了激光烧蚀电离质谱的化学深度剖析分辨率。后电离允许极低的烧蚀速率以及对纳米厚薄膜/涂层进行定量和化学计量分析。
