UV post‐ionization laser ablation ionization mass spectrometry for improved nm‐depth profiling resolution on Cr/Ni reference standard

Riedo, Valentine; Tulej, Marek; Riedo, Andreas; Lukmanov, Rustam; Ligterink, Niels F. W.; De Koning, Coenraad; Wurz, Peter (2020). UV post‐ionization laser ablation ionization mass spectrometry for improved nm‐depth profiling resolution on Cr/Ni reference standard. Rapid communications in mass spectrometry, 34(14) Wiley 10.1002/rcm.8803

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Rationale: 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.
Methods: 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.
Results: 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.
Conclusions: 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.

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