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

Abstract

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.

Current Progress in Femtosecond Laser Ablation/Ionisation Time-of-Flight Mass Spectrometry

“…Due to the nonlinear absorption of fs-radiation, the dependence of ablation efficiency and ion generation efficiency on material and wavelength is observed to be smaller than that of ns-lasers but still exists. While conducting the UV fs-laser ablation again less dependence on material properties and smaller fractionation effects compared to the IR fs-laser ablation can be observed [81,82].…”

“…The characteristics of fs laser pulses permit applications of more complex irradiation schemes such as short delay pulse sequences or tailored pulse shapes. One of the arrangements, known as double pulse fs-ablation, was particularly theoretically and experimentally particularly well-explored due to its application in material processing and synthesis [83][84][85], chemical analysis by LIBS [86], and was recently applied in fs-LIMS [82,87,88]. Femtosecond double pulse (DP) irradiation offers better control over ionisation yields and polyatomic species produced in laser ablation [87,88].…”

“…One of the arrangements, known as double pulse fs-ablation, was particularly theoretically and experimentally particularly well-explored due to its application in material processing and synthesis [83][84][85], chemical analysis by LIBS [86], and was recently applied in fs-LIMS [82,87,88]. Femtosecond double pulse (DP) irradiation offers better control over ionisation yields and polyatomic species produced in laser ablation [87,88]. This control is achieved by tuning the laser pulse energy of each laser pulse and inter-pulse delays (ps range) [85].…”

“…In a LIMS application, femtosecond double-pulse laser ablation can improve the quantitative chemical analysis and further improve the resolution of chemical depth profiling [82,87,88]. In addition, at delays in a range of tens of ps, DP leads to a significant reduction of polyatomic ions, by reheating produced plasma and ionising neutral gas, which reduces isobaric interferences and an increase of ion yields up to a factor of 10-50 compared to single pulse femtosecond ablation [82,87,88].…”

Improved plasma stoichiometry recorded by laser ablation ionization mass spectrometry using a double‐pulse femtosecond laser ablation ion source

“…[17][18][19][20] The application of a pulsed laser system to probe the analyte of interest has many advantages, as structures down to tens of nanometers can be targeted, layered materials can be chemically profiled by consecutive laser pulses, and sample contamination via handling can be minimized as no mechanical or chemical contact is required during preparation, among others. 9,21,22 With the replacement of, e.g., nanosecond or picosecond laser systems with powerful and stable ultra-fast pulsed femtosecond laser systems, the analytical capabilities and figures of merit of all laser-based measurement techniques have improved significantly, including, e.g., minimized matrix effects and element fractionation effects due to the absence of laser plasma plume interaction, improvement of spatial resolution due to reduced heat dissipation, increased reproducibility of measurements, enhanced ionization and stoichiometry, among others. [23][24][25][26][27] The application of femtosecond laser systems, however, shows drawbacks as well, at least for LIMS and LIBS systems.…”