Mass spectrometric analysis of the Mg plasma produced by double-pulse femtosecond laser irradiation
The capabilities of a double-pulse (DP) femtosecond (fs) laser ablation of solid materials as an ion source for application in a miniature Laser ionisation time-of-flight Mass Spectrometer (LMS) system designed for space research are investigated. The studies are conducted by irradiating a high-purity Mg sample with sequences of two femtosecond laser pulses. The positively charged fraction of the Mg plasma is analysed as a function of the inter-pulse delay in the range from 0 to 300 ps and for pulse energies in the range from 0.2 to 1 μJ. The DP ablation studies with both pulses of similar energy show a Mg+ ion yield enhancement within the inter-pulse delay range from 1 to 35 ps near the ablation energy threshold (AET) and from 1 to ∼300 ps if larger pulse energies are applied for the first ablation pulse. For the same total energies but different individual pulse energies, Mg+ ions are produced more efficiently if the weaker pulse is applied first. The analyses of Mg+, multiple-charged Mg and Mg cluster ion yields as a function of the inter-pulse delay and the pulse energies improve the understanding of the ablation mechanism and add some insights into the dynamics of the Mg-surface melting and cooling phases, thermal characteristics of the expanding plasma plume and atomic/cluster ion production mechanisms. By applying sufficiently high DP energies > (0.3 + 0.3) μJ the clusters which are likely produced in the initial ablation phase at the surface can be decomposed effectively in the heated plasma plume. Moreover, by tuning the inter-pulse delay one can also efficiently suppress the neutral-ion reactions in post-plasma chemistry. Our studies show that the DP femtosecond laser ablation ion source can improve the mass spectrometric analysis of solid samples by increasing the ion yield of atomic ions and reducing the abundance of cluster ions. This can improve the quantitative analyses of elements and their isotopes by increasing signal-to-noise ratios and reducing isobaric interferents arising from cluster ions. The current DP studies are conducted successfully with pulse energies lower than 1 μJ, which is sufficiently low to be realised in a compact, lightweight system using fibre technology based lasers, suitable for in situ space applications.