# Current Progress in Femtosecond Laser Ablation/Ionisation Time-of-Flight Mass Spectrometry
## 2 Laser-Matter Interaction in the Femtosecond Regime
### Diagnostics of the Ion Formation in fs-Laser Ablation  
Femtosecond LIMS studies on the dependence of the ion formation efficiency (ion yield) and the kinetic energy distributions are limited in number but more are expected in the near future. The ion yields produced in laser ablation are a complex function of laser ablation ion source parameters and are typically referred to the ionization efficiency, the fraction of ionized atoms in the ablated plume. Early studies by [PERSON] et al. [18] compared ns-and fs- induced ablation of brass samples indicating the presence of several ion speed components upon both types of ablations. Similar observations were made in fs-laser ablation of gold and copper samples by [PERSON] et al. [77], although the ratio of the high-/low- speed components differed from the ratios obtained by [PERSON] et al. [18]. In later diagnostic studies by [PERSON] et al., fs-ablation of Cu, Al, Mg, Cr, Fe, and W samples applying laser fluence in the range from about 0.5 J/cm\({}^{2}\) up to 75 J/cm\({}^{2}\) was conducted with three diagnostic techniques including a Faraday Cu (FC), a Langmuir Probe (LP) and imaging by ICCD camera. Up to a laser fluence of 10 J/cm\({}^{2}\), the ion angular distributions were found to be well described by the Anisimov expansion model [78]. At larger fluences, a broadening of the angular distribution of ions was observed. The light metal ions (Al, Mg) generally showed a narrow, forward-peaked velocity distribution, and high peak ion yield compared to heavier metals (Fe, W, Cu). In this study, ion yields were also investigated as a function of laser fluence (J/cm\({}^{2}\)) [79]. First, a clear spatial splitting of neutrals and ions, with the charged plume component expanding ahead of the neutral population, was observed with an increased relative abundance of atomic ions at higher fluences. Three different ion emission regimes were identified as a function of the laser fluence. Past the ablation threshold (\(\sim\)0.5 J/cm\({}^{2}\)), the first transition was observed at about 5-6 J/cm\({}^{2}\), with a maximum in the specific ion yield, i.e., the yield per unit fluence, and a second transition was observed at a high fluence of 50 J/cm\({}^{2}\) (Figure 2). For laser fluences <50 mJ/cm\({}^{2}\), the experimental results were in the agreement with the two-temperature model description of the fs laser-sample interaction and subsequent material decomposition mechanisms. However, for larger laser fluences, another complex mechanism was inferred [80]. These quantitative studies show that in typical LIMS experiments at a laser fluence of \(\sim\)1 J/cm\({}^{2}\), one would expect ion yields of about \(\sim\)10\({}^{10}\) ions/cm\({}^{2}\) for metallic and for high laser fluence instruments approximately 100 larger. Furthermore, the studies showed that the ion yield depends on the volatility of the elemental metallic samples and lesser dependence on IP (Table 1). Overall, the values of the ionisation efficiency (fraction of atoms ionised in the ablation plume) in nanosecond LIMS was summarised in the review with the ion yields in high laser fluence to lie in the interval from 10% to 80% (including multiple ions) [3]. For lower laser fluences above the laser ablation threshold, the ionisation efficiency is of the order of 10\({}^{-3}\) (Figure 2). So far, there are no systematic fs-ablation LIMS studies of ion yield as a function of wavelength. For ns-ablation, large differences between the ionisation yields dependent on wavelength and material are observed (large matrix effects). 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].