# Current Progress in Femtosecond Laser Ablation/Ionisation Time-of-Flight Mass Spectrometry
## 3 LIMS System: fs-Laser Ablation Ion Source and Time-of-Flight Mass Analyser
### Femtosecond Laser Radiation
When introducing negative group delay dispersion, the corresponding device is termed a compressor, while a device introducing positive group delay dispersion is termed a stretcher. The output pulse energy of the femtosecond oscillator (~1 n) per pulse) is low. Additional amplification of these pulses is provided by amplification stages. Two techniques, multipass and regenerative amplification, are typically employed. A second stage, typically an Nd:YAG pumped Ti:Sapphire crystal delivers the necessary energy via population inversion, which is used to amplify the low-energy pulses. The ns-pulse generated in the amplifier is energy efficiently transferred to the fs-pulse. The fs pulse must be chirped to several picoseconds. This pulse prolongation is achieved in the stretcher containing a grating that spatially disperses the fs-pulse 10\({}^{3}\)-10\({}^{4}\) times before injection into the amplifier. After the amplification process, the pulses have to be recompressed, compensating for additional phases accumulated during the amplification process [97; 98]. Due to a high saturation fluence that typically occurs in solid-state materials, only a small optical gain per pass can be achieved in the amplifier. To extract the energy stored in the medium, one must use multiple passes in the amplifier. Multi-pass amplification is based on a bow-tie type of amplifier, in which the different passes in the amplifier are separated geometrically. In regenerative amplification, a pulse is trapped in a laser resonator until it has extracted all the energy stored in the amplification medium, regardless of the gain per pass. Trapping and dumping the pulse is done using a Pockels cell and a broadband polariser. The Pockels cell is initially set to be equivalent to a quarter-wave plate. When a pulse is in the resonator, the voltage on the Pockels-cell crystal is switched so that it becomes equivalent to a half-wave plate. The pulse stays in the cavity until it reaches saturation, and a second voltage step is applied to the Pockels cell to extract the pulse. Because the regenerative amplifier operates at 1 kHz, and the seed laser at MHz repetition rates, the pulses have to be synchronised. This is accomplished by a radiofrequency (RF) divider. The MHz signal from the seed laser is divided down to a rate that can be sustained by the Nd:YAG laser. After amplification, the pulse must be compressed again in the temporal compressor functioning as a reversed stretcher. Most of the current commercial fs-laser systems are Ti:Sapphire systems with fundamental wavelength output at 775 or 800 nm. Table 2 summarises the characteristics of few femtosecond lasers used so far in LIMS instruments. More advanced temporally and intensity modified laser pulses (pulse-shaping method) can help to control ablation products and ionisation efficiency [90]. Using arbitrary pulse sequences (typically at the expense of temporal spread), one can achieve the required optimisation and control over the plasma parameters [103; 104]. Temporal tailoring of femtosecond laser pulses by adaptive feedback loops was demonstrated in material science [105]. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Ti:Sapphire [19]** & **Ti:Sapphire [21]** & **Ytterbium-Doped Semiconductor Laser** \\  & **(Spectra-Physics)** & **(Clark-MXR, Inc.)** & **S-Pulse, Amplitude Systems [20]** \\ \hline wavelength & 800 nm & 775 nm & 1030 nm \\  & & SHG, THG & \\ \hline duration & 45 fs (effect. 75 fs) & 180 fs & 500 fs \\ \hline repetition & 1 kHz & 1 kHz & 10 Hz \\ \hline output, max. & 2 m] & 1.2 mJ & 1 mJ \\ \hline Spot dia. & 8 μm & 5–40 μm & 40 μm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Femtosecond laser systems applied in LIMS. ence [103]. This method can enable better optimisation of laser interaction with the sample by adjusting the delivered energy. In the application of fs-radiation for fs-LIMS, one has to consider its several effects on the ablation efficiency.