Comparison of nanosecond and femtosecond laser-induced tip-enhanced ablations by experimental and theoretical methods

摘要

Laser-induced tip-enhanced ablation appears to be a promising nanometer-scale sampling approach. Recently we have developed a Near-Field Ablation and Ionization Mass Spectrometer, which is capable of direct ablation and ionization of materials at a scale of tens of nanometers under both nanosecond and femtosecond pulsed lasers. In spite of its great potential as a nanometer-scale sampling source, its underlying mechanism remains to be further explored.To figure out the difference between nanosecond and femtosecond laser-induced tip-enhanced ablations, we developed a three-dimensional theoretical code to explain the ablation phenomena on a Ti-coating sample. With calculated time-evolving and space-distribution features of sample temperature, the simulation justified the time needed to reach electron-lattice thermal equilibrium to be within several picoseconds, and showed that nanosecond laser ablation features much greater thermal effect. We also calculated the theoretical ablated pit profiles and volume after one laser pulse under both lasers. They all fitted well with experimental results. Afterwards, we compared the experimental mass spectra of various salts in the nanosecond and femtosecond laser experiments and studied the roles of properties via chemometrics works. Result again revealed that thermal properties contributed relatively more in the nanosecond laser model. Ionization potential was equally important in both experiments, which met well with the intensity distribution observed in the mass spectra. Moreover, the influences of lattice energy, molar mass and fusion heat were also explained. All these results proved and supported each other, which verifies our study to be reasonable and helps us get abetter understanding of this new technique.