Laser ablation is governed by a variety of distinct nonlinear mechanisms. Once the laser beam illuminates the sample, mass leaves the surface of a sample in the form of electrons, ions, atoms, molecules, clusters, and particles, each of the processes separated in time and space. An understanding of the fundamental mechanisms involved in each of these processes is critical for efficiently coupling the laser beam to the sample and removing mass in the appropriate form for analysis. Understanding laser-material interaction will allow ablation of stoichiometric vapor and control of the laser-induced plasma properties for optimum performance.
Laser ablation could be divided into three main processes: bond breaking and plasma ignition, plasma expansion and cooling, and particle ejection and condensation. These laser ablation processes occur over several orders of magnitude in time, starting with electronic absorption of laser optical energy (10-15 sec) to particle condensation (10-3 sec) after the laser pulse is completed. The following figure shows a summary of these three processes and various mechanisms occurring during each.
fs laser (1012 -1017 W/cm2)
Electronic excitation and ionization (10-15-10-13 s)
Coulomb explosion (10-13 s)
Electron-lattice heating (10-12 s)
ns laser (107 -1011 W/cm2)
Thermal vaporization (10-9-10-8 s)
Non-thermal ablation (10-9-10-8 s)
Plasma shielding (10-9-10-8 s)
Plasma expansion (10-11-10-6 s)
Plasma radiation cooling (10-6-10-4 s)
Nano particles formation (10-4-10-3 s)
Ejection of liquid droplet (10-8-10-6 s)
Solid exfoliation (10-6-10-5 s)