Understanding plasma ignition processes will help to determine optimum conditions for LIBS measurements. The plasma ignition processes include bond breaking and plasma shielding during the laser pulse. The plasma conditions, after the laser pulse terminates, will determine the expansion and cooling. Bond breaking mechanisms influence the amount and forms of energy (kinetic, ionization and excitation) that atoms and ions acquire. Plasma shielding can increase the energy by additional heating, before the laser pulse is finished. These mechanisms strongly depend on the laser irradiance and pulse duration, as described for nano, pico and femto-second lasers. Plasma shielding can be dominant when the laser irradiance reaches certain thresholds.
After the laser pulse ends, the induced plasma plume will continue to expand into the ambient. The electron number density and temperature of the plasma changes as the plasma expands. Plasma expansion depends on the amount and properties of the ablated mass, how much energy was coupled into the mass, the spot size of laser beam, and the environment (gases, liquid, and pressure). Most LIBS spectra are recorded from several hundreds of nanoseconds to several microseconds after the laser pulse. Understanding plasma expansion during this time period is critical for optimization of LIBS and LA-ICP-MS measurements.
Conventional LIBS measurements are made using nanosecond to microsecond delays after the laser pulse. Emission spectra at these times depend on the laser-induced plasma properties; when the plasma is hot and dense, the spectrum is mostly composed of continuum emission. During plasma expansion, the temperature and number density decrease; ionic lines then atomic emission lines appear.
A significant quantity of the ablated mass is not excited vapor, but in the form of particles. Particle formation occurs from condensed vapor, liquid sample ejection, and solid-sample exfoliation. The mass ablated as particles does not contribute to a LIBS measurement unless these particles can be re-evaporated and excited by the plume itself. Particles are important for laser ablation ICP analysis.