Space- and time-resolved dynamics of plasmas generated by laser double pulses interacting with metallic samples

Abstract

Significant improvements to the analytical performance of laser-induced breakdown spectrometry (LIBS) were achieved by the use of laser double pulses to ablate and excite the sample material to be analysed. To clarify the underlying physical phenomena the dynamics of the laser-induced plasma in air is studied using a high-speed electro-optic camera to observe the spatial and temporal development of the plasma geometry. A Mach–Zehnder interferometer was set-up to detect the spatio-temporal changes of the refractive index of the plasma. The velocities of the luminous plasma front and shock waves after the impact of the first laser pulse are consistent with the prediction of Sedov's model for spherical expansion. The dynamics caused by the second laser pulse of the double pulse differ significantly, indicating a modified state of the local atmosphere in the interaction region. Electron densities determined interferometrically amount to about 3 × 1018 cm−3 at the center of the hemispheric plasma geometry at a delay time of 2 µs. Whereas the electron density for single or double pulses is approximately the same, the volume of the plasma is greater by more than a factor of 3 after the interaction with the double pulse of the same total energy.