Influence of Resonant Ionization on Laser Erosion Plume Production

The influence of resonant ionization on production of erosion plume on both sodium and aluminum surfaces irradiated by similar to 20-ns laser pulses was experimentally studied. Laser radiation with intensity of 20 - 600 MW.cm(-2) was tunable in the region of 3S-3P transitions of an Na atom and 3P-3D transitions of an Al atom. It is shown that, with the laser wavelength being equal to those of the resonant transitions, the threshold of plasma production is 1.5 - 2 times lower, whereas the electron density is several times higher, as compared with the nonresonant case. The resonant curve width is measured to be 3 - 8 nm and is mainly determined by a field broadening due to saturation of transition. The obtained data allow one to explain the relatively low value of the breakdown threshold on an aluminum surface irradiated by a XeCl laser.