Two-pulse delayed 532-nm laser ionization of metals using collinear sub-threshold beams.

Two frequency-doubled Nd:YAG lasers collinearly aligned were used to ionize different metals at specific interpulse delays. The beams were independently operated in order to attain full control over the energy. Each laser beam was always set at fluences below the ionization threshold and an evaluation of the effect that the interpulse delay has on the material ionization and the LIMS signal was performed. The different metallic targets studied (Cu, Si, Al, Ti, Fe, 314 AISI stainless steel) exhibit a characteristic ionization feature consisting of a clear enhancement in the ionization yield at interpulse delays around 60 ns when analyzed as pure foils. In addition, an improvement in the spectral resolution is observed at the specific interpulse delay. Our results indicate that proper control of the energy allows optimization of the different steps in the ionization process and they suggest that the effect of the first laser pulse impinging on the surface enhances the way in which the second pulse interacts with the solid.