Influence of pre-pulse and main pulse energies on the resonant harmonic from Gallium laser-ablation plumes

We investigate the underexplored multiphoton ionization regime of resonantly enhanced harmonic generation in gallium laser-ablation plumes. Resonant harmonics generate quasi-monochromatic extreme ultraviolet radiation with a pronounced coherent intensity enhancement. In this experimental study, we systematically examine high-order harmonic generation yield by independently varying the energies of the pre-pulse, which forms the laser-ablation plume, and the main pulse, which drives the harmonic emission. This approach enables precise evaluation of how each pulse contributes to the overall efficiency of resonant harmonic generation. Throughout this study, we identify the optimal conditions for maximizing the generation efficiency of gallium resonant harmonics driven by a 400 nm laser. Our results demonstrate an effective method for generating intense, quasi-monochromatic femtosecond extreme ultraviolet radiation and offer valuable insight into the role of autoionizing resonances within the multiphoton ionization regime.