  It is well known that optical emission produced by femtosecond laser-induced
breakdown on a surface may be enhanced by using a pair of laser pulses
separated by a suitable delay. Here we elucidate the mechanism for this effect
both experimentally and theoretically. Using a bilayer sample consisting of a
thin film of Ag deposited on an Al substrate as the ablation target and
measuring the breakdown spectrum as a function of fluence and pulse delay, it
is shown experimentally that the enhanced signal is not caused by additional
ablation initiated by the second pulse. Rather, particle-in-cell calculations
show that the plasma produced by the first pulse shields the surface from the
second pulse for delays up to 100 ps. These results indicate that the
enhancement is the result of excitement of particles entrained in the plasma
produced by the first pulse.
