Double pulse ultrafast laser ablation of nickel in vacuum

We have studied ultrafast laser ablation of nickel using a pair of identical ~250 fs 527 nm laser pulses separated by ~1 to ~1000 ps. Scanning white light interferometry was used to measure the ablated volume, and an ion probe was used to measure the angular distribution of the ablation plasma plume and the total ion emission. As the delay of the second pulse increased from ~10 to 100 ps the ablated volume decreased by more than a factor of 2; indeed it falls to a value below the single pulse case. Conversely, it is found that the ion yield is sharply increased in this delay regime. It seems that both these features can be explained by the interaction of the second laser pulse with the ablated material produced by the first pulse.