Emission signal enhancement of laser ablation of metals (aluminum and titanium) by time delayed femtosecond double pulses from femtoseconds to nanoseconds

Femtosecond double pulses with an inter pulse delay ranging from 100 fs up to 2 ns are used to study the dynamics of laser ablation of metals under ambient conditions far above the ablation threshold. To that end femtosecond double pulses of 30 fs pulse duration at 785 nm having the same intensities are focused onto the sample with a NA 0.5 microscope objective. Signals from element specific spectral line transitions and from reflection of the plasma plume are recorded as function of delay between the two pulses. The corresponding ablation structures are analyzed via atomic force microscopy. Based on these different observables four different enhancement regimes of the element specific signals are identified and discussed with respect to different transient stages of the ablation process. Both metals (Al and Ti) show qualitatively the same transient behavior. A maximum signal enhancement of about five is achieved at an inter pulse delay around 800 ps. The ablation volume is approximately the same as compared to the corresponding single pulse ablation volume with doubled fluence. This result serves as a route to increase the spatial resolution of far-field spectrochemical imaging via laser-induced breakdown spectroscopy on the few μm scale and below.