Ablation of metals using ultrashort laser pulses in a pump-probe experiment: dynamics of laser induced particle emission from metal surfaces on the femto and picosecond time scale

The main part of this work deals with the dynamics of the laser ablation process of metals (Al, Ag, Fe and Ni) initiated by approx. 50 fs laser pulses. The phenomena have been investigated by interferometric time resolved pump and probe measurements. This work reports one of the first yield measurements of emitted singly charged ions and neutrals from a metal surface induced by laser light. The experiments have been performed using a two-pulse autocorrelation setup in which the differential yield of emitted metal ions is measured as a function of the temporal separation between a pair of excitation pulses with a reflectron-type time-of-flight (TOF) spectrometer. The intensity of each pulse is kept below the ablation threshold, thus only the combined interaction of both pulses causes particle emission. It must be pointed out, that the time information obtained in this way concerns only the initial excitation responsible for ablation, but does not yield information about the dynamics of the way this excitation is transferred into kinetic energy of the ablated particles. The high temporal resolution provided by our laser system and the advantages of ultra short laser pulses for laser ablation were the main motivation. This has been established recently, in particular for surface micro-processing, micro-machining and surgical application, where controlled removal of material is essential. The main results are presented and ideas are developed for interpreting the observed behavior. An unexpected behavior characterized by delayed ablation for a pump-probe beam delay in the range of several ps for Al, Ni and Fe has been observed. When the two beams are separated by more than 50 fs, Al and Ag behave differently: for Al the ion signal drops below the detection limit, while for Ag an exponentially decaying signal is observed. In all cases it was assured by blocking each of the two beams individually and adjusting the laser intensity that no emitted ions were detected. I