Pressure effects during excimer laser ablation of magnesium diboride targets

The expansion dynamics and kinetics of the plasma produced by excimer laser irradiation of MgB2 in Ar background gas has been studied by space- and time-resolved optical emission spectroscopy. The experimental results show that there is a distance-related pressure threshold above which the expansion dynamics changes from a free expansion to a shock wave regime, causing both a considerable increase of the fraction of excited Mg atoms and a simultaneous reduction of their kinetic flux energy. The analysis of the results within the framework of a model describing the plume expansion into an ambient gas indicates that the shock wave formation leads to plume heating, thus, influencing its excitation/ionisation kinetics. The production of a plume whose particles are characterised by a lower flux kinetic energy and a higher degree of internal energy can be particularly useful for MgB2 thin film deposition at lower substrate temperature, due to the high volatility of Mg.