Background pressure. The contribution of oxygen from the background was varied by changing the partial oxygen pressure (pO2) at constant total pressures. Ar was used to reach the total pressure aimed for, since it is inert and has an atomic weight close to the weight of molecular oxygen. In this way, the plasma plume size and shape was kept similar, meaning the flux of oxygen from the background could be changed independently from the Zr and Y fluxes from the plasma. Additionally, the fluxes of Zr and Y from the plasma could be changed independently by changing the laser repetition rate.