Ejection of positive ions from plasmas induced by laser ablation of Si and Nd1.85Ce0.15CuO4

Angle-resolved time-of-flight distributions of Si+ ions and of O+, Cu+, and Nd+ ions originating from laser ablation of Si and Nd1.85Ce0.15CuO4, respectively, were measured using a quadrupole mass spectrometer. From these distributions angle-resolved yield distributions of the ions were obtained. The time-of-flight distributions did not change with the detection angle; consequently, neither did the associated yields. The degrees of ionization of the laser-induced plasmas appeared to be about 5×10−7. However, it is argued that only a fraction of the order of 5×10−4 of the total number of ions could have been detected and that thus the actual degrees of ionization were of the order of 1×10−3. All these observations are explained in terms of ambipolar diffusion of the ions (and electrons) from the sheaths, with thicknesses of the Debye shielding distance, of the laser-induced plasmas in their initial stages. The results obtained indicate that in the case of the Si+ ions the main ionization mechanisms were collisional ionization and multiphoton ionization.