Kinetic energy of ions produced with first-, second-, and multi-shot femtosecond laser ablation on a solid surface

We report that the kinetic energy of samarium (Sm) atom and Sm+ ion produced by femtosecond laser ablation of solid samarium is strongly dependent on the number of ablation laser shots in the range from 1 to 10. By ablating the fresh surface (i.e. 1st shot), we find the kinetic energy of both Sm and Sm+ ion to be the largest (24 and 250 eV, respectively). Almost 10 times larger kinetic energy of Sm+ ion than that of Sm clearly indicates the contribution of Coulomb explosion in the acceleration process. From the second shot, kinetic energies of Sm and Sm+ ion are lower than those of the first shot and almost constant (ca. 12 and 80 eV, respectively). This behaviour suggests the change in the nature of the solid surface after femtosecond laser ablation, which can be explained by the amorphization of ablated sample surface reported in recent studies