Space- and time-resolved optical spectroscopy of plumes generated by laser ablation of NiO in a vacuum

Laser ablation of NiO at 266 nm has been studied by measuring absorption spectra and emission spectra which are resolved both temporally and spatially. The spectra were measured at several points 1 mm apart from the laser spot and with several detection angles. Ablated species in different electronic states such as the ground state, lower lying states, and excited states are distinguished from absorption and emission spectroscopy. Time-of-flight spectra of these species and their angular dependence are obtained. It is revealed that excited Ni neutrals are generated mainly by the recombination of Ni+ ions with electrons, and Ni+ ions ejected nearly normal to the surface are exclusively excited due to multiple collisional effect. The isotropic angular dependence of most probable velocities of Ni+ ions and Ni neutrals are explained by ambipolar diffusion of the plume. The mechanism of species ejection is also discussed.