This paper presents an experimental study of the ablation-plasmas and of the craters generated by focusing visible nanosecond laser pulses at normal incidence on a solid target of Er<sup>3+</sup>-doped Ti:LiNbO<sub>3</sub>, in atmospheric air. The laser irradiance was varied in the range of 0.25 to 2.5 TW/cm<sup>2</sup>, which is close to the plume-ignition threshold.
The spatial variation of the neutral Li lines and of the temperature along the axial direction within the plasma plume was evaluated by scanning axially the plume image with a fiber that is coupled at the spectrometer entrance. The results indicate that the intensity of the neutral Li lines increases when increasing the distance from the target's surface. The plasma temperature calculated by accounting for these lines intensities is almost constant (~14000 K) being non-dependent on laser-irradiance and distance from the target.
The spectroscopic study is augmented with an optical microscopy study of the ablated craters in order to
determine the correlation between the ablation rate in multi-pulse regime and the plasma spectrum. The results indicate
that monitoring the plasma spectrum at a fixed position above the target surface the lines intensity decrease with pulse
number, probably due to the confinement of the plume into the deep crater that are drilled in multi-pulse regime.