Ultra short laser pulse ablation from sodium chloride--the role of laser induced color centers

Ablation from sodium chloride single crystals by ultra short laser pulses is investigated, recording the emitted charged particles (electrons, negative and positive ions) by time-of-flight mass spectroscopy. The influence of irradiation parameters is analyzed, such as laser intensity and the number of shots per ablation site. The ablation craters are inspected by optical microscopy. In addition to material removal from the surface, the laser irradiation induces a coloration of the crystal along the beam path. The corresponding absorption spectrum reveals the generation of F color centers and their aggregates F2, F3, F4, probably decaying after minutes to Na-colloids. Our observations agree well with our earlier results on sapphire and barium fluoride, assuming multiphoton surface ionization followed by Coulomb explosion of the surface, due to positive charging. In addition, several laser generated defect types are identified (color centers, Na colloids and mechanical stress), and their role for emission behavior is discussed