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Ablation thresholds for the metallic elements Ti, Mo, Pt, Au, and Al were investigated using a XeCl excimer laser in the fluence range of 0.1–20 J/cm2 . The ablation thresholds were estimated from the dependence of the ablation rate and the diameter of the crater produced by the laser irradiation on laser fluence. Two ablation thresholds obtained from the ablation rate dependence on fluence agreed well with those obtained from the diameter dependence. The lower ablation threshold is related to the threshold calculated using a one-dimensional thermal diffusion model. The results suggest that even at such extremely low ablation rates, melting plays an essential role in excimer laser ablation.

The postoperative hyperopic shift of the excimer laser phototherapeutic keratectomy (PTK) for treatment of corneal superficial opacitis in the major complication of this technique. To investigate the causes of this hyperopic shift, we ablated the PMMA plates, hard contact lenses and the corneas of white rabbits by excimer laser which was set to perform a therapeutic ablation. The hyperopic shift when the PMMA plates were ablated with a depth 50 μm was about 1 diopter, and the greater the numbers of pulses during ablation, the greater the tendency toward hyperopia. Regression analysis shows that the steeper the basecurve of the hard contact lenses, the greater the tendency toward hyperopia. The hyperopic shift in ablated rabbit corneas was about 3 diopters after 2 weeks, and it was continued to 3 months. There was a tendency that the greater the numbers of pulses during ablation and the steeper the curvature of the cornea, the greater hyperopic shift. The causes. of this hyperopic shift of the PTK were considerd to be the following : 1) the differential distribution of the energy density of the laser beam ; 2) the ablation rates between central and peripheral cornea are different ; 3) re-epithelizations in the peripheral area were thicker than those in the central area.

Previous studies have reported altered dentinal structure and properties after laser irradiation. It was the aim of this investigation to determine the thermal and ablative effects of XeCl irradiation in dentin and then to investigate microstructural and physicochemical changes in the residual dentin structure. Extracted human molar tooth roots were bisected and coated with acid-resistant varnish, leaving a window. After irradiation of one half at 1 Hz, 15 ns pulse durations, fluences of 0.5 - 2 J/cm2, both halves were subjected to acidified gelatin gel at pH 4.5. The carious lesions were bisected and used to perform SEM and microhardness measurements.

Previous studies have reported altered dentinal structure and properties after laser irradiation. It was the aim of this investigation to determine the thermal and ablative effects of XeCl irradiation in dentin and then to investigate microstructural and physicochemical changes in the residual dentin structure. Extracted human molar tooth roots were bisected and coated with acid-resistant varnish, leaving a window. After irradiation of one half at 1 Hz, 15 ns pulse durations, fluences of 0.5 - 2 J/cm2, both halves were subjected to acidified gelatin gel at pH 4.5. The carious lesions were bisected and used to perform SEM and microhardness measurements.