POWER DENSITY INFLUENCE ON LASER-INDUCED GRAPHITE STRUCTURAL MODIFICATIONS

Graphite targets were irradiated with infrared (lambda = 800 nm) femtosecond (120 fs) and nanosecond (7 ns) laser pulses using comparable laser energy per pulse. Laser irradiation produced ablation and a superficial amorphisation of graphite in the centers of the irradiated zones whereas on their peripheries and in the depth of the target other structural modifications were observed. Micro-Raman investigations have shown a G-peak shift towards lower frequencies with the laser power density. That becomes particularly obvious for values in the range 10(7) - 10(9) W / cm(2) that also corresponds to a maximal increase of the sp(3) fraction. A maximum enlargement of the La - crystallite size at the surface of the target occurred for laser power density values ranging from 10 9 to 10 11 W / cm(2). Beyond these limits both sp(3) fraction and La decrease suggesting a strong depreciation of the crystalline structure. XPS investigations have shown that sp(2) - sp(3) transition takes place below the ablation threshold and is also affected by the energy density value: a 1000 times variation in the power density produces comparable effects with a 10 times variation in the energy density. 'In-depth' XPS investigations have shown that sp(3) fraction increases as a cumulative process with the number of the laser pulses and confirms observed interdependence between injected power density and energy density.