Characteristics of a surface plasma created during excimer laser photoablation

Although photoablation is considered to be a nonthermal mechanism of tissue removal, previous investigators have shown local temperature rises along the surface of corneal tissue following excimer laser photoablation. These temperature increases can be explained by a hot cloud of ionized gas (plasma) created by the products of ablation. Our study demonstrates that the plasma absorbs incident laser light energy and reduces the transmission and scatter of light at the ablated surface. The spectrum of light generated on the surface consists of a number of atomic lines and molecular bands, thereby verifying the presence of excited and ionized atoms and molecules. The magnitude of light scattering and absorption by the plasma increases with both energy density and the length of the incident pulse. Plasma shielding and absorption of incident light occurs for laser pulsewidths greater than several nanoseconds. The plasma emission spectrum is fairly independent of the atmospheric environment and of the type of material ablated, as long as the material is organic. This suggests that the plasma is created from the ablated material. The temporal decay function is multiexponential and dissipates within a fraction of a microsecond, precluding interaction of the plasma with subsequent pulses during excimer laser photoablation.