Abstract
Optical emission spectrum of Al plasma induced by a KrF excimer laser (248 nm) in a vacuum chamber is investigated using an optical multichannel analyzer (OMA). It is observed that different air pressures and delay times after the laser pulse play important roles in the characterization of emission spectrum from Al surface. The optimal conditions for spectroscopic study of Al plasma are estimated. A spectrum self-absorption effect can be observed clearly from the calibrated spectral line profiles at the initial stage of plasma expansion and high fluence. Assuming local thermodynamic equilibrium (LTE) conditions, the plasma temperature and density are estimated from two spectral lines (Al I, 309.2 and 396.1 nm) as a function of air pressure, delay time and laser fluence. The plasma temperature is derived to be in the range of 5500 to 10000 K using the relative intensities of the spectral lines. The electron density is estimated from Stark broadening profiles of the spectral lines, and found to range from 0.7×1018 to 3.5×1018 cm-3. The behaviors of the plasma temperature and density reveal a 1-D plasma expansion at a delay time shorter than 200 ns and the plasma shielding effect at high fluence.