SPECTROSCOPIC STUDIES OF LASER-PRODUCED PLASMAS FORMED IN CO AND CO2 USING 193, 266, 355, 532 AND 1064NM LASER-RADIATION

The wavelength dependence of laser-produced breakdown in air, CO and CO2 has been studied using the four Nd:YAG harmonics (266 nm, 355 nm, 532 nm and 1064 nm) and the ArF-excimer laser (193 nm). Breakdown thresholds at these wavelengths are reported for air, CO and CO2. A significant reduction in the breakdown thresholds for both CO and CO2 is apparent when comparing 193 nm with the four Nd:YAG harmonics. This reduction is attributed to the resonance-enhanced two-photon ionization of metastable carbon atoms generated in the laser focus at the ArF-laser wavelength. In addition to reporting breakdown thresholds, the laser-produced plasmas in CO and CO2 are characterized in terms of plasma temperatures and electron densities which are measured by time-resolved emission spectroscopy. Electron densities range from 9 x 10(17) cm-3 to 1 x 10(17) cm-3. Excitation temperatures range from 22 000 K at 0.2 mus to 11 000 K at 2 mus. Ionization temperatures range from 22 000 K at 0.1 mus to 16 000 K at 2 mus. Evidence is presented to indicate that, like ArF-laser-produced plasmas, Nd:YAG-laser-produced plasmas formed in CO and CO2 are in or near a state of Local Thermodynamic Equilibrium (LTE) soon after their formation.