Measurement of the properties of a CO2 laser induced air-plasma by double floating probe and spectroscopic techniques

The laser-induced breakdown spark has recently been advanced as a method for real-time, in-situ spectrochemical analysis of gases. Many of these analyses take place in ambient air. To better characterize this source, we have measured the temporal variation of temperature and electron density in an air plasma induced by a CO₂ laser operating at 0.5 and 0.8 J/pulse. The electron temperature was measured by the double floating-probe technique (DFP). An excitation temperature for oxygen atoms was determined spectroscopically by Boltzmann plots. Electron density in the plasma was measured from the Stark broadening of the 715.6-nm line of 01. At 0.5 J/pulse, the DFP temperature ranged from 175000 K at 5 μs to less than 10000 K at 25 μs, while the 01 excitation temperature ranged from 19000 K at 1 μs to above 11 000 K at 25 μs. The excitation temperature and electron density agree with values calculated by others from local thermodynamic equilibrium models of an air plasma. While the electron temperature from the DFP method is much higher than the excitation temperature at 5 μs, at times greater than 25 μs the two have converged, implying thermodynamic equilibration between the species.