Optical emission studies of nitrogen plasma generated by IR CO2laser pulses

Large-scale plasma produced in nitrogen gas at room temperature and pressures ranging from 4 × 103 to 1.2 × 105 Pa by high-power laser-induced dielectric breakdown (LIDB) has been investigated. Time-integrated optical nitrogen gas spectra excited from a CO2 laser have been measured and analysed. The spectrum of the generated plasma is dominated by the emission of strong N+ and N and very weak N 2+ atomic lines and molecular features of N+ 2(B2Σ+ u-X2Σ + g), N+ 2(D2Π g-A2Πu), N2(C3Π u-B3Πg) and very weak N2(B 3Πg-A3Σ+ u). The relative intensities of the 0-0 band heads in the N2(C-B) and N + 2(B-X) systems are very weak as compared with the chemiluminescence spectrum of nitrogen formed in a glow discharge. An excitation temperature Texc = 21 000 1300 K was calculated by means of the relative intensity of ionized nitrogen atomic lines assuming local thermodynamic equilibrium. Optical breakdown threshold intensities in N2 at 9.621 νm have been determined. The physical processes leading to the LIDB of nitrogen in the power density range 0.4 < J < 4.5 GW cm-2 have been analysed. From our experimental observations we can suggest that, although the first electrons must appear via multiphoton ionization or natural ionization, electron cascade is the main mechanism responsible for the LIDB in nitrogen.