A numerical investigation of the dependence of the threshold irradiance on the wavelength in laser-induced breakdown in N2

Studies of laser-induced breakdown in molecular nitrogen were carried out to investigate the dependence of the threshold irradiance on the wavelength at various pressures. The analysis was based on the numerical solution of the time-dependent Boltzmann equation for the electron energy distribution function (EEDF) and a set of rate equations describing the rate of change of the excited states population. The rate coefficients and cross-sections as functions of the electron energy were introduced into this analysis in order to probe the exact contribution of each physical process to the breakdown phenomenon. The calculations were performed under the experimental conditions of Davis et al. In this experiment the breakdown of nitrogen was measured at wavelengths of 1064, 532, 355 and 266 nm, over gas pressures in the range 25-760 Torr, with laser irradiances in the range 2 x 10(10) to 3 x 10(11) W cm(-2). The computed thresholds were found to be in good agreement with the measured ones at all wavelengths. The calculated EEDF and its parameters showed that, at lambda = 355 nm, vibrational losses are dominant. Collisional ionization of ground and excited state molecules was found to make a minor contribution to the breakdown phenomenon at 532, 355 and 266 nm. However, the contribution to this process at 1064 nm was more effective. Therefore, the breakdown phenomenon proceeds via an electron-cascade process that converts the molecules only into the excited states, whence murtiphoton ionization plays its role.