[en] The electron cascade model originally proposed by Evans and Gamal to study atomic gas breakdown is extended to molecular gas breakdown calculations, including molecular electronic excitations and vibrational excitation effects. The validity of this model is studied for nitrogen at pressures ranging from 7.6x10² to 3.3x10⁴ Torr irradiated by laser radiation of wavelength 1.06 μm and 10 ns pulse duration. The calculated threshold intensities are in good agreement with the experiment. Calculations of the electron energy distribution function have also been performed to reflect the important electron-molecule energy exchange processes in molecular gases. (author)