  We report on strong backward stimulated emission at 337 nm in Nitrogen gas
pumped by circularly polarized femtosecond laser pulses at 800 nm. A distinct
dependence of the backward UV spectrum on pump laser polarization and intensity
is observed, pointing to the occurrence of backward amplified spontaneous
emission inside filaments. We attribute the population inversion to inelastic
collision between the free electrons produced by the pump laser and neutral N2
molecules. The addition of Oxygen molecules is detrimental for the gain,
reducing it to near threshold at atmospheric concentration.
