  We report on the lasing in air and pure nitrogen gas pumped by a single 800
nm femtosecond laser pulse. Depending on gas pressure, incident laser power and
beam convergence, different lasing lines are observed in the forward direction
with rapid change of their relative intensities. The lines are attributed to
transitions between vibrational and rotational levels of the first negative
band of the singly charged nitrogen molecule-ion. We show that self-seeding
plays an important role in the observed intensity changes.
