Femotsecond laser pulse propagation in air

We present numerical results on the propagation of femtosecond pulses in io nizing air including multiphoton ionization, linear dispersion, space-time focusing and self-steepening. It is shown that all of these terms have an i nfluence on the pulse dynamics. Specifically, we demonstrate that the self- steepening of the pulse causes a strong blue-shifted spectrum which is in q ualitative agreement with the experimentally observed spectrum in air.