  We study the temporal and spatial dynamics of the large amplitude and
frequency modulation that can be induced in an intense, few cycle laser pulse
as it propagates through a rapidly ionizing gas. Our calculations include both
single atom and macroscopic interactions between the non-linear medium and the
laser field. We analyze the harmonic generation by such pulses and show that it
is spatially separated from the ionization dynamics which produce a large
dynamical blue shift of the laser pulse. This means that small changes in the
initial laser focusing conditions can lead to large differences in the laser
frequency modulation, even though the generated harmonic spectrum remains
essentially unchanged.

Atomic Physics