High-harmonic generation using ultrashort laser pulses with pulse dura
tions 25 to 200 fs is studied theoretically and experimentally. We obs
erve that the harmonic spectrum of argon taken with 25 fs laser pulses
contains harmonics up to 20 orders higher than for 100 fs laser pulse
s with the same intensity. We show that this increase in harmonics is
because the atom survives to higher laser intensities, due in part to
the nonadiabatic response of the atomic dipole to the fast rise time o
f our pulse.