Nonadiabatic Effects in High-Harmonic Generation with Ultrashort Pulses.

High-harmonic generation using ultrashort laser pulses with pulse durations 25 to 200 fs is studied theoretically and experimentally. We observe that the harmonic spectrum of argon taken with 25 fs laser pulses contains harmonics up to 20 orders higher than for 100 fs laser pulses 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 of our pulse. [S0031-9007(96)01018-6]