Propagation of ultrashort pulses in gaseous medium: breakdown of the quasistatic approximation

We compare the results of the propagation of strong ultrashort laser pulse in gas medium where the atomic ionization is calculated in quasi-static approximation, and by using ab initio solution of the Schroedinger equation. Our models reveal significant difference between the predictions of the two models which may have an impact on the phase matched high-harmonic and attosecond pulse generation. A novel equation that avoids the concept of ionisation rate is introduced, which can be used to accurately model the generation and propagation of attosecond-duration pulses. (C)2000 Optical Society of America.