The new plasma diagnostic technique of spectral blueshifting of femtosecond pulses is used for the first time to analyze quantitatively the ionization of noble gases under the influence of intense, femtosecond illumination. The two processes of strong-field tunneling ionization and electron impact ionization are found to play competing roles on these time scales.

• Received 5 August 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.3523