| A new method for studying the creation and evolution of atmospheric density, neutral plasmas on an ultrashort time scale, using the technique of spectral blueshifting of femtosecond laser pulses is developed. This method has important applications in the nascent fields of soft x-ray lasers and laser particle accelerators. Preliminary experimental results for single laser pulses are presented. Computer modelling using Keldysh strong-fields ionization theory and electron impact ionization theory is used to identify two separate features in the blueshifted spectra: A broad, low, blue peak which is caused by the primarily strong-field ionization early during the laser pulse; and a narrow, less shifted peak which is caused by the primarily collisional ionization that occurs during the peak of the laser pulse. Time-resolved measurements of this spectral shifting process are presented for the first time, and are seen to corroborate the conclusions inferred from the computer modelling of the single pulse data. |
