FEMTOSECOND GROWTH DYNAMICS OF AN UNDERDENSE IONIZATION FRONT MEASURED BY SPECTRAL BLUESHIFTING

We present a comprehensive report of time-resolved spectral blue shifts of 100-fs laser pulses caused by ionization of atmospheric density N2 and noble gases subjected to high (10(14) W/cm2-10(16) W/cm2) light intensities. Included are data for two experiments: 1) self-shifting of the ionizing laser pulses for varying peak intensities, pressures (1-5 atm.), and gas species; and 2) time-resolved blueshifts of a weak copropagating probe pulse for the same range of ionization conditions. The self-shift data reveal a universal, reproducible pattern in the shape of the blueshifted spectra: as laser intensity, gas pressure, or atomic number increase, the self-blueshifted spectra develop from a near replica of the incident pulse spectrum into a complex structure consisting of two spectral peaks. The time-resolved data reveal different temporal dependence for each of these two features. We present a quantitative model for a simplified cylindrical focal geometry which explains the presence of the two spectral features in terms of two distinct ionization mechanisms: collisionless tunneling ionization, which dominates early in the ionizing pulse profile, and electron impact ionization, which dominates during the intense maximum of the ionizing pulse. Transient resonant enhancements may also contribute to ionization near the peak of the pulse.