Spectral Blueshifts of Femtosecond Pulses in Atmospheric Density Kr and Xe Plasmas

Collisionless multiphoton ionization of atmospheric density gases with intense femtosecond pulses can produce a cold, dense plasma of highly stripped ions, thus providing a promising medium for XUV recombination lasers[1]. We have shown in several recent papers [2-4] that rapid ionization and the subsequent development of of plasma temperature in atmospheric density plasmas cause macroscopic refractive index changes which in turn induce characteristic spectral shifts on the ionizing pulse, or a time delayed probing pulse. Theoretical analysis of detailed features of these shifts in the lighter rare gases (He, Ne, Ax) has revealed several key aspects of femtosecond ionization: 1) an ab initio model of spectral shifts based on fundamental strong field ionization theory (e.g. Keldysh, Ammosov [5,6]); 2) the occurence of anomalously large blue shifts at higher pressures and intensities, when the ionization rate is significantly enhanced by impact-ionizing collisions of the quivering electrons [4]; and 3) evidence for low plasma temperatures, based on red shifts induced on a time-delayed probe [4], In this paper we present new results with Kr and Xe gases, which reveal a distinct "doublehumped" feature in the blue shifted single pulse spectra not seen in the lighter noble gases. A quantitative model is presented that attributes this effect to two characteristic rates at which multiple stepwise ionization occurs: a "fast" rate corresponding to each ionization stage during the early part of the pulse (e.g. Krn+ -> Krn+1)+) that produces the more shifted peak; and a "slow" rate which corresponds to the primarily impact ionization during the peak and latter parts of the pulse. The dwell time following the saturation of each ionization level in the early part of the pulse also contributes to a significant broadening of the bluer shifted part of the spectrum. This two-humped effect is more pronounced in Kr and Xe because of the lower ionization potentials, and therefore greater ionization rates; in addition, the larger number of levels ionized in these heavier noble gases cause a greater broadening of the further-shifted peak. Details of the double-humped blue shifted spectra depend on the role of impact ionizing collisions in the strong field, and also on the presence of chirp on the incident pulse.