Quasiresonant coherent multi-photon processes with participation of atomic discrete and continuum states

The mechanism of generation of new frequency components of gases in extremely strong light field (ESLF) is studied in the framework of phenomenological approach. A noble gas jet with concentration about 1018 cm-3 illuminating by ultrashort laser pulses producing light intensity about 1015-1016 W/cm2 is considered. The discrete excited ionic states can be populated. Two mechanisms of non-linear optical processes are discussed both appearing at quasiresonance conditions: 1) contribution of populated excited discrete states and 2) half-collisions. The role of population of excited discrete states as well as the quasiresonance with discrete spectrum are investigated. The general expression for cubic optical susceptibility in the case of moderately intense light field is written. The experimental study of non-linear properties of excited atoms and ions is performed. It is shown that 4-photon processes with one-two and three-photon resonances are much more efficient in quasiresonant conditions. 4-photon resonant processes are significantly reduced when lowering time delay to 100-300 ns. In this case the gas temperature is still high enough and high-lying discrete and continuum states are populated. Initial density of neutral gas is about 1019 cm-3 and discrete atomic states do not contribute significantly to the light scattering. The optical non-linearity is due to dependence of electron-ion collision frequency on the field intensity. In the dense low-temperature the plasma scattering approach has not any advantages but in low pressure gases in presence of pico- and femtosecond ESLF this approach is more adequate: during the light pulse the gas is fully ionized but is not yet a plasma. The optical non-linearity is due to half-collision of the outer electron with parent ion. The average value and projection of dipole moment on the field direction of the system oscillating at the frequencies nω (n=1, 3, 5..) is determined. 2 figs, 31 tabs