Cross-phase modulation in short light pulses as a probe for gas ionizationdynamics: The influence of group-delay walk-off effects

The method of probing the dynamics of gas ionization using cross-phase modu lation is considered. It is demonstrated that temporal walk-off effects may considerably lower the precision of such measurements in the case of very short pump and probe pulses. The first-order approximation of dispersion th eory is used to analyze the propagation of a probe pulse in a gas under con ditions of cross-phase modulation associated with the ionization of the gas by a high-power pump pulse. Expressions for the spectrum of the probe sign al and the mean frequency of this signal are derived within the framework o f this approach including cross-phase modulation and temporal walk-off effe cts. Calculations carried out for several rare gases and the atmospheric ai r demonstrate that the uncertainty of time resolved cross-phase-modulation measurements due to the temporal walk-off of the pump and probe pulses unde r characteristic conditions of gas ionization may reach several femtosecond s per centimeter of an ionizing gas.