To create a weakly conducting channel in air over 100 m, three-photon ionization is the most appropriate mechanism. A lower-order process requires too high energies to compensate for absorption. Theoretical modeling and experimental investigation of the generation of an ionized channel in air, using prechirped fs UV (248-nm) light pulses is presented. Theoretical estimates are made of the ionization cross-section of nitrogen and oxygen at 248 nm, and compared to experimental data. We calculate the optimum chirp that will produce sufficient pulse compression in normally dispersive air to compensate for the depletion due to the ionization. Important for the rapid formation of a streamer is an abrupt termination of the ionization process, resulting in a local field enhancement. A proper choice of the initial pulse parameters (duration, chirp, and energy) allows us to obtain a uniformly ionized channel of electron density to 1012/cm3 for the path—the thickness of a space-charge layer over the earth surface—which leads to the triggering of lightning.

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