Multiphoton processes in KrF-laser-induced gas breakdown

Ionization by multiphoton processes is the dominant generation mechanism of first electrons in the UV-laser-induced gas breakdown. A strongly simplified analytical model for the ion generation rate is used to give an estimate of the threshold intensity Ith. The result is compared to descriptions by Keldish [1] and Bebb [2] to check its limits of applicability. Solving the kinetic conservation equations for the focus volume gives an estimation of the breakdown intensity where the Debye criterion has been used in the sense of a plasma definition. As an example, breakdown experiments with a KrF-excimer laser have been carried out at different pressures in an argon atmosphere. The pressure dependency of the breakdown threshold intensity and its order of magnitude are predicted by the model.