The nonlinear photoionization of air was investigated with the help of a UV laser, discretely tunable in the range 355-200nm. It was found that two-step ionization of nitrogen and oxygen occurs at wavelengths in the range 355-200nm while two-photon ionization of oxygen occurs at wavelengths λ<205 nm. It is shown that in the case of the two-step process the cross section for photoionization decreases as the gas pressure increases.
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