Multiphoton ionization in intense ultraviolet laser fields

The mechanism of collision‐free multiphoton ionization of rare gases irradiated with ultraviolet radiation at an intensity of up to ∼1016 W/cm2 at 248 nm and a pulse length of ∼0.5 ps and ∼1015 W/cm2 at 193 nm and ∼5ps, respectively, has been studied by observing the electron energy spectra and ion charge state threshold intensities. The formation of multiply charged ions by a sequential process of ionization has been directly detected in the electron energy spectra in the form of a characteristic pattern of interwoven above threshold ionization (ATI) ladder line series. The threshold intensities for ion production have been compared with the Keldysh model and were found to be in good agreement for light ions (Ne) and consistently lower for heavier ions (Xe), scaling with the atomic number. These measurements, together with estimates of the ion population dynamics during the rise of the laser pulse can be reasonably understood on the basis of a single electron picture.