Multiphoton resonance effects in very high laser fields: twelve-photon ionisation of krypton at 1013W cm-2

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Abstract
Resonance effects in the multiphoton ionisation of atoms are described in terms of the characteristic times of the resonant and off-resonant processes. This description explains qualitatively why resonance effects in multiphoton ionisation of atoms appear differently in caesium atoms at a laser intensity I=108 W cm-2 and in rare gases at I=1013 W cm-2. Resonance effects in twelve-photon ionisation of krypton are investigated at 1013 W cm-2 by the use of a tunable-wavelength bandwidth-limited 15 ps laser pulse at 1.06 mu m. For the laser intensity used, the experimental results give no evidence of a significant enhancement in the number of ions Ni, while resonance effects appear through the variation of the slope K= delta ln Ni/ delta ln I which acquires values much larger than 12 (up to 40) within a frequency range less than 1 cm-1. New theoretical data on resonance effects in multiphoton ionisation of rare gases at 1013 W cm-2 are needed to explain the present experimental results quantitatively. In the absence of such theoretical data a simple model is used to fit the experimental results qualitatively.