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

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 N_i, while resonance effects appear through the variation of the slope K= delta ln N_i/ 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.