Electron energy measurements in multiphoton ionisation of xenon and neon

A retarding-potential method has been used to analyse the energy of electrons produced in the process of generating singly and doubly charged ions in Xe and Ne at 532 and 1064 nm. Energetic electrons observed in Xe at 1064 nm imply that up to 30 photons have been absorbed in the ionisation process, while only 11 photons are necessary to release one electron. The laser intensity dependence of energetic electrons emphasises the validity of perturbation theory. The high absorption rate of such large number of photons can be explained in terms of an interaction of the laser field with several electrons of the atomic shell. A tentative picture is given.