Laser-intensity effects in the energy distributions of electrons produced in multiphoton ionization of rare gases

The energy spectra of electrons produced in multiphoton ionization of rare gases display a large number of peaks corresponding to the absorption of much more than a minimum number of photons required to ionize the atom. A retarding-potential method has been used to analyze the energy of electrons produced in the multiphoton ionization of He, Ne, and Xe at 1064 and 532 nm in a broad range of laser intensities (1013–1015 W cm−2). The intensity dependence of energetic electrons emphasizes the validity of the IM law, where M is the total number of photons absorbed. The shape of the electron energy distribution can be completely changed when the laser intensity is increased. The maximum of the energy distribution is switched from the slowest electron peak to faster electron peaks. The distortion of the energy distribution is generally strongly asymmetric, with a steep gradient at the low-energy edge. At 1064 nm, this leads to the disappearance of the first peak in Xe at 4 × 1013 W cm−2, whereas in Ne the first three peaks disappear at 4.3 × 1014W cm−2 and the first thirteen peaks disappear at 7.8 × 1014W cm−2. These effects are still more marked in He in comparison with other higher-Z atoms. The present experimental results could be explained by a model based on continuum–continuum transitions.