Multiphoton ionization of hydrogen and rare gases

A 400-MW neodymium laser beam is focused in a vacuum chamber filled with a noble gas at about 10^-3mmHg. Operation at low pressure permits the isolation of the multiphoton effect from avalanche ionization. The ions produced at the focal point are collected in a Faraday cup. By varying the laser power through the focal point, we plot, in a log-log scale, the number of ions produced against the beam power through the focal point. The slope of the straight line drawn through the experimental points gives the number of photons simultaueously absorbed by an atom. For all gases, experimental values of are always much lower than values of K0corresponding to the ratio of atom ionization energy to photon energy. Considerations are given to justify this fact in the frame of recent theories.