Tunneling ionization of noble gases in a high-intensity laser field

Studies of multiphoton ionization of noble gases have been carried out using 1-μm, 1-ps laser pulses with intensities up to the mid- ${10}^{16}$ W/${\mathrm{cm}}^2$. To our knowledge, this work represents the first study of the production of highly ionized noble-gas ions done exclusively in the tunneling regime. It is found in this regime that the ionization at a given intensity depends on both the ionization potential and the charge state of the species. The onset of ionization occurs when the sum of the Coulomb and laser electric potentials causes the electron to be unbound.