Multiphoton ionization of rare gases at 1.06µ and 0.53µ

The interaction between an intense focused beam of optical photons and matter in gaseous form at low pressure (10^-3torr) brings into play some strongly nonlinear processes. These multiphoton processes occur through the simultaneous absoprtion of several quanta by an atom that may be thus either excited or ionized. The orders of nonlinearity of the interaction of a multimode -switched laser beam with rare gas atoms were measured with laser intensities up to 10¹³W ċcm^-2at 1.06μ and up to 10¹²W ċcm^-2at 0.53μ. The energy of the number of quanta corresponding to the order of nonlinearity is always close to the energy of an atomic level. The results seem to emphasize the particularly important role performed by bound states during the ionization process. Thus a two-stage ionization process seems far more probable than a single direct transition between the ground state and the continuum spectrum. Experimental values of multiphoton ionization probabilities are also given after having precisely determined the spatiotemporal intensity distribution function.