Experimental data are presented showing the channel closing of four-photon ionization of Mg. It is shown that, for circularly polarized light, the ionization versus intensity spectra exhibit sharp breaks from the normal I4 intensity dependence at the critical intensity where the channel closing occurs. Above the critical intensity, the population of Rydberg states which survives the laser pulse is observed. The residual Rydberg population is found to be greatly reduced for linearly polarized light due to the relatively large probability of ionization of the low-angular-momentum Rydberg states. The data are in good agreement with a model which includes averaging over the spatial profile of the laser.
J. G. Story et al., "Channel Closing in Multiphoton Ionization of Mg," Physical Review A, vol. 49, no. 5, pp. 3875-3880, American Physical Society (APS), May 1994.
The definitive version is available at https://doi.org/10.1103/PhysRevA.49.3875
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