Multiphoton ionization of atoms via a series of intermediate Rydberg states

Multiphoton ionization of atoms exposed to intense monochromatic radiation is studied under conditions ensuring resonant excitation of either one or two Rydberg series at a time due to transitions from a low-lying discrete level pre-populated by virtue of absorption of a quantum of a weak (probing) field. Our primary challenge is to check whether large sets of Rydberg levels involved in the process can stabilize the transient states and to explore the influence of intrinsic quantum defects and natural (autoionization-induced) widths of levels on stabilization. The role of the defects in the transition range of Rydberg level widths Gamma increases with the intensity of an external electromagnetic field f (in this range normal rise Gamma proportional to f(2) gives way to abnormal behavior Gamma proportional to 1/f(2)). Physical processes lying behind field-induced narrowing of Rydberg levels are analyzed. Calculations are carried out for the s and d Rydberg series of sodium atoms.