Microwave ionization of Rydberg atoms

Summary form only given. An atom can be ionized by a static field if the field depresses the potential below the binding energy W, leading to the requirement E=W/sup 2//4 in atomic units. The atomic units of field and energy are 5.14/spl times/10/sup 9/ V/cm and 27.2 eV. The ionization field is often expressed in terms of the principal quantum number n of the state in question as E=1/16n/sup 4/. In a microwave field with frequency far less than the separation /spl Delta/W=1/n/sup 3/ between adjacent n states, atoms other than H ionize at the much lower microwave field amplitude of E=1/3n/sup 5/. This field corresponds to the Inglis-Teller limit, where it is impossible to resolve spectrally adjacent n states due to Stark broadening in a plasma. In H ionization occurs as it does in a static field. The difference exists because the finite sized ionic core of a non hydrogenic atom breaks one of the symmetries found in H. In non hydrogenic atoms the microwave field drives a series of transitions through successively higher n states culminating in ionization. These transitions can be understood in terms of a Landau-Zener picture based on the variation of the energies of the atoms produced by the time varying field or as the resonant multiphoton absorption of the microwave photons. In either case, the atoms make transitions through real intermediate states en route to ionization. With short, four cycle, microwave pulses complete ionization does not occur with fields of E=1/3n/sup 5/, and population is left in intermediate states. The transition from ionization at fields near E=1/3n/sup 5/ to fields of E=1/16n/sup 4/ occurs when the frequency becomes low enough that the energies of the states vary adiabatically in the temporally varying field.