Resonance excitation and ionization of atoms

Numerous schemes can be used to carry into effect the resonance ionization of atoms. A feature common to all such schemes is the preliminary selective laser excitation of one or several intermediate atomic levels, and subsequent ionization of the excited atoms alone. Resonance excitation has been dealt with in the preceding chapters. The present chapter discusses the next step)—the ionization of the excited atoms. The molecular case differs substantially from the atomic one and will be considered in the next chapter. 

Qualitatively, the basic photoionization schemes may be classified as illustrated in Fig. 9.2. Excited states that are far from the ionization limit can, effectively, be ionized by laser radiation only. The following two possibilities exist in this case direct 

In accordance with eqn (2.74), a minimum energy fluence at the ionization stage is required in the schemes that make use of those resonance transitions from the excited state upward for which the ionization cross section is a maximum. With the proper choice of the sequence of transitions and intermediate quantum states, it is possible to attain a situation where all of the cross sections of the successive transitions lie in the region ai, a2. 10 cm and the critical energy fluences of the pulses are rather low - - 10 —10 J/cm or 10 -10 photons/cm. The direct 

The photoionization yield reaches a maximum when all the successive transitions are simultaneously saturated by several laser pulses. If the final excited state of a 

To realize effective multistep excitation of an atom by multiple-frequency radiation to the final bound state, the laser pulse duration must be shorter than the population relaxation times of the intermediate quantum states. In addition the laser pulse fiuence (photons/cm ) at the transition frequency ijJkn must ensure absorption 

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