Quantum defect alkali

In Chapter 3 we considered briefly the photoexcitation of Rydberg atoms, paying particular attention to the continuity of cross sections at the ionization limit. In this chapter we consider optical excitation in more detail. While the general behavior is similar in H and the alkali atoms, there are striking differences in the optical absorption cross sections and in the radiative decay rates. These differences can be traced to the variation in the radial matrix elements produced by nonzero quantum defects. The radiative properties of H are well known, and the radiative properties of alkali atoms can be calculated using quantum defect theory. 

Pulsed field ionization of an alkali atom differs from the description just given for H because of the finite sized ionic core, or equivalently, the nonzero quantum defects. There are three important effects. First, the zero field levels can only be spherical nim levels, not parabolic levels. Second, in the E > l/3n5 regime there are avoided crossings of states of different n. Third, ionization can occur at lower fields than in H. Specifically, in H blue states have higher ionization fields than red states, but in an alkali atom this is not the case due to nx changing ionization. 

Theoretical Simplicity. The theoretical simplicity of alkali atoms in their ground and valence electron excited states is well known they are well described by simple one electron "quantum defect" or "effective principal quantum number" ideas. Alkali atomic spectra traditionally follow the hydrogen atom in books on atomic spectroscopy. 

If such a wavepacket were formed in H, then the wavepacket would remain intact, with a fixed orientation in space, until some incoherent process (either spontaneous emission (see chapter 4) or collisions, discussed above) destroys the coherence. This arises because conservation of angular momentum for the excited electron applies strictly in this case. However, the experiment is performed in an alkali atom, which possesses a core, and there is a back reaction of the excited electron on this core (core polarisation), which depends on the degree of penetration of the excited electron into the core, i.e. on the quantum defect, which itself is a function of the angular momentum. Thus, the wavepacket precesses under the influence of a small potential due to the quantum defect of the alkali. It is found to follow a classical trajectory determined by the core polarisation potential. 

Z is the nuclear charge, Z the ionization stage. (For neutral atoms the ionization stage is Z = 1, for single ionized atoms Z = 2, and for multi-ionized atoms Z = -h 1.) The coefficients a,- are optimized numerically so as to reproduce the experimental field-free energy levels and hence quantum defects of the alkali-metal atom or alkali-like ion. 

Since the quantum defects of low angular momentum states are well known from optical spectroscopy and the high / quantum defects can be easily derived from the core polarization model, it is useful to present all the results together. In Figure 11 we show a plot of the alkali quantum defects vs. / on a logarithmic scale modified to include the s and p states. 

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