Forced autoionization

In Fig. 22.6(a) we show the spectrum of the transition from the Ba 5d6p F3 state to the 5d5/216d3/2 5/2 states which lie just above the Ba+5d 3/2 limit, and in Fig. 22.6(b) we show the spectrum from the same initial state to the 5d5/214d3/2 5/2 states which lie just below the Ba+ 5d3/2 limit. It is apparent that the envelopes of the excitations are identical. We have been discussing these states as if they were bound states, and in fact for our present purpose they might as well be. First, below the 5d3/2 limit the observed linewidths equal the laser linewidth, and second, there is no visible excitation of the 6seH continua below the 5d3/2 limit. It is also useful to note that a three channel quantum defect treatment of the 5d nd / = 4 series reproduces both the Lu-Fano plot of Fig. 22.2, and the spectra shown in Fig. 22.6. The experimental spectra were reproduced by QDT models using both the a and i channel dipole moment parametrizations described in Chapter 21. 

We can compare the forced autoionization resonance to the predictions of zero field QDT. The observed width, 15.5 cm-1, and the width from QDT, 15.3 cm-1, are in excellent agreement. However, the energy positions are different by 5 cm-1. Exactly why is not clear, but it is certainly the case that the Stark induced continuum is not in all respects like a zero field continuum. For example, with both lasers polarized parallel to the field, so as to excite m = 0 final states, the forced autoionization resonance analogous to the one shown in Fig. 22.9 exhibits structure due to the long lived, blue shifted, Stark states.20,21 

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Fig. 22.7 Schematic diagram of forced autoionization. Above the ionization limit the autoionizing state A, converging to a higher limit, is manifested as an autoionization resonance. Below the limit, in zero field the interaction of the perturber P with the Rydberg series results in the perturbation of the Rydberg series. Applying an electric field E depresses the ionization limit below P, and it appears as a forced autoionization resonance.

Fig. 22.8 The two different excitation schemes A and B used to observe the Ba 5d7d perturber as a forced autoionization resonance. Path A leads to a q parameter near zero, while B leads to a large q parameter (from ref. 21).

The Lorentz force F = q(v x B) causes the electron to process around the magnetic field direction B, and even if the total energy of the electron lies above the field-free ionization limit, the electron cannot escape except into the direction of B. This leads to relatively long lifetimes of such autoionizing states. The corresponding classical trajectories of the electron in such states are complicated and may even be chaotic. At present, investigations in several laboratories are attempting to determine how the chaotic behavior of the classical model is related to the term structure of the quantum states [567]. The question whether quantum chaos really exists is still matter of controversy [569-572]. 

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