Mechanism and timing of a ZEKE spectroscopy experiment

Deviations from predicted rotational intensity distributions are very common in ZEKE spectra. This is due to random near coincidence between extremely numerous rapidly- and slowly-autoionizing resonances (Rydberg series converging to excited rovibronic states of the ion). Since the waiting time between excitation and pulsed field ionization is long, and the very weak DC and stray electric fields present during the ZEKE waiting period can induce weak interactions 

As noted in Section 7.3, shape resonances also occur in the spectrum of vibrational transitions. They correspond to predissociation by rotation (resonances stabilized behind the nuclear motion centrifugal barrier, Vj(R)). Here the name shape resonance comes from the shape of the barrier on Vj(R). 

Both electronic and vibrational shape resonances arise from a direct process and can be explained by a single potential (McKoy, et al., 1984). Shape resonances (single Vi(r) or Vj(R)) differ from autoionization resonances and predissociation (with the exception of predissociation by rotation), which involve two potentials or two states with different quantum numbers. 

An important characteristic of shape resonances is that they cause non-Franck-Condon effects in vibrationally resolved photoionization spectra. These effects are a consequence of the strong R-variation of the transition moment caused by the R-dependence of the form of the continuum molecular orbital. 

Depending on the energy of the intersection between the diabatic states, the complementary state can be located below the ionization threshold, as it is for O2 in the (Oj X2ns) 3pau F-state (see Fig. 5.12) and for F2 (Orel et al., 1980) where the complementary state is a real bound state. However, when the complementary state is embedded in the electronic continuum, it gives rise to a shape resonance, and cannot be represented by a bound state. This complementary state corresponds to a very short lived resonance (the width corresponds to a lifetime of 10-16 to 10-17s). Consequently, the molecule lives for too short 

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Figure 8.4 Mechanism and timing of a ZEKE spectroscopy experiment, (Courtesy of W.A. Chupka).

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