The properties and variation of

We now consider how many q reversals are expected in a Rydberg series and the conditions under which they occur. Experimental data (such as those in fig. 8.18) demonstrate that there can be more than just one q reversal in a Rydberg series perturbed by an intruder level. One might ask is this a sign of perturbations due to more than one interloper, or can one interpret the two reversals as having a common origin  

Some insight is gained by asking how many q reversals can be expected 

General K-matrix expressions have been derived for the variation of q as a function of detuning, both in elastic scattering and in photoionisation [375, 377]. It turns out that as many as six q reversals can occur in the general case in photoionisation but that, in elastic scattering, there are at most two. 

K-matrix theory is a rather natural framework to analyse the behaviour of the q parameter because, in the case of a single channel, —q l is already the background K-matrix. 

The variation of q as a function of energy is plotted in fig. 8.20 for coupling strengths ranging from weak to strong coupling. These curves reveal the following characteristics  

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