I am writing this question here because I have a problem in understanding the Wigner Threshold law in Photodetachment and Photoionization.

The Wigner Threshold Law is given by:

$$\sigma=E^{L+1/2},$$

where $\sigma$ is the photodetachment cross section, E is the kinetic energy of the detached electron from the anion, and L is the detached electron angular momentum. 

I have the following questions:

What is the threshold in this law? For this threshold law to be valid, should the energy be above or below the threshold? Where the literature says "near threshold", does this mean above or below the threshold - and how far?

I found the threshold law for photodetachment but I couldn't find a threshold law for photoionization. Is there any threshold law for photoionization? It is known that the ejected electron and the neutral core (in the case of photodetachment) have an effective potential which is a sum of the interaction potential and a centrifugal potential:

$$V\_{\mathrm {eff}}=V(r) + h/2mr^2 [l(l+1)].$$

Is this centrifugal potential (second term) still valid in the case of photoionization? 

PS: If anyone can suggest a textbook or any other reference, having the derivation and details about the Wigner law on a fundamental level (a graduate level so that I can understand the basics very well), that would be good.