Soft X-ray threshold shapes extracted from Linde's rule in the optical alchemy approximation

The X-ray threshold exponents of free-electron metals are determined from impurity resistivity data in the optical approximation: an X-ray excited atom with its core hole is assumed to be identical to an atom 'optically transmuted' into one with an additional 'proton'. This leads to an approximate upper bound of alpha 0 approximately=0.21 for Na, Mg and Al L 2,3 edges.