Group theory and the link between expectation values of powers of $r$ and Clebsch-Gordan coefficients

In a recent paper [J.-C. Pain, Opt. Spectrosc. 218 , 1105-1109 (2020)], we discussed the link between expectation values of powers of r and Clebsch-Gordan coefficients. In this short note we provide additional information, reminding that such a connection is a direct consequence of group theory. The hydrogenic radial wavefunctions form bases for infinite dimensional representations of the algebra of the non-compact group O (2 , 1) and the expectation values r p and r − p ( p being pos-itive) transform as tensors with respect to this algebra. As shown a long time ago by Armstrong [L. Armstrong Jr., J . Phys. (Paris) Suppl. C 4 31 , 17 (1970)], analysis of matrix elements of r p and r − p reveals that the Wigner-Eckart theorem is valid for this group and that the corresponding Clebsch-Gordan coefficients are proportional to the usual SO (3) Clebsch-Gordan coefficients. This proportionality provides simple explanations of the selection rules for hydrogenic radial matrix elements pointed out by Pasternack and Sternheimer, and the proportionality of hydrogenic expectation values of r p and r − p to 3 jm symbols.