ABSTRACT
In this chapter we look at questions relating to the intensities of electronic transitions, first in the absence of hyperfine structure and then with it. Our emphasis is on relative intensities rather than absolute intensities. The latter are difficult to reliably compute and to measure. Two distinct types of transitions arise. Parity allowed transitions that require a change of parity of the one-electron orbitals, and parity forbidden transitions, which occur when there is no change in the parity of the one-electron states. Recall that the parity, P , of the states of an electron configuration is
P = (−1) ∑
(7-1) where the summation is over all one-electron orbitals of the configuration. The parity is said to be even or odd as P is even or odd. Closed shells are necessarily of even parity and may be omitted in the summation.
