ABSTRACT
Atomic line spectra reveal atomic structure and the fact that energy is quantized. Only transitions between the “fixed” excitation levels of an atom are possible. However, not all combinations of levels occur in actual spectra and not all spectral lines are of similar intensity. These observations point to selection rules (invoking parity, angular momentum, spin, etc.) and to the concept of transition probability, or the “A factor” Aki for a transition from level k to level i. The mean lifetime τ that appears in the exponential decay law of an excited level is the reciprocal of the sum of all transition probabilities from a given level:
τk = 1 ΣAki
.
