Narrowing of spectral lines beyond the natural or dephasing line width

Transient excitation and gated or delayed observation provides a
narrowing of the inherent line width of the transition. Possible
experimental one- and two-photon systems are discussed and the loss
of signal with line narrowing is calculated. The general case of
lines broadened by dephasing processes is treated. Contact is made
with the line narrowing in recent Raman type experiments.