Abstract
The responses of two-level atoms and of sodium atoms to trains of repetitively pulsed resonance radiation are computed. It is found that the optimum backscatter from mesospheric sodium for a given irradiance is attained when the pulse trains are phase modulated and when the lengths of individual pulses are in the 500–700-ps range, corresponding to a transform-limited bandwidth somewhat smaller than the Doppler-broadened hyperfine-structure pattern of the D2 line.