The lifetimes of the first excited states of the elements Na, K, Rb, and Cs were measured using the phase-shift method described by Brewer et al. [Rev. Sci. Instr. 33, 1450 (1962)]. The data were taken over a wide range of vapor densities to allow the effects of radiation entrapment and of scattered exciting light to be taken into account. The following values were obtained: Na 3p2P 16.1±0.3 nsec, K 4p2P 27.8±0.5 nsec, Rb 5p2P3/2 27.0±0.5 nsec, and Cs 6p2P3/2 30.5±0.7 nsec.
JOHN K. LINK, "Measurement of the Radiative Lifetimes of the First Excited States of Na, K, Rb, and Cs by Means of the Phase-Shift Method," J. Opt. Soc. Am. 56, 1195-1199 (1966)