Direct Measurement of the $5s5p\,{}^1P_1 \to 5s4d\,{}^1D_2$ Decay Rate in Strontium
Abstract
We investigate the decay process $5s5p\,{}^1P_1 \to 5s4d\,{}^1D_2 \to 5s5p\,{}^3P_2$ in a magneto-optical trap of Sr atoms operating on the $461\,\mathrm{nm}$ ($5s^2\,{}^1S_0 - 5s5p\,{}^1P_1$) transition by irradiating the trapped atoms with laser light resonant with the $448\,\mathrm{nm}$ ($5s4d\,{}^1D_2 - 5s8p\,{}^1P_1$) transition and observing the transient response of atom fluorescence. We measure, for the first time, the branching ratio of the $5s4d\,{}^1D_2 \to 5s5p\,{}^3P_2$ transition to be $0.177(4)$, which significantly deviates from the widely cited theoretical value of $0.322$ [C. W. Bauschlicher Jr. et al., J. Phys. B 18, 1523 (1985)]. Moreover, we determine the decay rate of the $5s5p\,{}^1P_1 \to 5s4d\,{}^1D_2$ transition to be $5.3(5)\times10^3\,\mathrm{s^{-1}}$, consistent within uncertainty with the widely cited experimental value [L. R. Hunter et al., Phys. Rev. Lett. 56, 823 (1986)], but substantially lower than the recent theoretical value of $9.25(40)\times10^3\,\mathrm{s^{-1}}$ [A. Cooper et al., Phys. Rev. X 8, 041055 (2018)]. These findings have significant implications for laser cooling of Sr and fluorescence detection of single atoms in optical tweezers. They also call for a reevaluation of theoretical frameworks used to calculate transition rates essential for evaluating blackbody radiation shifts in the Sr optical atomic clock.