High-precision laser absorption spectroscopy in a thermal vapor cell is used in the measurement of the ratio of the $D$-line transition strengths in the cesium atom. We find $\mathbf{|}〈{6p}^2{P}_{3/2}||\overrightarrow{r}||{6s}^2{S}_{1/2}〉{\mathbf{|}}^2/\mathbf{|}〈{6p}^2{P}_{1/2}||\overrightarrow{r}||{6s}^2{S}_{1/2}〉{\mathbf{|}}^2=1.9809\mathrm{}\left(9\right),$ which compares favorably with the ratio calculated from the most precise lifetime measurements of these states and the predictions of ab initio calculation. We also verify the predictions of impact-regime line-broadening calculations for densities of order ${10}^{18}$ atoms per ${\mathrm{m}}^3.$ Theoretical implications and future experimental promise are discussed.

• Received 29 December 1997

DOI:https://doi.org/10.1103/PhysRevA.58.1087