Measurement of the ratio of the cesium D -line transition strengths

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 $|〈{6p}²{P}3/2||{{→}}{r}||{6s}²{S}1/2〉{|}²/|〈{6p}²{P}1/2||{{→}}{r}||{6s}²{S}1/2〉{|}²=1.9809(9),$ 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 10¹⁸ atoms per m³. Theoretical implications and future experimental promise are discussed.