Combined experimental and theoretical study of the 6p²P_j→8s²S_1/2 relative transition matrix elements in atomic Cs

A combined experimental and theoretical study of transition matrix elements of the 6p²P_j→8s²S_1/2 transition in atomic Cs is reported. Measurements of the polarization-dependent two-photon excitation spectrum associated with the transition were made in an ≈200cm^-1 range on the low-frequency side of the 6s²S_1/2→6p²P_3/2 resonance. The measurements depend parametrically on the relative transition matrix elements, but also are sensitive to far-off-resonance 6s²S_1/2→np²P_j→8s²S_1/2 transitions. In addition, as the measured quantities are ratios of polarization-dependent intensities at a single-excitation frequency, they are quite insensitive to a variety of common-mode systematic effects; matrix-element ratios may then be determined to high accuracy. In the past, the matrix-element dependence has yielded a generalized sum rule, the value of which is dependent on sums of relative two-photon transition matrix elements. In the present case, best available determinations from other experiments are combined with theoretical matrix elements to extract the ratio of transition matrix elements for the 6p²P_j→8s²S_1/2(j=1/2,3/2) transition. The resulting experimental value of 1.423(2) is in excellent agreement with the theoretical value, calculated using a relativistic all-order method, of 1.425(2).