Measurement of the Spin-Orbit Perturbation in theP-State Continuum of Heavy Alkali-Metal Atoms: K, Rb, and Cs

Abstract

The spin-orbit interaction for the $P$-state continuum of heavy alkali metals was investigated in a photoionization experiment using spin-polarized alkali atoms and circularly polarized light. From the asymmetry in ion-counting rates corresponding to the two photon helicities, Fano's spin-orbit perturbation parameter $x$ was determined over a range of several hundred angstroms for K, Rb, and Cs. The spin-orbit perturbation was found to increase from K to Rb to Cs as expected, and the nonlinear behavior of $x$ as a function of the photon energy $E$ was demonstrated for K. Knowledge of $x(E)$ was used to establish accurate values for the position of the Cooper minimum and to estimate the magnitude of the cross section at the minimum. In addition, the $x(E)$ data for Cs were used to gain information about the spin polarization of photoelectrons in a Fano-type polarized electron source. Finally, extrapolation of $x(E)$ for cesium into the discrete spectrum indicated the existence of a pole in the function $\ensuremath{\rho}(E)$ which corresponds to the doublet line-strength ratio $\ensuremath{\rho}({E}_{\mathrm{nP}})=\frac{S(n{P}_{\frac{3}{2}})}{S(n{P}_{\frac{1}{2}})}$ at the discrete energies ${E}_{\mathrm{nP}}$. According to our extrapolation, the pole lies in the region of $n=10 \mathrm{to} 15$, in agreement with the early spectroscopic work of Sambursky (1928) and Beutell (1939), whose measurements were discounted by later investigators.