Outer-core emission spectra of heavy alkali metals

We report ${\mathrm{np}}^5$(n+1)$s^2$→${\mathrm{np}}^6$(n+1)s emission spectra of K (n=3), Rb (n=4), and Cs (n=5), and compare emission-band characteristics through the series Na–Cs using earlier data for Na. The normalized band profiles of the different alkali metals are remarkably similar when scaled by the Fermi energy $E_F$. However, the spin-orbit partner intensity ratios are far from the ideal value 2, reaching approximately 60 for Rb and above 65 for Cs, mainly owing to Coster-Kronig decays from higher levels. We confirm that the Mahan–Nozières–De Dominicis ‘‘anomaly’’ at $E_F$ is generally weaker in emission than in absorption. It decreases through the series to become undetectable for emission from Cs. A systematic increase of the core-hole-lifetime width occurs through the column of alkali metals from a reported estimate of 10 meV for Na to a value of 50 meV for Cs. A study of the Fermi-edge shape between 20 and 300 K reveals temperature-dependent phonon broadening in generally good agreement with theoretical predictions. Incomplete relaxation plays only a minor role in the edge processes of the heavy alkali metals. Additional Fermi-edge broadening and the shifted emission edges of surface atoms are observed for alkali-metal films 10–100 Å thick.