Outer-Core Emission Spectroscopy of the Heavy Alkalis and Alkali-Alkali Metal Alloys.

Electron-induced emission spectra from outer core np^5(n + 1)s^2 to np^6(n + 1)s recombination processes are reported for K (n = 3), Rb (n = 4), and Cs (n = 5), both as pure metals and when alloyed in Li, Na, and each other. The results for pure Rb and Cs allow the first comparisons of emission profiles, core lifetimes, and phonon broadening widths through the series of alkali metals Na, K, Rb, Cs. The emission profiles are remarkably similar and scale in width in close proportion to the Fermi energies. An exception is the MND "x-ray edge anomaly" response below threshold which decreased from Na to Cs and, for each alkali, is less pronounced than the corresponding feature in absorption. The spin orbit partner intensity ratio, J_{3/2 }:J_{1/2}, increased from ~10 for Na to > 65 for Cs, much different from the factor of two expected from the core hole degeneracy. For K and Rb, the data are consistent with Coster-Kronig decay reducing the J_{1/2} hole populations. Least -squares fits to the edge profiles of K, Rb, and Cs taken at high resolution and at temperatures from 15K to 300K agree with the temperature-dependent phonon broadening and core hole lifetimes of 15 +/- 10 meV, 25 +/- 10 meV, and 50 +/- 15 meV respectively. Emission intensity ratios observed for thin bilayer films support these findings. The anomalously large absorption edge widths observed in dilute alloys of K, Rb, and Cs in Li and Na are found also in the present emission spectra as well. The emission and absorption edges cross at half height, which establishes that some broadening mechanism other than quasiparticle excitations contributes to the edge width behavior. Phonon broadening is suspected, but the large Stokes shift normally associated with this mechanism is lacking. Changes observed in the K spin-orbit intensity ratio when alloyed in Cs are consistent with observed reductions in the K M _{rm III} core hole lifetime by Coster-Kronig processes, as deduced from the changes in emission intensity. Measurements of emission intensity show that the relative lifetimes of alkali cores, both as pure metals and as impurities in other alkalis, increases strongly with the observed emission band width. A dependence of the Auger matrix element on band width is the likely cause.