Collective Excitations in Atomic Shells

Collective excitations in atomic shells are studied with Xe and Cs as an example. The starting point of our method is the active part of the self-consistent charge density of the 4 d 10 5 s 2 5 p 6 electrons in Xe and of the 4 d 10 5 s 2 5 p 6 6 s 1 electrons in Cs. The active charge density is homogeneously smeared out within the shell R 1 < r < R 2 , where the radii R 1 and R 2 are found in a similar way as in the work by Mukhopadhyay and Lundqvist. Dipole-active collective excitations (atomic-shell surface plasmons) of the charge density within the shell are studied by sum rule techniques due to Lushnikov et al. Two dipole active surface plasmons are found whose peak positions and widths excellently agree with recent experimental data by West and Morton. This model result establishes an analog to the well-known plasmon pole approximation for electrons in metals.