Collective excitations in atomic shells

Collective excitations in atomic shells are studied with Xe and Cs as an example. The starting point of the method is the active part of the self-consistent charge density of the 4d105s25p6 electrons in Xe and of the 4d105s25p66s1 electrons in Cs. The active charge density is homogeneously smeared out within the shell R1<r<R2, where the radii R1 and R2 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. (Auth.)