  By using Supersymmetric Quantum Mechanics and Semiclassical Quantization, one
may argue that the low-lying excited states of any quantum system can be
modeled by a set of harmonic oscillators. In the present paper, we fit the
experimental excitation spectra of atoms with atomic number 2< Z< 36 to a
simple harmonic oscillator model with two parameters: the number of degrees of
freedom, d, and the effective frequency, \omega. The obtained \hbar\omega takes
values around 0.03 (in atomic units), whereas d shows clear shell filling
effects, that is, takes high values for the noble gases, suggesting collective
oscillations of the electrons occupying the last shell.
