  In recent researches of the dynamics of solitons, it is gradually revealed
that oscillation modes play a crucial role when we analyze the dynamics of
solitons. Some dynamical properties of solitons on external potentials are
studied with both numerical methods and analytical methods. In this paper, we
propose a method to deal with such oscillation modes of solitons in potential
wells. We show that oscillations of a soliton is described by the Klein-Gordon
equation with an external potential. Although this analysis does not seems to
give quantitative scattering amplitude of a soliton itself, it explains
qualitative pictures of scattering. As a result of our analysis, when a soliton
is scattered in a cyclic potential, the Laue pattern emerges. Furthermore,
since our analysis is based on the Klein-Gordon equation, a discrete frequency
spectrum of a soliton is obtained when it is bounded by some potentials. What
is especially important is that this analysis predicts a frequency spectrum of
a soliton in the Coulomb potential and then we find that this system absorbs
external waves with specific frequencies described by the Rydberg formula.
