  We study the second-order polaronic resonance between 2-LO-phonon states and
p-shell electron states in a quantum dot. We show that the spectrum in the
resonance area can be quantitatively reproduced by a theoretical model using
only standard material constants, without any free parameters. We propose also
a perturbative approach to the problem based on a quasi-degenerate perturbation
theory. This method not only considerably reduces the numerical complexity
without considerable loss of accuracy but also gives some insight into the
structure and origin of the resonance spectrum.
