  In this work an improved potential model for quarkonium spectra (charmonium,
bottomonium) is constructed. Beside the one-gluon exchange and a linear
confinement potential, the model includes systematically effects from two-gluon
exchange and the induced interaction (i.e. t-channel exchanges of
quark-antiquark bound states). These turn out to be important in order to
reproduce correctly the fine- and hyperfine structure of the charmonium and
bottomonium states. It is demonstrated that such a satisfactory description of
the quarkonium spectra can be obtained with parameters (quark masses, strong
coupling constant and string tension) consistent with values of lattice
simulations and QCD determinations.
