Quarkonium spectra and quantum chromodynamics

$c\overline{c}$ and $b\overline{b}$ spectra, including fine and hyperfine structures, are investigated with the use of a recently derived quantum-chromodynamic potential supplemented by a phenomenological longrange confining potential. Our theoretical results for all energy levels below the charm and bottom thresholds as well as their leptonic widths are in excellent agreement with experiments. We also give unambiguous theoretical predictions for unobserved $b\overline{b}$ energy levels, and present reasonable estimates for energy levels of $t\overline{t}$ Our values of the strong coupling constant ${\alpha }_s$ and the QCD scale parameter $\Lambda$ are consistent with those extracted from high-energy positron-electron annihilation.