Radiative recombination of 2D electrons with photoexcited holes in a single GaAlAs–GaAs heterojunction is used to illustrate the possibilities of the magneto-optic approach to the investigation of the ground state of strongly-correlated electrons in the ultraquantum limit. The method is used to determine the Coulomb gaps of quantum liquids and their hierarchy in the fractional quantum Hall effect. Wigner crystallization of 2D electrons is investigated and an analysis of time-resolved recombination spectra is used to reconstruct the phase boundary of the Wigner crystal.