Charged magnetoexcitons in two dimensions: Magnetic translations and families of dark states

We show that optical transitions of charged excitons in semiconductor heterostructures are governed in magnetic fields by a novel exact selection rule, a manifestation of magnetic translations. It is shown that the spin-triplet ground state of the quasi-two-dimensional charged exciton X- --- a bound state of two electrons and one hole --- is optically inactive in photoluminescence at finite magnetic fields. Internal bound-to-bound X- triplet transition has a specific spectral position, below the electron cyclotron resonance, and is strictly prohibited in a translationally-invariant system. These results allow one to discriminate between localized and free charged excitons.