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

We show that optical transitions of charged excitons in semiconductor heter ostructures are governed in magnetic fields by a novel exact selection rule , a manifestation of magnetic translations. It is shown that the spin-tripl et ground state of the quasi-two-dimensional charged exciton X--a bound sta te of two electrons and one hole-is optically inactive in photoluminescence at finite magnetic fields. Internal bound-to-bound X- tripler transition h as a specific spectral position, below the electron cyclotron resonance, an d is strictly prohibited in a translationally invariant system. These resul ts allow one to discriminate between free and disorder-affected charged exc itons.