Energy spectra and photoluminescence of charged magneto-excitons
  Charged magneto-excitons X- in a dilute 2D electron gas in narrow and symmetric quantum wells are studied using exact diagonalization techniques. An excited triplet X- state with a binding energy of about 1 meV is found. This state and the singlet are the two optically active states observed in photoluminescence (PL). The interaction of X-'s with electrons is shown to have short range, which effectively isolates bound X- states from a dilute e-h plasma. This results in the insensitivity of PL to the filling factor nu. For the "dark" triplet X- ground state, the oscillator strength decreases exponentially as a function of 1/nu which explains why it is not seen in PL. 
Arkadiusz Wojs, John J. Quinn, and Pawel Hawrylak