Far-infrared magnetospectroscopic studies of quasi-two-dimensional donor impurity states have been carried out on several GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well samples with different doping densities in the barrier with constant sheet doping in the well. Two distinct features on the low-frequency side of cyclotron resonance (CR) were observed. Extensive temperature- and carrier-density-dependence studies combined with statistical calculations of the occupancy of the various possible states have led to the assignment of these features to a $D^{-}$ triplet transition (∼13 ${\mathrm{cm}}^{-1\mathrm{}}$ lower than CR) and the $D^{0}2p^{-}-2s$ transition (∼9 ${\mathrm{cm}}^{-1\mathrm{}}$ lower than CR).

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