Internal transitions of negatively charged magnetoexcitons and many body effects in a two-dimensional electron gas.

Internal transitions of quasi-two-dimensional, negatively charged magnetoexcitons ( ${X}^{\ensuremath{-}}$) and their evolution with excess electron density have been studied in GaAs/AlGaAs quantum wells. In the dilute electron limit, due to magnetic translational invariance, the optically detected resonance spectra are dominated by bound-to-continuum bands in contrast to the negatively charged donor system ${D}^{\ensuremath{-}}$, which exhibits strictly bound-to-bound transitions. With increasing excess electron density Landau-level filling factors $\ensuremath{\nu}l2$ the ${X}^{\ensuremath{-}}$-like transitions are blueshifted; they are absent for $\ensuremath{\nu}g2$. The blueshifted transitions are explained in terms of a new type of collective excitation\char22{}magnetoplasmons bound to a mobile valence band hole.