  A theory of shake-up processes in photoabsorption of an interacting
low-density two-dimensional electron gas (2DEG) in strong magnetic fields is
presented. In these processes, an incident photon creates an electron-hole pair
and, because of Coulomb interactions, simultaneously excites one particle to
higher Landau levels (LL's). In this work, the spectra of correlated charged
spin-singlet and spin-triplet electron-hole states in the first hole LL and
optical transitions to these states (i.e., shake-ups to the first hole LL) are
studied. Our results indicate, in particular, the presence of optically-active
three-particle quasi-discrete states in the exciton continuum that may give
rise to surprisingly sharp Fano resonances in strong magnetic fields. The
relation between shake-ups in photoabsorption of the 2DEG and in the 2D hole
gas (2DHG), and shake-ups of isolated negative X^- and positive X^+ trions are
discussed.

Mesoscale and Nanoscale Physics