Observations of Final State Interactions and Landau Damping in the Two-Dimensional Electron Gas

We report the measurement of new features of electron-electron interactions in inelastic light scattering by intersubband excitations of the 2D electron gas. The experiments are carried out in modulation doped GaAs single quantum wells of ultra-high electron mobility (Mu ~ 3-5x6 sup 6 cm sup 2/Vsec). The spin-density and charge-density excitations have widths, W 0.2 meV, that are much narrower than those reported in lower mobility systems. Such sharp lines allow the identification of a third type of spectral feature which is due to single particle intersubband transitions. The single-particle character is verified by the measurement of its dependence on in-plane wavevector and perpendicular magnetic field. The energy spacing between single-particle and spin-density excitations is in the range 2-3 meV. They are a direct measurement of large final state interactions of about ten percent of the transition energies.