An optical pumping technique was used to enhance and localize nuclear
magnetic resonance (NMR) signals from an n-doped GaAs/Al0.1Ga0.9As mul
tiple quantum well structure, permitting direct radio-frequency measur
ements of gallium-71 NMR spectra and nuclear spin-lattice relaxation r
ates (1/T-1) as functions of temperature (1.6 K < T < 4.2 K) and the L
andau level filling factor (0.66 < nu < 1.76). The measurements reveal
effects of electron-electron interactions on the energy levels and sp
in states of the two-dimensional electron system confined in the GaAs
wells. Minima in 1/T-1 at nu approximate to 1 and nu approximate to 2/
3 indicate energy gaps for electronic excitations in both integer and
fractional quantum Hall states. Rapid, temperature-independent relaxat
ion at intermediate v values indicates a manifold of low-lying electro
nic states with mixed spin polarizations.