  We use terahertz pulses to induce resonant transitions between the
eigenstates of optically generated exciton populations in a high-quality
semiconductor quantum-well sample. Monitoring the excitonic photoluminescence,
we observe transient quenching of the $1s$ exciton emission, which we attribute
to the terahertz-induced $1s$-to-$2p$ excitation. Simultaneously, a pronounced
enhancement of the $2s$-exciton emission is observed, despite the $1s$-to-$2s$
transition being dipole forbidden. A microscopic many-body theory explains the
experimental observations as a Coulomb-scattering mixing of the 2$s$ and 2$p$
states, yielding an effective terahertz transition between the 1$s$ and 2$s$
populations.
