  We investigated the temperature-dependent optical conductivity of NaV2O5 in
the energy range 4 meV-4 eV. The intensities and the polarization dependence of
the detected electronic excitations give a direct indication for a
broken-parity electronic ground-state and for a non-centrosymmetric crystal
structure of the system in the high-temperature phase. A direct two-magnon
optical absorption process, proposed in this Letter, is in quantitative
agreement with the optical data. By analyzing the optically allowed phonons at
various temperatures above and below the phase transition, we conclude that a
second-order change to a larger unit cell takes place below 34 K.
