  The C=C stretching modes in resonance Raman spectra and infrared reflectivity
were measured at temperatures between 15 K and 300 K using various
polarizations in k-(BEDT-TTF)_2Cu[N(CN)_2]Br, its fully and partially
deuterated analogues, and ^{13}C-substituted analogue. The infrared- and
Raman-active bands were re-assigned based on the factor group analysis. We
found a Raman-active EMV-coupled \nu_3 mode near 1420 cm_{-1}, which shows a
large downshift and broadening through the inter-dimer EMV interaction. The
mixing of the bridge and ring C=C stretching vibrations depends upon the
symmetry of the crystal mode. In contrast to deuterated crystals, the
non-deuterated crystals show a splitting in the \nu_2, \nu_3, and \nu_{27}
bands.
