We report on room-temperature Raman measurements on kappa-di[bis-ethyl
enedithiotetrathiafulvalene] copper dicyanamide bromide (kappa-(BEDT-T
TF)(2)Cu[N(CN)(2)]Br) and the isotopic analog C-13(6)-(BEDT-TTF)(2)Cu[
N(CN)(2)]Br, in which the six central carbon atoms in the inner rings
of the BEDT-TTF molecule are replaced with C-13 and assign the feature
s to the symmetric intramolecular modes of vibration, by comparison wi
th a recently published study on neutral BEDT-TTF. We then used these
data to help us make a more definitive assignment that has been previo
usly possible of the features in the low-temperature IR conductivity s
pectra of (BEDT-TTF)(2)Cu[N(CN)(2)]Br. It was also necessary to measur
e the low-temperature IR conductivity of additional isotopic analogs o
f BEDT-TTF, namely those containing C-13(6), C-13(2), S-34(8) and d(8)
. We found, as expected, that the spectra are dominated by the electro
n-phonon activated totally symmetric (A(g)) modes, but also that the n
ature of the strongest of these, nu(3)(A(g)), is changed dramatically
in (BEDT-TTF)(2)Cu[N(CN)(2)]Br from what it is in neutral BEDT-TTF. We
also found, unexpectedly, that one of the stronger IR features is mos
t likely due to an activated non-totally-symmetric (B-3g) mode, instea
d of a normally IR-active mode, which had been previously assigned, an
d further that several other (B-3g) modes appear to be activated in th
e IR spectra.