Infrared and Raman studies of the organic superconductor kappa-(BEDT-TTF)(2)[Cu(NCS)(2)] and its C-13(4) isotopic analog

We report on low-temperature infrared reflectivity measurements as well as room-temperature Raman measurements of kappa-(ET)(2)[Cu(NCS)(2)] and its C-13(4) isotopic analog, in which the four carbon atoms in the terminal ethylene groups have been substituted. The measured isotopic frequency shifts are used to support previous assignments of the infrared features to A(g) and B-3g modes. Two Raman spectrometers were used, one using a visible laser and the other an infrared laser. The spectra are very dissimilar, due to the resonant nature of the scattering. The electronic resonance responsible for the infrared-laser Raman scattering appears to be the same as that occurring in the infrared spectrum. We have reassigned nu(9)(A(g)) in the infrared spectrum and successfully compare our frequency shifts due to electron-phonon coupling with those recently calculated. (C) 1997 Elsevier Science B.V.