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Abstract

The resonant Raman spectra of powder samples of protonated, partially and fully deuterated TTF - TCNQ and protonated TSeF - TCNQ have been measured as a function of temperature (10K-295K) using a Brucker RFS 100 spectrometer with an infrared laser. A number of previously unobserved lines appear in the spectra of TTF - TCNQ below approximately 200 K . The intensity of these lines increases with decreasing temperature. However they show no discontinuity at the three phase transtions, which establish the three-dimensional order of the fluctuating charge - density - wave (CDW) in TTF - TCNQ . These new lines are assigned to the normally Raman - inactive (and infrared - active) B3u modes on the basis of the measured isotopic shifts. The appearance of these B3u out-of- plane intramolecular distortion modes of the TCNQ molecule are in agreement with the results of a recent x-ray study which found that such out-of plane distortions of the TCNQ molecule occur with the appearance of the CDW on the TCNQ chain. Only the intramolecular modes of the TCNQ molecule are resonant and thus observed. A few of the new lines also appear in the low - temperature spectra of TSeF - TCNQ but are generally much weaker in intensity.