It is not clearly understood how and why the Raman intensity increases drastically with an increasing chain length of polyenes. We therefore investigated the vibrational intensities of the C = C stretching vibrational modes of four polyene systems of X-(HC=CH)(n)-Y, where X/Y=H/H, H/NH2, H/NO2, and NH2/NO2. The investigation was done using nodal mode analysis (based on the number of nodes formed by the alternations of stretches and contractions) combined with ab initio frequency calculations. The C=C stretching/contracting mode without node is found to have the strongest Raman intensities regardless of polyene systems because of the long-range cooperation effect by the concurrent stretch/contraction motion of all C=C bonds. The corresponding IR spectra have also the strongest intensities for the nonsymmetric polyene systems, whereas are inactive for the symmetric polyenes (by the exclusion rule). The intensities of the nonconcurrent C=C stretching/contracting modes (particularly for the Raman spectral tend to decrease drastically (in proportion to [n/(m + 1)](4)) with increasing node number m, though weak (or zero) intensities appear somewhat alternately because the molecules have approximate (or exact) centrosymmetry. (C) 1997 American Institute of Physics.