with a local site-impurity potential in the framework of the su, schrieffer, and Heeger (SSH) model, both the dopant dependence and the chain-length dependence of the vibrational modes related to charged solitons in a finite trans-polyacetylene chain have been studied. The numerical calcultions showed that the well-known Goldstone mode will be pinned by the dopant potentials as well as by the chhain ends. Two localized modes were found in the acoustic-frequency branch. The results are in agree-ment with the infrared- active vbibrational modes also observed in lightly doped polyacetylene cases. A comparison with the phtoexcitation measurements has been given.