We present the results of a spectroscopic study carried out on a solid sample of 5,5 "-bis(dicyanomethylene)-5,5 "-dihydro-Delta(2,2＇ :5＇ ,2 ") -terthiophene. This compound bears a heteroquinonoid structure in its ground state and could be considered as a model of the structural changes induced by the different types of charge carriers in doped polythiophene. We have identified two very strong Raman bands that are indicative of the attainment of a pure heteroquinonoid structure in alpha-oligothienyl compounds. The large frequency downshifts of these Raman bands with respect to the pure heteroaromatic structure are a measure of the extent of the electronic modification caused by the quinoidization of the ring molecular system.