  We study effects of different configurations of adsorbates on the vibrational
modes as well as symmetries of polyacenes and poly-p-phenylenes focusing on
lithium atom adsorption. We found that the spectra of the vibrational modes
distinguish the different configurations. For more regular adsorption schemes
the lowest states are bending and torsion modes of the skeleton, which are
essentially followed by the adsorbate. On poly-p-phenylenes we found that
lithium adsorption reduces and often eliminates the torsion between rings thus
increasing symmetry. There is spontaneous symmetry breaking in
poly-p-phenylenes due to double adsorption of lithium atoms on alternating
rings.
