Neutral and Doped Polymers

When a positive polaron is formed, one electron is removed from the -WQ level (Fig. 18b). Thus, a positive polaron is expected to have the following intragap transitions  

When a negative polaron is formed, one electron is added to the +wg level (Fig. 18c). In this case, three transitions are also expected. Within the Fesser-Bishop-Campbell model, the subgap absorptions due to the Pi and P2 transitions are much stronger than that due to the P3 transition, because the P3 transition is symmetry forbidden. Quantitatively, the intensities of these subgap absorptions are given as a function of the ratio aig/Ag. 

The transition from the — ao level to the +o q level is missing, because the level is unoccupied. When a negative bipolaron is formed, two electrons are added to the -b o level and thus two transitions are again expected (Fig. 18e). Within the Fesser-Bishop-Campbell model, the subgap absorption due to the BPj transition is much stronger than that due to the BP3 transition, 

Soliton excitations are possible only in degenerate polymers such as /rans-polyacetylene. For a neutral soliton, a nonbonding electronic level is occupied by one electron. Thus, a neutral soliton is expected to have a single intragap transition (Fig. 18f)  

---