Evidence of dynamic Jahn-Teller distortions in two-dimensional crystallinemolecular films

The surface electronic structure in crystalline copolymer films of vinylide ne fluoride (70%) with trifluoro-ethylene (30%) has been studied by photoem ission and inverse photoemission as a function of alkali metal (sodium) dop ing. Sodium doping introduces at least two new states into the band (HOMO-L UMO) gap well away from the Fermi level. While the sodium-doped copolymer i s observed to resemble an n-type semiconductor, the change in electronic st ructure with temperature suggests that dynamic distortions lead to a photoe mission initial state splitting of the lower Hubbard-like bands. There is a decrease in the effective Debye temperature with sodium doping which may, in turn, enhance the dynamic Jahn-Teller distortion. [S0163-1829(99)06603-5 ].