Evidence of dynamic Jahn-Teller distortions in two-dimensional crystalline molecular films

The surface electronic structure in crystalline copolymer films of vinylidene fluoride (70%) with trifluoro-ethylene (30%) has been studied by photoemission and inverse photoemission as a function of alkali metal (sodium) doping. Sodium doping introduces at least two new states into the band (HOMO-LUMO) gap well away from the Fermi level. While the sodium-doped copolymer is observed to resemble an n-type semiconductor, the change in electronic structure with temperature suggests that dynamic distortions lead to a photoemission 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].