Electron Spin Resonance Studies of Trans-Polyacetylene

The degenerate ground state in trans-polyacetylene is expected to give rise to excitations on the polyene chain in the form of topological solitions. These excitations can be neutral or charged. They exhibit reversed spin -charge relations; a neutral soliton has spin one-half while a charged soliton has spin zero. It has been predicted that charged solitons may be generated by doping or by photo-excitation. In this thesis, ESR studies of trans -polyacetylene after doping or during photoexcitation are presented. These studies are an attempt to verify the reversed spin-charage relation of the excitations. Coordinated transport, optical absorption and ESR studies were carried out on thin films of trans-polyacetylene doped with sodium. The Pauli susceptibility remains low ((chi)(,p) < 10('-8) emu/mole) for y < 0.05 while the conductivity reaches metallic levels, i.e. (sigma) (TURN) 1-10 (OMEGA)(' -1)cm('-1). This indicates that the charge carriers are spinless and the excess charge is stored in soliton states. ESR studies of trans-polyacetylene during photo -excitation were carred out to complement photoinduced absorption studies, done under similar conditions. These studies show that the photoexcited states are spinless. An upper limit on the number of photogenerated neutral solitons is set N(,N)('s) (LESSTHEQ) 10('16)/cm('3). The branching ratio, R, for photogeneration of neutral solitons to charged solitons is determined; R < 2 x 10('-3). These studies, in conjunction with photoinduced absorption studies, show that the same charged species is generated upon doping and photoexcitation. These excitations are charged solitons which exhibit the reversed spin-charge relation.