ELECTRON-POSITRON MOMENTUM DENSITY IN TTF-TCNQ

We present measurements of the positron two-dimensional angular correl ation of annihilation radiation (2D-ACAR) in TTF-TCNQ. We report also theoretical simulations of the 2D-ACAR in which the electron wave func tions were expressed as TTF or TCNQ molecular orbitals obtained from s elf-consistent quantum chemical calculations. The positron wave functi on was calculated taking the charge transfer from TTF to TCNQ as a par ameter. The best agreement with the experiment is obtained for a charg e transfer of 0.7 electrons from the TTF to the TCNQ molecules. This i s larger than the value of 0.55 obtained from a study of the Kohn anom aly. We investigate also the shape and position of the Fermi surface a nd conclude that a simple planar Fermi surface is consistent with our measurements.