Dynamic structure factor and excitons in TPD

We present a momentum dependent electron energy-loss study of the electronic excitations in TPD [N,N’-diphenyl-N,N’-bis(3-methyl-phenyl)-1,1’biphenyl-4,4’diamine] and compare our results to calculations based upon semiempirical post Hartree-Fock methods. Our experiment probes the dynamic structure factor $S(\mathbf{q},\omega )$ and the experimental results are in good agreement with the calculations indicating that the size of the primary gap exciton along the molecule is comparable to the length of the molecule. Calculations of the excited state bond-length relaxation can be interpreted in terms of the formation of polaron excitons.