Valence electronic structure of gas-phase 3,4,9,10-perylene tetracarboxylic acid dianhydride: Experiment and theory

We present gas phase ultraviolet photoemission spectra of 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA) and compare them to condensed phase spectra. Contrary to common expectations for organic solids, we observe that the two differ in many details, necessitating a reexamination of our theoretical understanding of the PTCDA electronic structure. Thus, we use density functional theory with several exchange-correlation functionals, as well as the GW method. We find that the local density approximation and the generalized gradient approximation fail to describe the experimental data because of underbound orbitals that are concentrated on the anhydride groups. We show that this is corrected by either a hybrid functional or the GW method and use the results to interpret the experimental data.