We have investigated the crystalline structure of PTCDA overlayers on reconstructed Au(111) surfaces by the combination of scanning tunneling microscopy (STM) with in situ reflection high energy electron diffraction (RHEED). Large domains of PTCDA (up to 200 nm) are found on the still reconstructed Au surface. These structurally well-characterized PTCDA samples have been optically examined in detail, mainly by cw-and time-resolved fluorescence. An important finding is the occurrence of a new spectral feature in the emission around a wavelength of 590 nm. Temperature and intensity dependent measurements reveal that the very broad emission around 700 nm (commonly considered to be caused by excimer emission) consists of several sub-bands with different origin.