Molecular orientation and photo-degradation of PTCDA films on TiO$_{2}$(110)

The molecular orientation and photo-degradation process of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) films on TiO$_{2}$(110) were investigated by near edge X-ray absorption spectroscopy (NEXAFS). As the incidence angle of p-polarized synchrotron light with respect to the substrate surface normal increases, the intensity of the $\sigma $* resonances diminishes and the $\pi $* resonances is greatly enhanced. This finding indicates that the molecular orientation of the PTCDA film is flat on the TiO$_{2}$(110) surface. NEXAFS results of pristine and photo-degraded PTCDA films exposed to synchrotron white light at the VLSPGM beamline of CAMD revealed the photo-degradation mechanism. We found that the intensity of $\sigma $* states diminishes and the intensity of $\pi $* states of increases upon the PTCDA film exposed to white light for 30 minutes.