Origin of indium-[perylene-3,4,9,10-tetracarboxilic dianhydride] interface states studied by outermost surface spectroscopy using metastable atoms

Metastable atom electron spectra (MAES) and ultraviolet photoelectron spectra (UPS) of indium-[perylene-3,4,9,10-tetracarboxilic dianhydride (PTCDA)] system prepared on a ${\mathrm{MoS}}_{2}$ single crystal substrate were measured as a function of the In overlayer thickness $({\ensuremath{\Theta}}_{\mathrm{In}}).$ As observed by a previous UPS experiment, a new band was observed in the original PTCDA energy gap region even by the MAES which detects the outermost surface selectively. The ${\ensuremath{\Theta}}_{\mathrm{In}}$ dependence of this new band intensity measured by the MAES gives a maximum at ${\ensuremath{\Theta}}_{\mathrm{In}}\ensuremath{\sim}1\AA{},$ suggesting that four In atoms are reacting with one PTCDA molecule at the $\mathrm{C}\mathrm{=}\mathrm{O}$ parts. From the result of the density functional theory (DFT) methods and the enhanced intensity of the new band in the MAES, it was concluded that the new band originates from \ensuremath{\pi} state consisting of In $5pz$ AO's.