Electronic Excitation at Molecule/Substrate Interfaces
소속 :
연사 : Prof. Toshiaki Munakata (Osaka University)
일시 : 2012-06-04 15:00 ~
장소 : 503동 404호
일 시 : 2012년 6월 4일, 3:00 PM
장 소 : 503동 404호
-Abstract-
Electronic excitation at interface between organic molecular film and substrate is a general importance for the area of organic electronics and light conversion processes. We have examined the resonance excitation between occupied and unoccupied energy levels at the interface of organic film and substrate. We employ two-photon photoemission (2PPE) spectroscopy to probe the unoccupied energy levels. Figure 1 shows 2PPE spectra for the 1 ML lead phthalocyanine (PbPc) film formed on HOPG graphite. The peaks aligned at fixed intermediate energy arise from unoccupied levels. Other peaks, shifting with photon energy, arise from occupied levels. The splitting of the peak indicates resonant excitation. From these spectra, all the occupied and unoccupied levels in the vicinity of the Fermi level are resolved. The intensity of the HOMO and LUMO+2 peaks are shown in Fig.2 by the area of the circles. On crossing the resonance, the HOMO peak suddenly disappears. The LUMO+2 peak is not observed below the resonance and suddenly becomes intense at the resonance. The intensity variation is not symmetric around the resonance, and is clearly different from the convolution of the occupied and unoccupied density of states. The discrete levels of molecule strongly interact with continuous substrate bands. The intensity variation is a result of quantum interference between transition pathways. We also show that the molecule/substrate interaction causes strongly enhanced molecular excitation mediated by the image potential state which is located outside the molecule.
장 소 : 503동 404호
-Abstract-
Electronic excitation at interface between organic molecular film and substrate is a general importance for the area of organic electronics and light conversion processes. We have examined the resonance excitation between occupied and unoccupied energy levels at the interface of organic film and substrate. We employ two-photon photoemission (2PPE) spectroscopy to probe the unoccupied energy levels. Figure 1 shows 2PPE spectra for the 1 ML lead phthalocyanine (PbPc) film formed on HOPG graphite. The peaks aligned at fixed intermediate energy arise from unoccupied levels. Other peaks, shifting with photon energy, arise from occupied levels. The splitting of the peak indicates resonant excitation. From these spectra, all the occupied and unoccupied levels in the vicinity of the Fermi level are resolved. The intensity of the HOMO and LUMO+2 peaks are shown in Fig.2 by the area of the circles. On crossing the resonance, the HOMO peak suddenly disappears. The LUMO+2 peak is not observed below the resonance and suddenly becomes intense at the resonance. The intensity variation is not symmetric around the resonance, and is clearly different from the convolution of the occupied and unoccupied density of states. The discrete levels of molecule strongly interact with continuous substrate bands. The intensity variation is a result of quantum interference between transition pathways. We also show that the molecule/substrate interaction causes strongly enhanced molecular excitation mediated by the image potential state which is located outside the molecule.
