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Enhancement of Signal Intensity for Inverse Photoelectron Spectroscopy by Surface Plasmon Resonance of Ag Nanoparticles

Information concerning the unoccupied states of condensed matter is of great relevance to its electronic, optical and chemical properties. These unoccupied states can be directly examined by inverse photoelectron spectroscopy (IPES), which is often regarded as the time-inversion process of photoelectron spectroscopy. The fundamental drawback of IPES is its low signal intensity. Other spectroscopic intensities are enhanced by surface plasmon resonance (SPR), so the intensity of the IPES signal may also be enhanced by SPR. This was, however, impossible because the photon energy involved in conventional IPES exceeds 9 eV and is much higher than the SPR energy of existing materials. In 2012, we developed low-energy IPES (LEIPS), in which the photon energy is <5 eV, which can then be matched with the SPR energy. We demonstrate a five-fold enhancement of the LEIPS signal from a prototypical organic semiconductor, copper phthalocyanine (CuPc), by SPR of Ag nanoparticles.