One- And two-photon photo emission micro-spectroscopy for organic films

We have constructed a laser-based microspot photoemission spectrometer which achieved lateral resolution of 0.3 μm and energy resolution of 30 meV. The light source at wavelength of 140 nm (photon energy of 8.86 eV) was generated as the 6-th harmonics of a titanium sapphire laser of 100 fs pulse duration. The high-energy resolution is the characteristics of our apparatus. The space-charge effect which affects the energy resolution and limits the sensitivity was described. The apparatus was applied to reveal spatial inhomogeneity of copper phthalocyanine films caused by intermolecular interaction and inter-layer interaction. We found that the highest occupied molecular orbital is peaking at the binding energies of 1.13, 1.23, and 1.38 eV depending on the film thickness and the sample positions. The peaks were assigned to originate from isolated molecules, ordered monolayer, and the second layer, respectively. The apparatus can also be operated as microspot two-photon photoemission spectrometer which probes unoccupied electronic states. A surface image due to the unoccupied first image potential state of Cu(111) facets showed that the lateral resolution for two photon photoemission with a light of 280 nm wavelength was 0.4 μm, smaller by 1/√2 than the diffraction limited spot size.