Invited Paper SA-MoA9
Electronic States of Functional Molecular Materials Probed by Low-energy Excitation

Monday, October 19, 2015, 5:00 pm, Room 112

Understanding the impacts of electron-phonon coupling as well as weak intermolecular interaction on the electronic state is required to discuss the mechanism of charge transport in functional molecular materials. Ultraviolet photoelectron spectroscopy (UPS) is known to be a powerful technique to study the electronic states. However, the experimental study of fine features in the highest occupied molecular orbital (HOMO) state has not been progressed till recently due to difficulty in the sample preparation, damages upon irradiation and so on, though it can offer a variety of key information, that is essential to comprehend charge-hopping transport and small-polaron related transport in the ordered monolayer film [1] as well as to coherent band transport in the molecular single crystal [2,3]. We present recent findings regarding on the precise measurements of electronic states for large aromatic organic molecular materials by using low-energy excitation UPS.

Use of low-energy excitation realizes unique experiments to find out the followings as an advantageous for the organic materials; i) low-kinetic energy of photoelectron may achieve resonance excitation channel to open eyes for many events accessed, e.g. on a degree of electron cloud localization, inelastic scattering phenomena at the interfaces and a breaking the sudden approximation, and ii) high-photoionization cross section and bulk-sensitive probe may detect a quasi-particle state in very narrow HOMO band. Combining all findings above mentioned, characteristics of an electron in the functional molecular material has been embossed.

[1] S. Kera et al., Prog. Surf. Sci. 84 (2009) 135.

[2] N. Ueno et al., Prog. Surf. Sci. 83 (2008) 490.

[3] S. Ciuchi et al., Phys. Rev. Lett. 106 (2011) 166406.