Very narrow photoemission bandwidth of the highest occupied state in a copper-phthalocyanine monolayer

We observed a very narrow bandwidth of the highest occupied molecular orbital (HOMO) state in ultraviolet photoemission spectra (UPS) of copper-phthalocyanine monolayer deposited on graphite. The HOMO band in UPS consists of three components which may originate from the vibrational coupling. The full width at half maximum of each component was found to be ∼150 meV at 295 K. This HOMO-bandwidth leads to an estimation that the lifetime of the HOMO hole should be at least longer than 2.2 fs, which may be dominated by the electron transfer rate from the substrate to the molecule.