The Observation of an Unoccupied Shockley-Type Surface State at the H$_2$-phthalocyanine/Ag(111) Interface

A free-electron interface state has been observed at the H$_2$-Phthalocyanine/Ag(111) interface using time- and angle-resolved two-photon photoemission. Energetically the interface state is located $\sim$0.21 eV above the Fermi level and angle-resolved measurements yield an effective mass of 0.5 $m_e$. These measurements, in conjunction with density functional theory calculations allow us to assign the interface state as being a metal/molecule hybrid state derived from the Shockley state of the clean Ag(111) surface. Time-resolved measurements of two different crystalline phases of H$_2$-Phthalocyanine monolayers reveal that the lifetime of the interface state is sensitive to the bonding geometry of the molecule. The results of this study add to a mounting body of evidence that suggests that the Shockley surface state is robust and persists after deposition of organic $\pi$-conjugated materials in the form of an uplifted interface state, instead of being `quenched'.