Hybridization of an unoccupied molecular orbital with an image potential state at a lead phthalocyanine/graphite interface

Abstract

The interaction of a molecular orbital with a surface state is important to understand the spatial distribution of the wave function at the molecule/substrate interface. In this study, we focus on hybridization of an unoccupied state of lead phthalocyanine (PbPc) with the image potential state (IPS) on a graphite surface. The hybridization modifies the energy-momentum dispersions of the IPS on PbPc films as observed by angle-resolved two-photon photoemission. On the PbPc 1 monolayer film, the IPS band forms a band gap and back-folding appears at the first Brillouin zone boundary due to the periodic potential by the adsorbate lattice. The modification of the dispersion is accompanied by the intensity enhancement of the IPS. We attributed the origin of the modified dispersion and intensity enhancement to a hybridization of the IPS with a molecule-derived unoccupied level. From the photon energy-dependent measurement on multilayer films, we have found the diffuse unoccupied molecular level in the vicinity of the IPS. The tail part of the IPS wave function in the substrate is enhanced by the hybridization with the unoccupied state, and thus strengthens the transition from the occupied substrate band to the hybridized IPS.

Hybridization-Induced Image Potential States with Large Effective Mass in Lead Phthalocyanine Overlayers on Graphene

“…Although an IPS, in general, shows a parabolic band whose effective mass m * is equal to the electron mass m e , two-photon photoemission (2PPE) spectroscopy measurements have revealed that the lowest IPS (LIPS) of PbPc on graphite exhibits a significant increase in the effective mass as m */ m e ≃ 2.2 ± 0.3, , which suggests the influence of the PbPc overlayer. Recently, a similar increase in the effective mass has also been observed for a PbPc overlayer on single crystalline graphite (SCG) …”

“…A similar square pattern has long been observed with scanning tunneling microscopy for MPc on highly oriented pyrolytic graphite (HOPG), − which can be regarded as a model system of graphene. In particular, Munakata and co-workers have studied intensively the properties of lead phthalocyanine (PbPc) on HOPG ,,,− including the image potential states (IPSs), , a set of unoccupied states that exist universally not only on metal but also on insulator − and semiconductor − surfaces.…”

“…A similar square pattern has long been observed with scanning tunneling microscopy for MPc on highly oriented pyrolytic graphite (HOPG), − which can be regarded as a model system of graphene. In particular, Munakata and co-workers have studied intensively the properties of lead phthalocyanine (PbPc) on HOPG ,,,− including the image potential states (IPSs), , a set of unoccupied states that exist universally not only on metal but also on insulator − and semiconductor − surfaces. Although an IPS, in general, shows a parabolic band whose effective mass m * is equal to the electron mass m e , two-photon photoemission (2PPE) spectroscopy measurements have revealed that the lowest IPS (LIPS) of PbPc on graphite exhibits a significant increase in the effective mass as m */ m e ≃ 2.2 ± 0.3, , which suggests the influence of the PbPc overlayer.…”

“…These behaviors are reminiscent of the wet electron states observed at the water–TiO 2 interface in that an unoccupied state extended around a molecule is stabilized by forming a delocalized state in the molecular layer. Note that the hybridization with the LUMO+10 has been mentioned as a possible cause of the increased effective mass of the LIPS of PbPc on graphite in ref . However, the effective mass of the quasiparabolic band in Figure d is calculated to be m */ m e = 1.22 (1.66) in the Γ–X (Γ–Y) direction, which is not large enough to explain the IPS with large m * observed experimentally for the adsorbed system.…”

“…Thus, the molecular structure is a key factor for the effective mass of IPSs in molecular adsorption systems but has been overlooked so far. Indeed, the increased effective masses of IPSs in several molecular adsorption systems including PbPc on graphite , have been explained by employing the Kronig-Penny model, which does not take into account the molecular structure.…”