Two-dimensional metal-organic porous networks (2D-MOPNs) are highly ordered
quantum boxes for exploring surface confinements. In this context, the electron
confinement from occupied Shockley-type surface states (SS) has been vigorously
studied in 2D-MOPNs. In contrast, the confinement of excited surface states,
such as image potential states (IPSs), remains elusive. In this work, we apply
two-photon photoemission to investigate the confinement exemplarily for the
first image state in a Cu-coordinated T4PT porous network (Cu-T4PT). Due to the
lateral potential confinement in the Cu-T4PT, periodic replicas of the IPS as
well as the SS are present in a momentum map. Surprisingly, the first IPS
transforms into a nearly flat band with a substantial increase of the effective
mass (> 150 %), while the band dispersion of the SS is almost unchanged. The
giant confinement effect of the excited electrons can be attributed to the
wavefunction location of the first IPS perpendicular to the surface, where the
majority probability density mainly resides at the same height as repulsive
potentials formed by the Cu-T4PT network. This coincidence leads to a more
effective scattering barrier to the IPS electrons, which is not observed in the
SS. Our findings demonstrate that the vertical potential landscape in a porous
architecture also plays a crucial role in surface electron confinement