Session 3-3

Abstract

Image-potential states represent quantized electronic states localized at metal surfaces, forming a Rydberg-like series of energy levels influenced by Coulomb forces and repulsive surface potentials. The high-order energy states in this Rydberg-like series are closely spaced, posing a challenge for their energetic resolution without employing advanced spectroscopic techniques such as quantum beat spectroscopy. This study utilizes time-resolved two-photon photoemission spectroscopy, using a pump photon of ℏω_UV=4.71 eV and a probe photon of ℏω_IR=1.57 eV, to investigate the image-potential states of the weakly coupled graphene/Ir(111) system. In the static 2PPE spectra, the first three image-potential states are clearly observed, while the energy levels of the fourth and higher-order states are estimated. Population and relaxation processes of the first three states are characterized using a density matrix formalism. In contrast to previous findings, we identified a clear quantum beat phenomenon in the transient spectrum for the fourth and higher-order states, with a beating frequency of 1.35 THz (5.6 meV). The quantum beating spectra reveal an energy difference between closely lying states, smaller than the intrinsic energy resolution of our experimental setup, underscoring the precision of quantum beat spectroscopy in resolving these features.