First evidence of transient interactions between $\pi\to\pi^*$ optical excitations and image potential states in graphite

TL;DR

This study provides the first experimental evidence of transient interactions between $\pi o\pi^*$ optical excitations and image potential states in graphite, revealing strong coupling effects observable via non-linear photoelectron spectroscopy.

Contribution

It introduces a novel experimental observation of IPS and optical excitation interactions in graphite, supported by a self-energy model linking many-body effects to spectral variations.

Findings

IPS photoemission intensity varies with photon energy

Effective mass and linewidth show strong changes near $\pi o\pi^*$ saddle points

High carrier density induces transient many-body effects

Abstract

Here we report the experimental evidence of the interactions between the excitations of the $\pi\to\pi^*$ optical transition and the image potential states (IPS) of highly oriented pyrolitic graphite (HOPG). By using non-linear angle resolved photoelectron spectroscopy (NL-ARPES) we show that the IPS photoemission intensity, the effective mass, and the linewidth exhibit a strong variation when the photon energy is tuned across the $\pi\to\pi^*$ saddle points in the 3.1 - 4.5 eV photon energy range. A model based on the self-energy formalism is proposed to correlate the effective mass and the linewidth variations to transient many body effects, when a high carriers density (in the $10^{20}$ cm$^{-3}$ range) is created by the absorption of a coherent light pulse. This finding brings a clear evidence of a high IPS-bulk coupling in graphite and opens the way for exploiting the IPS as a sensitive, nonperturbing probe for the many-body dynamics in materials.