Direct resolution of unoccupied states in solids via two photon photoemission

TL;DR

This paper demonstrates that two-photon photoemission spectroscopy can directly resolve unoccupied electronic states in solids, revealing new band structure information through non-linear effects and resonant transitions.

Contribution

It introduces a novel approach using non-linear two-photon photoemission to access unoccupied states, supported by ab-initio calculations and experimental data.

Findings

Resonant transitions obey strict selection rules.

Structures in unoccupied states are observable in low-energy laser spectroscopy.

Similar resonances are predicted in ultraviolet photoemission spectra.

Abstract

Non-linear effects in photoemission are shown to open a new access to the band structure of unoccupied states in solids, totally different from hitherto used photoemission spectroscopy. Despite its second-order nature, strong resonant transitions occur, obeying exact selection rules of energy, crystal symmetry, and momentum. Ab-initio calculations are used to demonstrate that such structures are present in low-energy laser spectroscopy experimental measurements on Si previously published. Similar resonances are expected in ultraviolet angle-resolved photoemission spectra, as shown in a model calculation on Al.