One-step theory of two-photon photoemission

TL;DR

This paper develops a perturbative, relativistic theoretical framework for two-photon photoemission spectroscopy, enabling quantitative, material-specific analysis of non-equilibrium electronic phenomena in solids, exemplified by Ag(100).

Contribution

It introduces a one-step, perturbative approach based on Green functions for realistic, quantitative modeling of two-photon photoemission in various materials.

Findings

Provides a formalism for time-dependent photocurrent calculation.

Applicable to simple metals and complex compounds like topological insulators.

Demonstrates the approach with an application to Ag(100) surface.

Abstract

A theoretical frame for two-photon photoemission is derived from the general theory of pump-probe photoemission, assuming that not only the probe but also the pump pulse is sufficiently weak. This allows us to use a perturbative approach to compute the lesser Green function within the Keldysh formalism. Two-photon photoemission spectroscopy is a widely used analytical tool to study non-equilibrium phenomena in solid materials. Our theoretical approach aims at a material-specific, realistic and quantitative description of the time-dependent spectrum based on a picture of effectively independent electrons as described by the local-density approximation in band-structure theory. To this end we follow Pendry's one-step theory of the photoemission process as close as possible and heavily make use of concepts of multiple-scattering theory, such as the representation of the final state by a time-reversed low-energy electron diffraction state. The formalism is fully relativistic and allows for a quantitative calculation of the time-dependent photocurrent for moderately correlated systems like simple metals or more complex compounds like topological insulators. An application to the Ag(100) surface is discussed in detail.