Evidence for Umklapp electron scattering emission from metal photocathodes

TL;DR

This study provides evidence for an additional electron emission process in metal photocathodes, likely due to a momentum-resonant Franck-Condon mechanism involving inelastic Umklapp scattering, supported by first-principles simulations.

Contribution

It introduces a new photoemission process involving Umklapp electron scattering, supported by experimental data and first-principles modeling.

Findings

Evidence of an additional emission process near the photoemission threshold.

The process is consistent with a momentum-resonant Franck-Condon mechanism.

Simulations align with measured spectral dependencies of quantum efficiency.

Abstract

Comparison of the measured spectral emission properties of single-crystal Cu(001) and W(111) photocathodes to established photoemission theories reveal evidence for an additional one photon emission process predominantly affecting electron emission near and below the photoemission threshold. This additional photoemission process is postulated to be due to a momentum-resonant Franck-Condon mechanism mediated by inelastic Umklapp electron scattering. An initial first-principles simulation of this emission process (involving the electron thermal effective mass, the inelastic electron mean free path at the vacuum level, and the number of Fermi surfaces in the metal), when combined with a direct one-step band emission model, is consistent with the measured spectral dependencies of both the quantum efficiency and mean transverse energy of electron photoemission from the two single-crystal metal photocathodes.