Inverse of the Photoelectric Effect in Al

Theoretical predictions for inverse photoemission from free-electron-like materials are tested by measurement of the energy and angle dependence of the emitted photon intensity from Al. The predicted cross-section minimum at the plasma frequency ωₚ is observed as well as a characteristic increase below ωₚ due to the inverse of the surface photoelectric effect. Above ωₚ there is an additional bulklike contribution from the radiative decay of bulk plasmons. Other materials with a well-defined plasma resonance, e.g., Ge and Si, exhibit a similar cross-section behavior.