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 p is observed as well as a characteristic increase below p due to the inverse of the surface photoelectric effect. Above p 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. © 1988 The American Physical Society.