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 ${\omega }_p$ is observed as well as a characteristic increase below ${\omega }_p$ due to the inverse of the surface photoelectric effect. Above ${\omega }_p$ 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.

• Received 9 February 1988

DOI:https://doi.org/10.1103/PhysRevLett.60.2070