Surface-electronic-structure information from bulk plasmon photoexcitation in free-electron metal films

Bulk-plasmon-photoexcitation (BPPE) phenomena (particularly the shapes of resonance peaks associated with standing-plasma-wave excitation) in thin free-electron-metal films are shown to be sensitive to surface electronic structure. A formalism is developed which permits the microscopic evaluation of corrections to the classical theory of the optical properties of a jellium solid, through first order in the wave vector of an incident electromagnetic wave. This formalism permits the evaluation of effects due to BPPE in both the ordinary optical as well as in the photoemissive properties of thin films, and moreover, allows one to determine their dependence on the forms of the one-electron surface potential barriers $V\left(z\right)\mathrm{}$ at the two film surfaces. Theoretical results based on the random phase approximation (RPA) to a jellium film's dielectric response are compared to the photoyield data of Anderegg et al. taken on potassium films. The experimental BPPE resonance features are found to be considerably narrower than the theory predicts—for all the forms of $V\left(z\right)\mathrm{}$ that were tried—casting some doubt on the RPA's ability to describe surface dielectric phenomena quantitatively. The asymmetry of Anderegg et al.'s films, i.e., the fact that their upper and lower film surfaces were inequivalent, is shown to require a partial reinterpretation of their data; in particular, their films were probably only about half as thick as they supposed. Moreover, the argument that leads to this conclusion also provides a simple explanation for the alternation of BPPE resonance peak strengths which appears to be a fairly systematic feature of their data. Directions for further experimental and theoretical work on BPPE phenomena are suggested.