Optical absorption and plasma resonance in very thin films of calcium

The transmittance and reflectance of very thin films of calcium have been measured at several angles of incidence and for both $s$ and $p$ polarization, in the energy range 2-5.6 eV. The imaginary part of the complex dielectric constant, ${\epsilon }_2$, has been calculated making the approximation that the films were thin, plane, and parallel, and the experimental results have been compared with theoretical calculations of Lopez-Rios and Sommers. The interband transitions at 4.4 and 5.1 eV have been confirmed but we found no experimental evidence for that predicted at 2.9 eV. Using obliquely incident $p$-polarized light we have detected a surface plasmon at 3.6 eV, in good agreement with the measurements of energy losses in calcium.