The optical and dielectric constants of Rb have been determined from reflectance measurements for photons of energy between 3.3 and 10.7 eV. Reflectance measurements were made as a function of incident angle at a Rb-substrate interface in an ultrahigh-vacuum system. The refractive index ($n$) was determined from the critical angle for total internal reflection, and the absorption coefficient ($k$) from the slope of the reflectance curve at the critical angle. The real part of the dielectric function (${\epsilon }_1$) and the optical conductivity ($\sigma =\frac{\omega {\epsilon }_2}{4\pi }$) were derived from $n$ and $k$. In $\sigma$, we find a broad strong peak centered at 7 eV. It is attributed to absorption processes involving plasmon-assisted interband transitions and to direct interband transitions to "$f$" bands. ${\epsilon }_1$ is analyzed in terms of a nearly-free-electron model to obtain a value for the contribution of core polarization to ${\epsilon }_1$ of $4\pi n_0\alpha =0.\mathrm{}25\pm 0.02$, and an effective mass of $m_{\mathrm{eff}}=(1.\mathrm{}03\pm 0.02)m_0$.

• Received 13 September 1971

DOI:https://doi.org/10.1103/PhysRevB.5.2118