Optical Properties of Rb between 3.3 and 10.5 eV

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 $m_{\mathrm{eff}}=(1.\mathrm{}03\pm 0.02)m_0$.