What is the Bandgap energy of Rubidium? Could anyone please tell me the bandgap energy for alkali metals like rubidium and cesium?  
 A: An early paper is "Energy Bands of the BCC Metals rubidium and cesium", A. M. Radwan, Cryst. Res. Technol. 23(6) 785-791 (1988). The results shown are focused on the bands near the Fermi Energy. It shows the parts of the next higher band(s) that are overlapping with the conduction bands.
There is also "Method for Performing LCAO Band Structure Calculations in Crystalline Solids: Application to Rubidium", Werner Obermayr, Int. J. Quantum Che. 78 212-225 (2000). It also shows the bands close to the Fermi Energy.
Google pulls up a 2001 masters thesis by A.J. Zukaitis that shows the deeper valence bands. 
A: The optical spectra of the alkali metals show a kind of gap: interband transitions have an onset. In first approximation this is explained by the empty-lattice band structure where the periodicity makes possible vertical transitions from the conduction band from states with a wave vector smaller than the Fermi vector.
Here is an image with the extended zone scheme. The gap is 0.64 times the Fermi energy. 
