We develop the tight-binding model to study electronic and optical properties of graphane. The strong sp3 chemical bondings among the carbon and hydrogen atoms induce a special band structure and thus lead to the rich optical excitations. The absorption spectrum hardly depends on the direction of electric polarization. It exhibits a lot of shoulder structures and absorption peaks, which arise from the extreme points and the saddle points of the parabolic bands, respectively. The threshold optical excitations, only associated with the 2px and 2py orbitals of the carbon atoms, are revealed in a shoulder structure at ∼3.5 eV. The first symmetric absorption peak, appearing at ∼11 eV, corresponds to energy bands due to the considerable hybridization of carbon 2pz orbitals and H 1s orbitals. Also, some absorption peaks at higher frequencies indicate the bonding of 2s and 1s orbitals. These results are in sharp contrast to those of the sp2 graphene systems.
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