For an electron in the valence band, the Hamiltonian is H = ( p + e A ) 2 2 m 0 + V ( r ) , where V ( r ) is the potential of the crystal, e and m 0 are the charge and mass of an electron, and p is the momentum operator. Here we consider process involving one photon and first order in A . The resulting Hamiltonian is H = H 0 + H ′ = [ p 2 2 m 0 + V ( r ) ] + [ e 2 m 0 ( p ⋅ ⋅ A + A ⋅ ⋅ p ) ] , where H ′ is the perturbation of light.