1. The problem statement, all variables and given/known data The efficiency of electron excitement by photons is described as being related to how much different orbitals share the same k-space or momentum space. Is this similar to orbital shape (eg spheres, dumbells, ec)? 2. Relevant equations 3. The attempt at a solution I have seen the equation for electron or hole momentum in different materials, but is difficult to interpret. Any answers would be v. much appreciated. Many thanks. JB.