Scattering experiments often consist of a particle with initial crystal momentum k → → incident on a solid. The particle passes through a potential distributed in space, V ( r → → ) , and exits with crystal momentum k → → ′ . This situation is described by Fermi's golden rule, which gives the probability of transition per unit time, Γ Γ ( k → → ′ , k → → ) , to the energy eigenstate E k → → ′ from the energy eigenstate E k → → due to the weak perturbation caused by our potential V ( r → → ) .