Before revision: The elastic scattering is governed by two kinematic conditions - conservation of energy and the energy of the momentum component parallel to the crystal: Ef = Ei => ki² = kG² = kGz² + k||G² k||G = k||i + G Here G is a reciprocal lattice vector, kG and ki are the final and initial (incident) wave vectors of the helium atom.
After revision: The elastic scattering is governed by two kinematic conditions - conservation of energy and the energy of the momentum component parallel to the crystal: Here G is a reciprocal lattice vector, kG and ki are the final and initial (incident) wave vectors of the helium atom.