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One‐dimensional volume diffraction may be analyzed in a manner similar to the problem of an electron subject to a periodic potential. The quantity analogous to the diffraction Nath parameter ρ is then the ratio ΔW0/V, where V is the strength of the electron potential and ΔW0 the spacing between the free‐electron‐energy eigenstates. The crossover between the Bragg and Raman–Nath diffraction regimes is marked by a violation of the (1/ρ)≪1 condition, and corresponds to the requirement that (V/ΔW0)≪1 in order to preclude electron eigenstate mixing. Bound‐state perturbation theory as developed for use in quantum mechanics may be applied to the diffraction problem to obtain a (1/ρ) expansion for the intensities of diffracted grating orders.