Variations of neutral fractions for Na⁺ scattering on Cu (001) with perpendicular exit velocity. The incident angle with respect to the surface plane is 45° and the exit angle is 45°. Experiment: (◼).^[19] Theory: the blue dotted line represents the calculated result from Ref. [19]; the black solid and red dashed lines denote the calculated results by using the modified BN model with and without the parallel velocity effect, respectively.