By considering single-step excitation and the consequent de-excitation of a projectile nucleus along with the centre-of-mass pair correlation in a target, the authors derive an analytical expression for the effective N- alpha amplitude for alpha -nucleus scattering. This necessarily results as a correction to the rigid-projectile approximation. They found that the corrections coming from the aforementioned considerations are small in alpha -⁴⁰Ca and alpha -¹²C elastic scattering at 1.37 GeV, and thus are unable to explain the success of the phenomenological effective N- alpha amplitude. An expression for the monopole inelastic N- alpha amplitude is also derived in this paper.