The distorted wave impulse approximation (DWIA) is a non-relativistic model used at intermediate energies to compensate for the effects of a mean nuclear potential. Basic scattering reaction calculations often assume that the incident nucleon behaves as a plane wave until it interacts with the a nucleon in the nucleus. In fact the potential field of the nucleus, which is usually given by an optical potential, will distort the nucleon wavefunction.
The optical potential is typically a phenomenological potential with parameters based on elastic scattering data. It contains both real and imaginary pieces, where the imaginary part models the absorption of certain reaction channels. The form of the nucleon wavefunction that interacts with a constituent of the nucleus can be found by solving the Schrödinger equation with an optical potential.
The primary effect of the use of this optical potential comes from the absorptive term. Using a realistic optical potential it has been shown [Ich89] in a calculation of nuclear spin responses that the states with lower excitation energy and lower angular momentum transfer are attenuated most strongly by the imaginary part of the optical potential. This leads to a reduction in the predicted spin longitudinal enhancement, because the dominant multipolarities for the transverse response are higher than for the longitudinal. As the momentum transfer increases the multipolarities which are least attenuated become larger.