  The theory of specular X-ray reflectivity from a rough interface based upon
the reflection function method (RFM) is proposed. The RFM transforms the second
order differential equation for the wave amplitude into the non-linear first
order differential equation of Riccati type for the reflection function. This
equation is solved in the approximation of the abruptly changing potential,
which is justified for the typical angles of X-ray reflectometry. The
reflectivity is represented as a series. The first term of this series
reproduces the Nevot-Croce approximation and second one gives the phase
correction for greater angles. It is shown that the phase correction can be
used to obtain the degree of interface asymmetry. The X-ray reflectometry model
profiles for Fe/Cr superlattice are used to illustrate the method.
