  We develop an analytic model of vector correlations in rotationally inelastic
atom-diatom collisions and test it against the much examined Ar--NO ($\mathbf
X^2 \Pi$) system. Based on the Fraunhofer scattering of matter waves, the model
furnishes complex scattering amplitudes needed to evaluate the polarization
moments characterizing the quantum stereodynamics. The analytic polarization
moments are found to be in an excellent agreement with experimental results and
with close-coupling calculations available at thermal energies. The model
reveals that the stereodynamics is governed by diffraction from the repulsive
core of the Ar--NO potential, which can be characterized by a single Legendre
moment.
