The Ba(6{ital s}6{ital p} {sup 1}{ital P}{sub 1}{r arrow}{sup 3}{ital P}{sub 2}) inelastic transition induced by argon has been investigated in a crossed beam as a function of the collision energy between 0.14 and 0.40 eV. The differential cross sections have been obtained by the Doppler technique probing the Ba(6{ital s}6{ital p} {sup 3}{ital P}{sub 2}) with a single-mode cw laser. Oscillations are observed in the differential cross sections that are assigned to Stueckelberg interferences resulting from a crossing between the {sup 1}{Pi}{sub 1} and {sup 3}{Sigma}{sub 1} Ba-Ar potential-energy curves. A very large polarization effect is observed by comparing the differential cross sections corresponding to a {ital p}{sigma} or a {ital p}{pi} preparation of the Ba({sup 1}{ital P}{sub 1}) orbital with respect to the relative Ba-Ar velocity.