Angularly resolved vibrational excitation in Na2--He collisions

We report angle-resolved measurements of v/sub i/ = 0 → v/sub f/ = 1 vibrational transitions in Na2--He collisions at an energy of 90 meV. The agreement with calculated cross sections using an ab initio surface is good, both in the angular variation of the cross section as well as with respect to its magnitude relative to the vibrationally elastic process. The calculated (v/sub i/ = 0, j/sub i/ = 0) → (v/sub f/ = 1, j/sub f/ ) differential cross sections are discussed in some more detail. They show structure, in addition to the rainbow oscillations, related to the fact that the vibrational transition probability vanishes for a specific approach angle