4-VECTOR STUDY OF THE SPIN-CHANGING COLLISION BA[(6S)(1)(6P)(1), (1)P(1)-](6S)(1)(6P)(1), (3)P(2)] H-2/

The spin-changing (6s)1(6p)1, P-1(1) --> (6s)1(6p)1, P-3(2) transition of barium induced by single collisions with H-2 has been studied usin g a crossed beam apparatus. An m(j)-selective preparation and detectio n of barium was achieved using the pump and probe technique in the pre sence of a magnetic field that breaks the degeneracy of the barium mag netic sublevels. The probe laser was scanned across the (6s)1(6p)1, P- 3(2, m)j' --> (6s)1(7s)1, S-3(1, m)j'' multiplicity. Analysis of the D oppler profiles of each line reveals the angular distribution of scatt ered barium atoms, the polarization of which is selected both before a nd after the collision. This is therefore a four-vector correlation ex periment, and it is the first to document spin-changing transitions of alkaline-earth-metal atoms induced by a molecular target. It must be emphasized that all four vectors in the present experiment do not refe r to the same quantization axis, whereas in the standard notion of n-v ector correlation experiments, the n-vectors refer to the same axis. T he results of the present work are discussed in the light shed by othe r results obtained on the same experiments with only two and three vec tors controlled. A collision mechanism is proposed that accounts for a ll the existing experimental results on this process.