State selective reactions prepared through the excitation of orbital states in van der Waals complexes of Ca–HX*

We have observed the chemiluminescent reaction Ca*+HX→CaX*+H, where the reactants are prepared in a van der Waals complex formed in a supersonic expansion. This preparation, combined with tunable laser excitation, allows access to well-defined electronic states of the reaction complex corresponding to different orientations of the calcium excited orbital. In the case of the Ca–HBr complex, a remarkable effect of this preparation is observed on the branching ratio to the final excited states A 2Π and B 2Σ of CaBr: Depending upon the selected state of the complex, the A/B ratio varies by a factor of 2. This is interpreted by the conservation of the orbital orientation during the reaction involving the departure of the hydrogen.