Transition state observation of excited harpoon reactions, within Ca-HX van der Waals complexes

The observation of reactions through the excitation of van der Waals precursors allows for the spectroscopic investigation of the reaction surface of the excited state adducts. This method has been applied to the reaction of calcium D-1 and P-1 with the various hydrogen halide molecules, the reaction being endothermic with ground state calcium by less than 1 eV. The complexes were produced by supersonic expansion following laser ablation of the calcium atom. The chemiluminescent channel producing the CaX (A (2) Pi, B (2) Sigma) molecules is found to be an important channel and was investigated through action spectra. These spectra reveal the local electronic excitation of the calcium atom within the complex with the local orbital symmetry A' or A '' of the C-s molecular complex. The band structure has been analyzed in terms of the intermolecular modes, the bending mode being prominent in the A '' state correlating with (CaP1)-P-1. This state can be characterized by a single generic two dimensional reaction surface (bending + H-X reaction coordinate) common to the HX molecules, This surface is characterized by an identical entrance valley in the bending and reaction coordinates opened to the products through a transition state characterized by a channel of variable size depending upon the halogens. The experimental results have been successfully simulated on this model surface by performing one- and two-dimensional wave packet propagation calculations. The position of the bands in the spectrum are related to the bending modes and their bandwidth to the opening in the channel. These excited state reactions appear through their various characteristic as a generic model for excited state harpoon type reactions and display resonances despite the high cross sections for the reactions observed in collisions. (C) 1996 American Institute of Physics.