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 p recursors allows for the spectroscopic investigation of the reaction s urface of the excited state adducts. This method has been applied to t he reaction of calcium D-1 and P-1 with the various hydrogen halide mo lecules, the reaction being endothermic with ground state calcium by l ess than 1 eV. The complexes were produced by supersonic expansion fol lowing laser ablation of the calcium atom. The chemiluminescent channe l producing the CaX (A (2) Pi, B (2) Sigma) molecules is found to be a n important channel and was investigated through action spectra. These spectra reveal the local electronic excitation of the calcium atom wi thin 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 t he 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 react ion coordinate) common to the HX molecules, This surface is characteri zed by an identical entrance valley in the bending and reaction coordi nates opened to the products through a transition state characterized by a channel of variable size depending upon the halogens. The experim ental results have been successfully simulated on this model surface b y performing one- and two-dimensional wave packet propagation calculat ions. The position of the bands in the spectrum are related to the ben ding modes and their bandwidth to the opening in the channel. These ex cited state reactions appear through their various characteristic as a generic model for excited state harpoon type reactions and display re sonances despite the high cross sections for the reactions observed in collisions. (C) 1996 American Institute of Physics.