On the quenching of Na(2P) by HCl: nonadiabatic effects

The electronic structure aspects of the nonadiabatic reactive quenching process Na(2P)+HCl→NaCl+H were studied within an adiabatic states approach. Portions of the 1, 2 2A′ potential energy surfaces (PES’s) were characterized using state averaged-MCSCF/CI wave functions. Two minima on the 2 2A′ PES were found, one in the entrance channel at ΔR(NaCl)=0.89 a0, ΔR(HCl)=0.01 a0, ∠NaClH=95.4° and a secondary minimum in the interaction region at ΔR(NaCl)=0.32 a0, ΔR(HCl)=0.60 a0, ∠NaClH=90.9°. On the 1 2A′ PES a saddle point was located at ΔR(NaCl)=0.34 a0, ΔR(HCl)=0.64 a0, ∠NaClH=55.1°. Here ΔR(X)=R(X)−Re(X). The global and secondary minima on the 2 2A′ PES are stable with respect to the Na(2P)+HCl asymptote by 0.24 and 0.13 eV, respectively, while the 1 2A′ saddle point is 0.75 eV above the ground state Na(2S)+HCl asymptote. Nonadiabatic coupling matrix elements 〈Ψ(2 2A′)‖(∂/∂Rα)Ψ(1 2A′)〉 were determined in the vicinity of the minima on the 2 2A′ PES and the saddle point on the 1 2A′ PES. The region of maximum nonadiabatic coupling as measured by the Massey strength does not correspond to any of these extrema. It was found in the neighborhood of ΔR(NaCl)=0.33 a0, ΔR(HCl)=0.70 a0, ∠NaClH=100°. This region is stable by 0.10 eV relative to the Na(2P)+HCl asymptote and is characterized by close approach of the 1 2A′ and 2 2A′ PES’s.