Semiempirical potential surfaces and dynamical considerations for collisions between alkali metals and molecular oxygen: Li+O2 and Na+O2

Abstract

The intermediates in both the inelastic and reactive collisions of alkali metals with molecular oxygen are the M+O−2 ion pairs. We develop here analytic forms for the lowest energy diabatic potential energy surfaces of both A′ and A″ electronic symmetry for the Li+O−2 and Na+O2− systems. An exponential–rational approximant functional form, which is found to provide an excellent description of the ionic alkali monoxide potential curves, is adapted to the alkali dioxide ion pairs. The degree of polarization of the O−2 charge distribution is modeled from available theoretical data for the homologous LiF2 system. With only a few variable parameters it is possibile to fit extremely well those features of the M+O−2 surfaces which are presently known either from experiments or ab initio calculations. The topologies of the A′ and A″ surfaces are substantially dissimilar. The lower energy surface (A″) for both LiO2 and NaO2 possesses a pronounced minimum in isoceles triangular geometry, as expected from the matrix isolation experiments; whereas the A′ surface displays little variation with the MOO angle. The fitted ion pair surfaces are used to discuss certain expected features of the M+O2 collision dynamics. Also, we show how it may be possible to relate the coupling matrix elements for the MO2 ionic–covalent surface crossings to equivalent matrix elements for the alkali monoxide systems, which are significantly easier to determine.