The influence of the rate of bond stretching on the differential cross sections for ion-pair formation in K + XY collisions

Experimental differential cross sections are presented for ion-pair formation in collisions between K and I₂, IBr, Br₂, ICl, Cl₂. In this sequence the vibrational frequency of the halogen molecule is increased gradually from one system to the next. Therefore this is a suitable set of cros sections for analysing the influence of the rate at which the halogen bond stretches after transition to the ionic state. The experimental data are compared with trajectory calculations on diabatic and on adiabatic potential energy surfaces. The results of the two types of calculations differ significantly; the adiabatic model gives the better agreement with experiment. The essential difference between the two models is that in the diabatic model stretching of the halogen bond starts only when the crossing seam R_c between the ionic and covalent configurations is reached, while in adiabatic scattering, pre-stretching of the halogen bond occurs as the alkali atom approaches the crossing seam. As a result of pre-stretching the electron affinity is increased, which in turn affects the endoergicity and the total and differential cross sections for ion-pair formation. At energie below the threshold for ion-pair formation, the cross section for reactive scattering is also affected.