Role of potential structure in nonadiabatic collisions with applications to He⁺+Ne(2p⁶)→He⁺+Ne(2p⁵3s) and Na+I→Na⁺+I^-

Abstract

The first-order functional-sensitivity densities δσ₁₂(E)/dV_ij(R) from close-coupling calculations are used for a quantitative probe of the role of structure in crossing diabatic curves used to model nonadiabatic collisions. Application to the excitation of Ne by He+ shows a region of significance for δσ₁₂(E)/ δV₁₂(R) as a prominent Gaussian-like profile around the crossing point (R^∗) in accord with the δ(R-R^∗) idealization of the Landau-Zener-Stueckelberg (LZS) theory. Similarly, the densities δσ₁₂(E)/δV₁₁(R) and δσ₁₂(E)/δV₂₂(R) mimic dδ(R-R^∗)/dR-type behavior with one being the negative of the other in the neighborhood of R^∗, in qualitative agreement with the LZS theory. However, all three sensitivity profiles identify a much broader area of importance for the curves than the loosely defined avoided-crossing region. Also, although the sensitivities themselves decrease with increasing energy, the domain of importance of the curves increases. Examination of the functional-sensitivity densities δσ₁₂(E)/δV_ij(R) for the chemi-ionization collision Na+I→Na⁺+I^- reveals regions of potential-function importance very different from that predicted by the LZS theory. The chemi-ionization cross section is about ten times more sensitive to the ionic curve than the covalent curve. Also, the domain of sensitivity of the ionic curve is larger compared to that of the covalent curve. The density δσ₁₂(E)/δV₁₂(R) for chemi-ionization shows that the area of maximum potential significance is not at the crossing point itself but the regions bracketing it on both sides. Also, the dominant sign dependence of the coupling sensitivity is unexpectedly negative. The results offer other observations about the domain of validity of the intuitive pictures rooted in the LZS theory. The significance of these results to the inversion of inelastic cross-section data is briefly discussed.