Angularly resolved rotationally inelastic scattering ofNa2-Ne: Comparison between experiment and theory

The results of a systematic experimental and theoretical investigation of the differential cross sections for vibronically elastic, rotationally inelastic scattering of ${\mathrm{Na}}_2$ from Ne at a center-of-mass collision energy of 190 meV are presented. The experimental cross sections cover the range of rotational transitions $\Delta j=2\mathrm{}$ to $\Delta j=20\mathrm{}$ for a variety of initial rotational levels including the initially rotationless level $j_i=0\mathrm{}$. The data document clearly the major features of rotationally inelastic scattering for collisional systems with steeply repulsive, strongly anisotropic interaction potentials and many energetically open channels, such as main and supernumerary rotational rainbows. The experimental curves are transformed into the center-of-mass reference frame using a constrained minimalization procedure. They are compared with those calculated within the infinite-order sudden approximation from an ab initio potential surface which includes configuration interaction. The results show that the theoretical curves faithfully reproduce both the form and relative magnitudes of the experimental cross sections. An analysis of the sensitivity of the agreement between theory and experiment as the potential is systematically varied indicates that the experimental data place a limit of about ±5% on the accuracy of the calculated anisotropy. The steepness of the ab initio potential could be varied by - 10% or + 25% and still lead to an acceptable agreement of calculated and experimental cross sections. The variation of the cross section at the rainbow maximum with $\Delta j$ and $j_i$ is shown to be an unreliable test of the accuracy of the potential-energy surface unless a large range of $\Delta j$ is studied. Finally, it is found that all data from a large set of calculated but also partially experimentally verified integral cross sections fall near a single curve when plotted as reduced cross sections versus transferred energy $\Delta E$, according to a power-gap fitting law. Different power exponents are, however, needed to describe the variation of the cross sections with $\Delta E$ for very small and larger energy transfer.