Intermolecular potentials for the metastable Ne*-rare gas and Ne*-molecule systems

Abstract

The absolute value and the velocity dependence of the total cross section Q( g) has been measured in a crossed beam machine for the Ne*-Ar, Kr, Xe and Ne*-O 2, N 2, CH 2 and CO 2 systems, using a mixed beam containing Ne*( 3P( 0) and Ne * ( 3P 2) fine structure states in a 1:5 ratio. The range of velocities is typically 1000 ⩽ g ⩽ 8000 m s −1, always including the interesting N = 1 glory oscillation. The results for the Ne* -rare gas systems are in excellent agreement with the predictions of the ion-atom Morse-Morse-spline-van der Waals potentials of Gregor and Siska, both with regard to the absolute value (1.5%), position of the N = 1 glory maximum (2.7%) and the amplitude of the N = 1 glory maximum (4.3%). The predictions of the potentials proposed by Hausamann are less satisfactory, most likely due to the specific switchover function used to connect the well area at R/R M ≈ 1.1 to the van der Waals long-range attractive branch at R/R M ≈ 2 ( R M is the well position). By using a semiclassical scaling method the potential parameters ϵ (well depth), R M (well position) and C 6 (van der Waals constant) have been determined for the Ne*-molecule systems, using the Gregor and Siska IAMMSV potential for the Ne*-Xe system as a reference. The well parameters are (ϵ (meV), R M (Å)) = (3.21, 5.43), (4.24, 5.17), (13.55, 4.74) and (7.08, 5.44) for the Ne*-N 2, O 2, CO 2 and CH 4, systems, respectively. For the C 6 values we observe a fair scaling with the polarisibility α of the molecule. For the Ne*-CO 2 system we observe a damping of the amplitude of the glory oscillations, which increases rapidly with decreasing velocity. This damping is interpreted in terms of the probability for ionisation along the glory trajectory, providing useful information for determining a complex potential for this system.