Absolute Total Scattering Cross Sections of 7Li–Ar

Abstract

The Li–Ar absolute total scattering cross section was measured to an accuracy of 1%–2% over the relative velocity range 1–4× 105 cm/sec, corresponding to relative energies of 0.03–0.5 eV. The absolute cross section determines the radial scale of the interatomic potential, by reference to the average distance between target molecules. The present study is intended as an exemplary system investigation of inferences that can be drawn from more accurate absolute total cross sections, since currently it is believed that this class of total cross section is known to an absolute accuracy of 5%–10% [R. B. Bernstein and J. T. Muckerman, Advan. Chem. Phys. 12, 414 (1967)]. The primary finding is the value of the interatomic potential with an accuracy of about 5% over a span of the relatively large interatomic distances 6–8 Å. At 7 Å the Li–Ar potential is −1.97 × 10−15 erg± 5%. In this region half of the error arises from the systematic error in the cross section and half from the ambiguity about the proper potential model to apply. The results should be useful for critically testing future model potential calculations and, in conjunction with differential scattering and other data, for giving a more exact and complete understanding of the potential. An apparatus was used that had been previously applied to molecule-atom scattering, where angular resolution corrections were negligible and where absolute pressures were measured with a McLeod gauge modified to physically eliminate trap pumping error. A deconvolution procedure that took into account the effects of target motion upon the glory structure of the total cross section was applied to the data. An economical method for applying nonlinear least squares fitting of model interatomic potentials is described and is applied to the data.