Messung der Anisotropie des van der Waals-Potentials durch Streuung von Molekülen in definiertem Quantenzustand

Abstract

A beam of TlF molecules in the (1,0) rotational state was produced by an electrostatic four pole field. This primary beam was crossed at right angles by a secondary beam in a scattering chamber. By changing the direction of an electric field in the scattering chamber it is possible to produce a (1, 0) or (1, 1) state with respect to the secondary beam direction. In this way it was possible to measure the ratio of the total scattering cross-sections,$\frac{{Q\left( {1, 1} \right)}}{{Q\left( {1, 0} \right)}}$, for He, Ne, Ar, and Kr as scattering gases. The result, which should be independent of the scattering gas, is$\frac{{Q\left( {1, 1} \right)}}{{Q\left( {1, 0} \right)}} = 1.0133 \pm 20 and 1.0140 \pm 50$ for Ar and Kr resp., whereas for Ne and He the measured ratios are considerably smaller.

The results were interpreted in terms of a van der Waals potential of the form$V = - \frac{A}{{R^6 }}\left( {1 + q \cos ^2 \Theta } \right)$, whereR is the distance between the scattering partners and Θ is the angle between the internuclear axis andR.A andq are constants. With the Schiff approximation it is possible to calculate the scattering cross section as a function of the angle between the internuclear axis and the collision direction. Using the rotator eigenfunctions the ratio of the matrix elements of this function was calculated for various assumed values ofq. The above experimental result for$\frac{{Q\left( {1, 1} \right)}}{{Q\left( {1, 0} \right)}}$ for Kr and Ar leads to the anisotropy factor,q=0.40±0.07-A detailed estimate of all interactions contributing to the van der Waals potential shows that it is possible to separate out the dipol-dipol dispersion potential from the observed potential; usingLondon's expression for the dispersion potential of asymmetric molecules one gets for the polarisabilities parallel and perpendicular to the intermolecular axis of the TlF molecule:$\alpha _ \shortparallel = 7.8 {\AA}^3$ and$\alpha _ \bot = 5.5 {\AA}^3$.