Molekularstrahlmessungen von Stoßquerschnitten für Übergänge zwischen definierten Rotationszuständen zwei-atomiger Moleküle

Abstract

Previously reported experimental results for inelastic cross sections for rotational excitation of TlF molecules in low-lying, well defined rotational states are interpreted in terms of a time dependent perturbation theory formulation of the high energy approximation. In order to calculate inelastic cross sections for the observed small angle scattering the Born approximation and the classical deflection function are shown to be applicable. In this approximation the ΔJ selection rules are characteristic of the individual terms in the expansion of the potential, whereas the ΔM selection rules depend on the orientation of the molecule with respect to the scattering trajectory. An approximation for dealing with a scattering gas consisting of molecules is introduced and the appropriate orientation averaging is carried out for the case of a generalized electrostatic potential. The measured results for the transition TlF(2.0)→ (3.0) in collisions with the rare gases and CH₄ and SF₆ are more than a factor three larger than calculated results for the induced dipole-quadrupole (α, μ, Q) interaction. Rough argeement is found between calculated results for a dipole-quadrupole interaction and the experimental results for the above mentioned transition produced by the scattering gases O₂, N₂ (air), N₂O, and H₂O. Finally, the dipole-dipole potential appears to provide an explanation for the large inelastic cross sections observed with NH₃ and CF₂Cl₂. Calculated inelastic and total cross sections are however considerably larger (about a factor 2) than the measured results with NH₃. Possible explanations are discussed.