| Abstrakt: |
The energies and eigenfunctions of hyperfine levels belonging to the J=|K|=1 rotational manifold of the methylhalides have been calculated as a function of the strength of an applied electric (dc) field. From the eigenfunctions, the molecular orientational distribution function P(cos [variant_greek_theta]), where [variant_greek_theta] is the angle of the principal axis relative to the electric field, has been obtained. In the case of interest (J=1), the Legendre expansion of P(ρ), with ρ≡cos [variant_greek_theta], can be characterized by two moments only: the orientation and the alignment. The calculated moments have been compared with data available from different molecular beam experiments, without recourse to a specific collision model. For the passage of the molecules through different electric fields, the adiabatic approximation has been assumed to apply, except for curve crossings with extremely small gaps. For methylfluoride and methylchloride, the calculated orientations are in agreement with the measured steric effects of the reactive collisions with Ca atoms, down to a field strength of a few volts per centimeter.At lower field strengths the orientation falls off more rapidly than predicted by the theory. This behavior is tentatively attributed to the variation in the direction of the electric field, and hence in the axis of quantization, across the reaction volume, at low field strengths. This may, in addition, cause a breakdown of the adiabatic approximation. In the case of methyliodide, quantitative agreement between the theoretical and the experimental asymmetry parameters of photodissociation is obtained if the latter ones are provisionally corrected for systematic errors. Contrary to earlier preliminary conclusions, the results show that within experimental error the adiabatic approximation is applicable. Up until now, no experimental data has been available for methylbromide. The calculations... [ABSTRACT FROM AUTHOR] |
