ISSN:
1089-7690
Quelle:
AIP Digital Archive
Thema:
Physik
,
Chemie und Pharmazie
Notizen:
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 cursive-theta), where cursive-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 cursive-theta, can be characterized by two moments only: 〈P1〉 the orientation and 〈P2〉 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 clearly demonstrate the inadequacy of restricting theoretical calculations to limiting cases, i.e., weak field and strong field, particularly because the Stark curves reveal the presence and the precise position of avoided crossings. Dependent on the field strength used for state selection and focusing, these crossings may, in principle, have a marked effect on the orientation obtained, given that the final orientation field at the reaction site remains the same. In addition, knowledge of the detailed Stark curves suggests a novel and feasible way to reverse the orientation of the molecules without interference with the detection system, which may be particularly useful if ionic products are formed.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1063/1.460200
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