Electric field dependence of reactivity of state-selected and oriented methylhalides

Abstract

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 ϑ), where ϑ 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 ϑ, 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.

Slice imaging of photodissociation of spatially oriented molecules

“…For example, an electric field strength of about 1500 V / cm is required to fully decouple the hyperfine interaction and to saturate the degree of orientation in CH 3 I molecules in the ͑JKM͒ = ͑111͒ state. 10 Photodissociation of oriented CD 3 I molecules, followed by imaging of the CD 3 photofragments, was first demonstrated in 1995 using the original ion imaging technique with grids. 11,12 The molecules were oriented in an electric field perpendicular to the extraction field.…”

Dynamical Steric Effect in the Decomposition of Methyl Chloride on a Silicon Surface

“…The orientation of CH 3 Cl molecules is constructed by a homogeneous orientation electric field in front of the surface. A field strength of 350 V=cm is enough to obtain almost perfect orientation for CH 3 Cl [21,22]. Figure 1(a) shows the focusing curve for a CH 3 Cl beam at E i 120 meV impinging directly upon the quadrupole mass spectrometer (QMS).…”

Electrostatic Hexapole State-Selection of the Asymmetric-Top Molecule Propylene Oxide

“…In the case where orientation after focusing may be difficult, 52,53 "brute force" may be required. [54][55][56][57] The enantiomeric selected forms can be used for investigations of chirality effects.…”