Tracks of symmetric top molecules in hexapole electric fields

The trajectories of symmetric top molecules are computed in hexapole electric fields. The transmission of a hexapole for linear Stark effect focusing is compared with that due to the exact interaction of the electric field with the symmetric top molecule. The use of the exact interaction in place of the linear Stark effect produces significant changes in the position and shape of the transmission peaks. Focusing curves are also calculated assuming the Stark effect calculated to second order. The nuclear quadrupole-electric field gradient interaction is found to significantly affect the focusing of CH3I, and it is possible to produce a beam of rotational state selected and partially spin polarized CH3I. The focusing for the usual experimental hexapole composed of cylindrical rods is compared with the focusing for an ideal hexapole, and a finite difference technique is used to determine the cylindrical rod radius that will provide the best approximation to an ideal hexapole field. This best rod radius is 0.565 of the hexapole radius, and focusing curves are shown for this choice of rod radius. A computer program to calculate trajectories for experimental hexapole arrangements with exact Stark interactions is available.