Hexapole state-selection and orientation of asymmetric top molecules:: CH2F2

Molecular beams of the b-type asymmetric top molecule methylenefluoride (CH2F2) were focused and rotationally state-selected with an electrostatic hexapole. The focusing behavior is mediated by the dependence of rotational energy on electric field strength, the Stark effect. The matrix quantum theory needed to calculate the rotational energies of asymmetric top molecules within an electric field are summarized. These Stark energies were calculated and parameterized for the lowest 165 CH2F2. Excellent agreement was found between classical trajectory simulations incorporating these calculated energies and experimentally measured hexapole focusing spectra. Based on this agreement, the rotational state distribution transmitted by the hexapole as a function of hexapole voltage has been ascertained. Comparisons are made with simulated focusing spectra of the a-type rotor, formaldehyde (H2CO). The theoretical formalism needed to describe the orientational probability distribution functions (opdf's) of hexapole-selected asymmetric top wave functions is developed and applied to the experimentally selected states of CH2F2. Calculated opdf's demonstrate the remarkable control we have over the orientation by varying the "orienting" field strength in the region following the hexapole selector. (C) 1999 American Institute of Physics. [S0021-9606(99)00919-8].