Focusing and state selection of NH3 and OCS by the electrostatic hexapole via first- and second-order Stark effects

Abstract

Neither the linear molecule OCS nor the rapidly inverting pyramidal NH3 have conventional first-order Stark effects (SE) and so are not ideally focusable by an electrostatic hexapole field. However, strong focusing and state selection of these molecules by such a hexapole field has been achieved using the UCLA apparatus, consisting of a pulsed supersonic beam source, 3 m hexapole rods, and a mass spectrometer detector. For NH3, a single intense peak in the focused intensity curve I(V0) is obtained (where V0 is the ‘‘rod voltage’’). From the observed linear velocity dependence of V0, characteristic of focusing via second-order SE, the peak is assigned as the ‖JKMJ〉=‖11±1〉 state (the upper state of the JK=11 inversion doublet with para nuclear symmetry), previously quadrupole focused by Kay and Raymond (1986). In contrast, ND3 focuses like a symmetric top, as expected, with a quadratic velocity dependence characteristic of first-order SE. For OCS, the first three peaks in the I(V0) curve, showing quadratic velocity dependence, are assigned to the ‖JlMJ〉 states ‖111〉, ‖212〉, and ‖313〉, focused via first-order SE for the l-doubled excited bending vibrational state, analogous to that found by Stolte et al. (1986) for N2O. An intense peak at higher V0 is due primarily to the ‖JMJ〉=‖10〉 state of the ground vibrational state, i.e., the linear OCS focused by second-order SE, following the theory of English (1971).