Focusing and state selection of NH3and OCS by the electrostatic hexapole via firsthyphen; and secondhyphen;order Stark effects

摘要

Neither the linear molecule OCS nor the rapidly inverting pyramidal NH3have conventional firsthyphen;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 curveI(V0) is obtained (whereV0is the lsquo;lsquo;rod voltagersquo;rsquo;). From the observed linear velocity dependence ofV0, characteristic of focusing via secondhyphen;order SE, the peak is assigned as the Verbar;JKMJrang;=Verbar;11plusmn;1rang; state (the upper state of theJK=11inversion doublet with para nuclear symmetry), previously quadrupole focused by Kay and Raymond (1986). In contrast, ND3focuses like a symmetric top, as expected, with a quadratic velocity dependence characteristic of firsthyphen;order SE. For OCS, the first three peaks in theI(V0) curve, showing quadratic velocity dependence, are assigned to the Verbar;JlMJrang; states Verbar;111rang;, Verbar;212rang;, and Verbar;313rang;, focused via firsthyphen;order SE for thelhyphen;doubled excited bending vibrational state, analogous to that found by Stolteetal. (1986) for N2O. An intense peak at higherV0is due primarily to the Verbar;JMJrang;=Verbar;10rang; state of the ground vibrational state, i.e., the linear OCS focused by secondhyphen;order SE, following the theory of English (1971).