Three-dimensional hexapole focusing of pulsed molecular beam for state selection

We theoretically investigate three-dimensional(3D) focusing of pulsed molecular beam using a series of hexapoles with different orientations. Transversely oriented hexapoles provide both the transverse and longitudinal focusing force and a longitudinally oriented one provides only the transverse force. The hexapole focusing position are designed to realize the simultaneous focusing in three directions. The additional longitudinal focusing compared with the conventional hexapole can suppress the effect of chromatic aberration induced by the molecular longitudinal velocity spread, thus improving the state-selection purity as well as the beam density. Performance comparison of state selection between this 3D focusing hexapole and a conventional one is made using numerical trajectory simulations, choosing CHF_3 molecules as a tester. It is confirmed that our proposal can improve the state-selection purity from 68.2% to 96.1% and the beam density by a factor of 2.3.