TIGHT FOCUSING OF BEAMS OF POLAR POLYATOMIC-MOLECULES VIA THE ELECTROSTATIC HEXAPOLE LENS

Interest in orientational control of molecule-surface scattering has stimulated consideration of tight focusing of oriented molecule beams. Various hexapole field configurations are compared for their ability to provide the highest practicable flux densities of focused (and oriented) molecules onto the smallest possible element of target surface. The two important criteria are (1) the demagnification factor (i.e., the ratio of the size of the focused image relative to that of the beam source) and (2) the focused fraction (i.e., the fraction of the beam flux entering the hexapole that is focused on the target element). Using these considerations one can design configurations to maximize beam flux densities at microscopic targets. Focusing experiments with beams of CH3Cl and CHF3 using various combinations of hexapole lenses are reported, confirming several of the key results of the present analysis. Using two hexapole fields of lengths l1 and l2 in series with the voltage on each lens controlled independently is found to be superior to the traditional single hexapole field of length l0 (= l1 + l2) for achieving tight focusing.