Mass filter and focusing ion detector for use with time-of-flight velocity distribution measurements

A novel mass filter and focusing ion detector for time-of-flight studies of dissociative ionization fragments is described. The filter-dector combination is particularly suited for measuring the kinetic energy released in the molecular dissociation process. The mass filter utilizes a computer-controlled, time-dependent retarding potential synchronized with, and located a distance L from, a pulsed electron-bombardment ion source. Ions arriving at the filter at time t have a kinetic energy of mL(2)/2t(2). The retarding potential varies with time as mL(2)/2et(2), thus deflecting ions with masses less than m. Operation of the filter in a mode which provides unity mass resolution at 16 amu has been demonstrated. The detector consists of a channel electron multiplier and a focusing electrode configuration approximating that of a charged sphere above a grounded conducting plane. The planar input aperture of the detector is located just behind the mass filter along the ion flight path. The detector features high efficiency, large input aperture, low background, and simple operation. Ion transit times through the detector are small and essentially independent of the initial ion kinetic energy.