Transverse ionization of a seeded molecular beam: Effects of space-charge on mass spectra and velocity measurements

Abstract

Optimization of the signal of the minor component of a seeded molecular beam results in ion source conditions that allow space-charge trapping. Space-charge trapping of the ions from XeF 6 seeded into rare gases at or below a few mol% has two deleterious effects: the relative intensities of the ionic species are inaccurately represented, and the translational temperature of the molecular beam cannot be accurately determined. Ion residence times are sufficiently long to allow second or third ionization events, yielding large relative intensities for Xe 3+, and Xe 2+. When this situation is corrected, the relative intensities of these latter species are significantly reduced. Ionic species containing more than one xenon atom are found to be fragmentation products from ionization of the van der Waals' dimer Xe 2F 12, and not a result of ion—molecule reactions. Accurate determination of the translational temperature of the molecular beam by the time-of-flight method requires elimination of space-charge trapping of the ions in the ionizer. The translational temperature of an argon molecular beam generated from a 0.0025-cm diameter orifice with one atm pressure was 6 K, in good agreement with other time-of-flight techniques.