A modulated retarding-potential-difference technique has been used to define the energy width of an electron beam located in the source region of a time-of-flight mass spectrometer. Negative ions with energies in the region 0-3 eV are produced by dissociative electron attachment to gas molecules in this source and are permitted to react for a controlled continuously variable length of time with neutral molecules to produce secondary negative-ion products. From the observed time development of the reactions, the reaction rate constant may be obtained, and the cross section then calculated if the primary-ion velocity is known. Results are presented for the reactions ${\mathrm{O}}^{-}$ + ${\mathrm{NO}}_2$ → ${\mathrm{NO}}_2^{-}$ + O, ${\mathrm{H}}^{-}$ + ${\mathrm{H}}_2$O → ${\mathrm{OH}}^{-}$ + ${\mathrm{H}}_2$, and ${\mathrm{D}}^{-}$ + ${\mathrm{D}}_2$O → ${\mathrm{OD}}^{-}$ + ${\mathrm{D}}_2$ at a number of primary ion energies, and for the reactions ${\mathrm{O}}^{-}$ + ${\mathrm{N}}_2$O → ${\mathrm{NO}}^{-}$ + NO, ${\mathrm{HCOO}}^{-}$ + S${\mathrm{F}}_6$ → S${\mathrm{F}}_5^{-*}$ + (HCOOF), and S${\mathrm{F}}_6^{-}$ + HCl → ${\mathrm{F}}_2$${\mathrm{Cl}}^{-}$ + (S${\mathrm{F}}_4$H) at single energy points.

• Received 28 April 1969

DOI:https://doi.org/10.1103/PhysRev.184.81