Guided ion beam measurement of the product branching ratios for the ion-molecule reaction N++O2 as a function of collision energy

Abstract

A quadrupole-octopole-quadrupole mass spectrometer has been constructed for comparing ion-molecule reaction product intensities as both the internal excitation and the kinetic energy of the reactant ion are varied. Such comparisons require an ion beam with a known kinetic energy distribution and, most importantly, they require product intensity measurements made without significant bias in detection of the different product channels. To assess the characteristics of our instrument, we have studied the ion-molecule reaction N++O2 that is known to yield three different product channels: N+O+2, NO++O, and NO+O+. Ion beam trajectory simulations combined with experimental measurements show that the spread in the kinetic energy of the reagent ions has a fixed value in the range of 0.6 to 1.1 eV full width at half maximum in the center of mass (c.m.). Relative cross sections for the three different product channels are reported as a function of c.m. collision energy. A comparison of the observed product branching ratios with those determined previously by other workers shows that no serious product discrimination occurs over the collision energy range of 1.5 to 10.0 eV c.m. Discrepancies in the product branching ratios below 1.5 eV c.m. are believed to be caused by the overall collision energy uncertainty that results from both the ion beam kinetic energy spread and the thermal motion of the O2 reactant.