Electron-impact dissociation of [Formula Presented] ions: Measurement of [Formula Presented] fragment ions

Absolute cross sections for electron-impact dissociation of [Formula Presented] producing [Formula Presented] fragment ions were measured in the 3–100 eV range using a crossed electron-ion beams technique with total uncertainties of about 10% near the cross-section peak. Although the measured energy dependence agrees well with two sets of storage ring measurements, the magnitude of the present results lies about 15–25 % below the other results at the cross-section peak near 40 eV. Below 10 eV, the present data tend to exceed the storage ring data, consistent with the presence of excited states in the [Formula Presented] ion beam. For energies above 29 eV, the cross sections should also include contributions from dissociative ionization, though no clear onset of this channel is evident in any of the three sets of measured data.