Angular scattering and internal energy deposition in kiloelectronvolt collision activated decomposition reactions of some low mass cations

Abstract

The scatter profiling technique has been used to determine angular distribution functions for molecular ions of acetone, methane, propyne, ethanol dimethyl ether, 1-propanol and nitromethane which undergo collisional activation with He target atoms and which subsequently dissociate into specific product ions. The mean internal energy depositions, ε ̄ , associated with each reaction have been determined by translational energy loss (MIKE) spectrometry, the angular distributions allowing correction for energy losses to target atom kinetic energy. For dissociation of 5 keV acetone ions into CH 3 CO + ions, energy losses and kinetic energy release distributions associated with “height resolved” MIKE spectra showed that larger internal energy depositions were associated with larger scatter angles. For reactions of other ions, studied at 4 keV, the most probable scatter angles, θ c, (and root-mean-square angles) were found to correlate with the square roots of the energy uptake values. Dissociation reactions of methane ions studied at 4 and 8 keV, show that such angles are also inversely proportional to the collision energy E and suggest a relationship ε ̄ α (θ c E) 2 for He target gas. In contrast a relationship of the form ε ̄ α θ c was found for Ar target gas. These results suggest that collisional activation of small molecular ions involves principally electronic (near vertical) transitions rather than roto-vibrational excitation.