Double coincidence studies of molecular dissociation induced by heavy ion impact

Abstract

Multielectron removal from an atom or molecule may be accomplished with high efficiency by the impact of a fast, highly-charged, heavy ion. When a diatomic molecule suffers the loss of electrons, it will generally dissociate into ions having considerable amounts of kinetic energy as a result of their mutual Coulomb repulsion. In the present work, experiments designed to examine the yield and kinetic energy distributions associated with different charge division pathways for the dissociation of multicharged CO, N2 and O2 molecular ions were performed. A beam of 96 MeV Ar[^14+] ions was directed through a differentially pumped gas cell containing the molecules of interest into a microchannel plate detector. Dissociation products produced in ionizing collisions were accelerated out of the gas cell by an electric field into a time-of-flight (TOF) spectrometer. Upon reaching the end of the flight tube, the ions were detected by another set of microchannel plates. An acceptable binary dissociation event was one for which both dissociation-product ions were detected. The TOF of the first ion to reach the detector and the time difference between the arrival of the first ion and its partner were recorded on magnetic tape event-by-event so as to maintain their correlation. Separation of the time-difference (Δt) distributions into sets correlated with the charge of the first ion was accomplished by off-line sorting of the event-by-event data. Transformation of the Δt distributions into total kinetic energy distributions (TKED) required a detailed simulation of the ion trajectories in order to construct the response matrix of the spectrometer system. The average total kinetic energies were determined for all the observed dissociation channels and used to calculate the average excitation energies of the parent molecular ions. The TKED of CO[^2+] was in very good agreement with previous measurements obtained in a photoionization study using synchroton radiation. The present measurements indicate that states of higher excitation are populated in ion-molecule collisions. Moreover, the average total kinetic energies were systematically higher than the point-charge Coulomb energies for fully screened ions separated by the equilibrium bond length.