COLLISION ENERGY DEPENDENCE OF THE REACTIONS OF METASTABLE NEON WITH SMALL MOLECULES

The reaction dynamics of Ne* (2p⁵3s³P₂, ³P₀) + X → [NeX]⁺ + e⁻(X = H₂, CO, N₂, NO, O₂, CO₂, and C₂H₂) were studied with supersonic beams at various collision energies (E) via electron spectroscopy. Increasing E decreases the interparticle distance at which ionization occurs, allowing for exploration of the reaction potential energy surfaces via the kinetic energy ε of the ejected electron. Data were fit to give vibrational populations and line shifts (Δεs), the difference between the excitation energy of Ne* and the vibronic energy of the target molecules, where vibronic excitation is due to Ne*. The resulting populations were compared to calculated or experimental Franck-Condon factors (FCfs), and vibrational progressions were identified. Deviation from Franck-Condon (FC) behavior was observed in all cases except for C₂H₂, and all spectra at all E showed a blue shift except CO₂. With increasing E, Δεs for H₂⁺, CO⁺, and N₂⁺ increased with increasing E, while Δεs decreased for NO⁺ and C₂H₂⁺. The CO2⁺ spectra revealed a nearly constant red shift for the lowest three E and a blue shift for the highest E. O₂⁺ showed a very small blue shift, but the O₂⁺ populations were not determined due to an underlying continuum. Penning, excitation transfer, and ion-pair mechanisms are the most widely accepted for the reactions of metastable atoms. The closed-shell structure of H₂, CO, and N₂ and the large, increasing Δεs suggest that their Ne* reactions proceed via the Penning mechanism. The open- shell structure of NO and its decreasing Δεs indicates changing dynamics and possibly also competition between all three mechanisms for Ne* + NO. The very small Δεs for O₂⁺ implies the excitation transfer mechanism for Ne* + O₂. Ne* reactions with CO₂ and C₂H₂ both exhibited constant Δεs values for more than one E. This suggests that an excitation transfer mechanism is at work in these systems, but changes in Δεs at other E indicate that competing mechanisms may also be relevant. Lastly, a retrospective on authoring a solutions manual for a freshman chemistry textbook is offered