The metastable and collision-induced dissociation of propane, CO2-cluster, and NH3-cluster ions

Abstract

Metastable decay reactions occurring in the mass spectrum of C₃H₈ and the cluster species (NH₃)₂, (NH₃)₃, and (CO₂)₂ were investigated by decoupling the acceleration and deflection fields of a reversed Nier–Johnson geometry double‐focusing mass spectrometer. It is shown that the present experimental setup (molecular beam‐electron impact ion source‐deflection optics) permits a clean separation of molecular beam processes from background processes. The general reliability of our instrument is demonstrated by the study of the well‐characterized metastable ions in propane, which corrobated and extended previous measurements. Moreover, the ability to differentiate and study quantitatively both collision‐induced and unimolecular decay processes with the combined techniques of our instrument is demonstrated. Our tentative conclusion from the present study of ammonia and carbon dioxide clusters is that small cluster ions should be described by a loosely coupled oscillator model. The simple dissociation of (CO₂)⁺₂, (NH₃)⁺₂, and (NH₃)₂H⁺ are collision‐induced in the present work, supporting this model. On the other hand, the observation of true (unimolecular decay) metastables for the rearrangement decay of (NH₃)⁺₂ to NH⁺₄ and (NH₃)₃⁺ to NH⁺₄. NH₃, and for the dissociation of (NH₃)₃H⁺ to (NH₃)₂H⁺, suggests that energy exchange within a statistical ensemble of strongly coupled oscillators controls the decomposition kinetics of these species.