Construction and calibration of a custom time-of-flight mass spectrometer and its use in measuring the reaction kinetics of transition metal ion-organic interactions.

Abstract

A unique instrument was constructed and used to generate and interrogate jet cooled neutrals, ions and their respective clusters. The instrument is a result of the artful marriage of supersonic expansions combined with time of flight spectroscopy. Ionization occurs in a large main chamber. The ions are separated via a kinetic energy pulse from a custom built linear particle accelerator. The revelatory hardware for our instrument is a microchannel plate detector (MCP). The MCP is mounted on the exit of a custom designed and built hemispherical energy analyzer (sector), which acts as an energy filter. This filtering characteristic of the sector permits study of selected ionized fragments. To test the instrument, the 2-photon resonant, 3-photon ionization spectrum of gaseous atomic copper was measured. The (n = 9 – 21) Rydberg series was observed in 2-photon excitation. The term energies of this series converged to copper's lowest ionization threshold with an apparent quantum defect of 0.92. The state which couples the ground state of copper to the Rydberg series is a non-stationary state composed primarily of the spin-orbit components of the lowest 2P° atomic states. Additionally, the time dependence of the gaseous unimolecular decomposition of the jet-cooled adduct ion, Ni⁺•Acetone was monitored by selective detection of the daughter fragment, Ni⁺CO. Various photon energies were supplied to initiate dissociation of the adduction. The energies employed in this reaction were well below that required to fragment C-C ς-bonds. First-order unimolecular decomposition rate constants, k(E) ranged from 55000 – 113000 s⁻¹. The rate constants decreased with decreasing amounts of internal excitation. Ni⁺ cation is implicated as a catalytic necessity to activate the bond and cause molecular fragmentation. These experiments represent the first direct kinetic study of such catalytic type reactions.