Rovibrationally selected ion-molecule collision study using the molecular beam vacuum ultraviolet laser pulsed field ionization-photoion method: Charge transfer reaction of N _2 ~+(X ~2Σ _g ~+; V ~+ 0-2; N ~+ = 0-9) ~+ Ar

摘要

We have developed an ion-molecule reaction apparatus for state-selected absolute total cross section measurements by implementing a high-resolution molecular beam vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) ion source to a double-quadrupole double-octopole ion-guide mass spectrometer. Using the total cross section measurement of the state-selected N _2 ~+(v ~+, N ~+) ~+ Ar charge transfer (CT) reaction as an example, we describe in detail the design of the VUV laser PFI-PI ion source used, which has made possible the preparation of reactant N _2 ~+(X ~2Σ _g ~+, v ~+ = 0-2, N ~+ = 0-9) PFI-PIs with high quantum state purity, high intensity, and high kinetic energy resolution. The PFI-PIs and prompt ions produced in the ion source are shown to have different kinetic energies, allowing the clean rejection of prompt ions from the PFI-PI beam by applying a retarding potential barrier upstream of the PFI-PI source. By optimizing the width and amplitude of the pulsed electric fields employed to the VUV-PFI-PI source, we show that the reactant N _2 ~+ PFI-PI beam can be formed with a laboratory kinetic energy resolution of ΔE lab ± 50 meV. As a result, the total cross section measurement can be conducted at center-of-mass kinetic energies (E cm's) down to thermal energies. Absolute total rovibrationally selected cross sections σ(v + = 0-2, N ~+ = 0-9) for the N _2 ~+(X ~2Σ _g ~+; v ~+ = 0-2, N ~+ = 0-9) ~+ Ar CT reaction have been measured in the E cm range of 0.04-10.0eV, revealing strong vibrational enhancements and E cm- dependencies of σ(v ~+ = 0-2, N ~+ = 0-9). The thermochemical threshold at E cm = 0.179eV for the formation of Ar + from N _2 ~+(X; v ~+ = 0, N ~+) ~+ Ar was observed by the measured σ(v + = 0), confirming the narrow ΔE cm spread achieved in the present study. The σ(v ~+ = 0-2; N ~+) values obtained here are compared with previous experimental and theoretical results. The theoretical predictions calculated based on the Landau-Zener-Stückelberg formulism are found to be in fair agreement with the present measured σ(v ~+ 1 or 2; N ~+). Taking into account of the experimental uncertainties, the measured σ(v ~+ 1 or 2, N ~+) for N ~+ = 0-9 at E _(cm) = 0.04-10.0eV are found to be independent of N ~+.