Rovibrationally selected ion-molecule collision study using the molecular beam vacuum ultraviolet laser pulsed field ionization-photoion method: Charge transfer reaction of N2+(X2Σg+; v+=0-2; N+=0-9) + Ar

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-2 Sigma(+)(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 Delta E-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 sigma(v(+) = 0-2, N+ = 0-9) for the N-2(+)(X-2 Sigma(+)(g); v(+) = 0-2, N+ = 0-9) + Ar CT reaction have been measured in the Ecm range of 0.04-10.0 eV, revealing strong vibrational enhancements and E-cm-dependencies of sigma(v(+) = 0-2, N+ = 0-9). The thermochemical threshold at E-cm = 0.179 eV for the formation of Ar+ from N-2(+)(X; v(+) = 0, N+) + Ar was observed by the measured s(v(+) = 0), confirming the narrow Delta E-cm spread achieved in the present study. The sigma(v(+) = 0-2; N+) values obtained here are compared with previous experimental and theoretical results. The theoretical predictions calculated based on the Landau-Zener-Stuckelberg formulism are found to be in fair agreement with the present measured sigma(v(+) = 1 or 2; N+). Taking into account of the experimental uncertainties, the measured sigma(v(+) = 1 or 2, N+) for N+ = 0-9 at E-cm = 0.04-10.0 eV are found to be independent of N+. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4750248]