Rovibrationally selected ion-molecule collision study using the molecular beam vacuum ultraviolet laser pulsed field ionization-photoion method: Charge transfer reaction of N{sub 2}{sup +}(X {sup 2}{Sigma}{sub g}{sup +}; v{sup +}= 0-2; N{sup +}= 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{sub 2}{sup +}(v{sup +}, N{sup +}) + 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{sub 2}{sup +}(X {sup 2}{Sigma}{sub g}{sup +}, v{sup +}= 0-2, N{sup +}= 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{sub 2}{sup +} PFI-PI beam can be formed with a laboratory kinetic energy resolution of {Delta}E{sub lab}={+-} 50 meV. As a result, the total cross section measurement can be conducted at center-of-mass kinetic energies (E{sub cm}'s) down to thermal energies. Absolute total rovibrationally selected cross sections {sigma}(v{sup +}= 0-2, N{sup +}= 0-9) for the N{sub 2}{sup +}(X {sup 2}{Sigma}{sub g}{sup +}; v{sup +}= 0-2, N{sup +}= 0-9) + Ar CT reaction have been measured in the E{sub cm} range of 0.04-10.0 eV, revealing strong vibrational enhancements and E{sub cm}-dependencies of {sigma}(v{sup +}= 0-2, N{sup +}= 0-9). The thermochemical threshold at E{sub cm}= 0.179 eV for the formation of Ar{sup +} from N{sub 2}{sup +}(X; v{sup +}= 0, N{sup +}) + Ar was observed by the measured {sigma}(v{sup +}= 0), confirming the narrow {Delta}E{sub cm} spread achieved in the present study. The {sigma}(v{sup +}= 0-2; N{sup +}) values obtained here are compared with previous experimental and theoretical results. The theoretical predictions calculated based on the Landau-Zener-Stueckelberg formulism are found to be in fair agreement with the present measured {sigma}(v{sup +}= 1 or 2; N{sup +}). Taking into account of the experimental uncertainties, the measured {sigma}(v{sup +}= 1 or 2, N{sup +}) for N{sup +}= 0-9 at E{sub cm}= 0.04-10.0 eV are found to be independent of N{sup +}.