ABSOLUTE INTEGRAL CROSS SECTIONS AND PRODUCT BRANCHING RATIOS FOR THE VIBRATIONALLY SELECTED ION-MOLECULE REACTIONS: N$_{2}^{+}$(X2Σ$_{\rm g}^{+}$;v+= 0-2) + CH4

Abstract

Absolute vibrationally selected integral cross sections (σ v +'s) for the ion-molecule reaction N(X 2Σ; v + = 0-2) + CH4 have been measured by using the newly developed vacuum ultraviolet (VUV) laser pulsed field ionization-photoion (PFI-PI) double-quadrupole-double-octopole ion guide apparatus. By employing a novel electric field pulsing scheme to the VUV laser PFI-PI source, we have been able to prepare reactant N ions in single-vibrational quantum states with not only high intensity and high purity but also high kinetic energy resolution, allowing integral cross section measurements to be conducted in the center-of-mass kinetic energies (E cm's) from 0.05 to 10.00 eV. Three primary product channels corresponding to the formations of CH, CH, and N2H+ were identified. After correcting for the secondary reactions involving CH and CH, we have determined the σ v + values of the formation of these primary product ions, σv+(CH), σv+(CH), and σv+(N2H+), and their branching ratios, [σv+(CH): σv+(CH): σv+(N2H+)]/σv+(CH + CH + N2H+), v + = 0-2, in the E cm range of 0.05-10.00 eV, where σv+(CH + CH + N2H+) = σv+(CH) + σv+(CH) + σv+(N2H+). The branching ratios are found to be nearly independent of the v + state and E cm. Complex v +-state and E cm dependences for σv+(CH), σv+(CH), and σv+(N2H+) along with vibrational inhibition for the formation of these product ions are observed. The vibrational effects on the σv+ values are sufficiently large to warrant the inclusion of the vibrationally excited reactions N(X 2Σ; v + ≥ 1) + CH4 for a more realistic modeling of the ion and neutral densities observed in the atmosphere of Titan. The cross-sectional data obtained in the present study are also useful for benchmarking theoretical calculations on ion-neutral collision dynamics.