ABSOLUTE INTEGRAL CROSS SECTIONS AND PRODUCT BRANCHING RATIOS FOR THE VIBRATIONALLY SELECTED ION-MOLECULE REACTIONS: N-2(+)(X-2 Sigma(+)(g); v(+)=0-2) + CH4

作者：Xu Yuntao^*; Chang Yih Chung; Lu Zhou; Ng C Y

来源：Astrophysical Journal, 2013, 769(1): 72.

DOI：10.1088/0004-637X/769/1/72

• astrochemistry
• methods: laboratory
• planets and satellites: atmospheres
• planets and satellites:individual (Titan)
• techniques: spectroscopic

Absolute vibrationally selected integral cross sections (sigma (v+)'s) for the ion-molecule reaction N-2(+)(X-2 Sigma(+)(g); 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-2(+) 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 CH3+, CH2+, and N2H+ were identified. After correcting for the secondary reactions involving CH3+ and CH2+, we have determined the sigma (v+) values of the formation of these primary product ions, sigma (v+)(CH3+), sigma (v+)(CH2+), and sigma (v+)(N2H+), and their branching ratios, [sigma (v+)(CH3+) : sigma (v+)(CH2+) : sigma (v+)(N2H+)]/sigma (v+)(CH3+ + CH2+ + N2H+), v(+) = 0-2, in the E-cm range of 0.05-10.00 eV, where sigma (v+)(CH3+ + CH2+ + N2H+) = sigma (v+)(CH3+) + sigma (v+)(CH2+) + sigma (v+)(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 sigma (v+)(CH3+), sigma (v+)(CH2+), and sigma(v+)(N2H+) along with vibrational inhibition for the formation of these product ions are observed. The vibrational effects on the sigma (v+) values are sufficiently large to warrant the inclusion of the vibrationally excited reactions N-2(+)(X-2 Sigma(+)(g); 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.

• 出版日期2013-5-20