A state-to-state study of the electron transfer reactions Ar+(2P3/2,1/2)+N2(X̃,v=0)→Ar(1S) +N+2(X̃,v′)

Abstract

The vibrational state distributions of N+2(X̃,v′) ions resulting from the reactions, Ar+(2P3/2)+N2(X̃,v=0)→Ar(1S0) +N+2(X̃,v′) [reaction (1)] and Ar+(2P1/2)+N2(X̃,v=0)→Ar(1S0) +N+2(X̃,v′) [reaction (2)], over the center-of-mass collisional energy (Ec.m.) range of 0.25–41.2 eV in a crossed ion–neutral beam experiment have been probed by the charge exchange method. The experimental results obtained for reaction (1) are in accord with the predictions of the semiclassical multistate calculation of Spalburg and Gislason that N+2 ions are formed predominantly (≳85%) in the v′=1 state and that the production of N+2(X̃,v′=0) becomes more important as Ec.m. is increased. The experiment also supports the theoretical results for reaction (2) at Ec.m.=1.2 and 4.1 eV showing that ≳80% of N+2 product ions are in the v′=2 state. However, the calculation is found to either over-estimate the populations for N+2(v′<2) or underestimate the populations for N+2(v′>2) resulting from reaction (2) at Ec.m.=10.3 and 41.2 eV. Absolute spin-orbit-state-selected total cross sections for reactions (1) and (2), σ3/2 and σ1/2, respectively, at the Ec.m. range of 0.25–115.3 eV have also been measured using a tandem photoionization mass spectrometer which is equipped with a radio frequency (RF) octopole ion guide reaction gas cell. The measured values for σ3/2 at Ec.m.=4.1, 10.3, and 41.2 eV and σ1/2 at 41.2 eV are in reasonable agreement with the theoretical cross sections. However, the experimental values for σ3/2 at 1.2 eV and σ1/2 at 1.2, 4.1, and 10.3 eV are approximately a factor of 2 higher than the theoretical predictions. A model analysis, which takes into account possible collision-induced spin-orbit mixings of the reactant Ar+ states in the RF octopole gas cell, shows that the values for σ1/2/σ3/2 and σ1/2 determined using the ion beam–RF octopole gas cell arrangement can be strongly susceptible to gas cell pressure effects whereas the experimental values for σ3/2 are reliable. The values for σ1/2 deduced by multiplying the values for σ3/2 and the ratios σ1/2/σ3/2 determined in the crossed ion–neutral beam experiment are in agreement with the theoretical cross sections. Both σ3/2 and σ1/2 are found to increase as Ec.m. is increased from 41.2 eV. This observation is interpreted as due to the formation of N+2 in the à 2Πu state at high Ec.m. . Combining the measured vibrational state distributions of product N+2(X̃,v′) ions and the absolute state-selected total cross sections, absolute state-to-state total cross sections for reactions (1) and (2) at selected Ec.m. are determined.