Chemiluminescent ion–molecule reactions of O+ ions with CH4, C2H4, C2H6, C3H8

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Abstract
UV emission in the wavelength range 3060–3300 Å was observed from OH(A 2Σ+), produced in a beam experiment by reactive collisions of O+ ions with hydrocarbon molecules at energies from 1 to about 150 eVCM. The rotational population distribution in the v′=0 and 1 levels of OH(A) was determined from computer simulations of the observed spectra. At low energies (?10 eVCM) the v′=0 distributions peak at rotational quantum numbers of N′=2–6 with CH4, but at about N′=20 with C2H4, C2H6, C3H8. The luminescent reaction is first order in the target gas pressure with C2H4, C2H6, C3H8, but second order with CH4. These results are rationalized in terms of a direct H− transfer reaction in the former case. With CH4, OH+ is formed in the primary reaction step, which is followed by a luminescent charge transfer reaction to give OH(A). Cross sections were measured for OH(A), CH(A), CH(B), Hβ and C(1P0) production as a function of collision energy. The OH(A) cross section suggests a transition from stripping to a sequential quasihard‐sphere collision mechanism. The energy dependence of the cross section for luminescence from O++H2, D2 collisions was also measured.