Flowing Afterglow Studies of the Temperature Dependencies for Dissociative Recombination of O₂⁺, CH₅⁺, C₂H₅⁺, and C₆H₇⁺ with Electrons

Abstract

A temperature-variable flowing afterglow with an electrostatic Langmuir probe has been used to determine the rate coefficients,R e , for the recombination of a series of molecular ions with electrons. The R e for O 2 + , CH 5 + , C 2 H 5 + , and C 6 H 7 + have been determined at temperatures ranging from 80 to 600 K. Data for O 2 + over the temperature range of 100-500 K establish a single-power law dependence, which is consistent with previous data in this temperature range. The hydrocarbon data show that the R e are large at room temperature, being 1.1 × 10 -6 , 1.2 × 10 -6 , and 2.4 × 10 -6 cm 3 s -1 for CH 5 + , C 2 H 5 + , and C 6 H 7 + , respectively, and exhibit significant dependencies on temperature consistent with theoretical models based on the direct and indirect mechanisms. The change between these two dependencies occurs at a temperature of ∼300 K. The dissociative recombination of these ions is significant to molecular synthesis in interstellar clouds and the ionosphere of Titan, which is to be probed by the NASA Huygens-Cassini spacecraft that reaches the Saturnian system in July 2004. The relevance of the present data to these media is briefly discussed.

Experimental studies of the dissociative recombination processes for the C6D6+and C6D7+ions

“…The thermal rates at 300 K was found to be 1.0 ± 0. agreement with previous results for the undeuterated isotopomers (Abouelaziz et al 1993;McLain et al 2004). The lowest formation enthalpy for the undeuterated C 6 H + 6 and C 6 H and 854 kJ mol −1 respectively (Lias et al 1988).…”

“…Thus, our findings are in line with a formation of benzene in the interstellar medium, circumstellar envelopes and planetary ionospheres through ion-neutral reactions leading to C 6 H + 7 followed by DR yielding benzene. Also the rate constant previously measured in flowing afterglow experiments (Abouelaziz et al 1993;McLain et al 2004) and used in models of Titan's ionosphere (Vuitton et al 2008) has been corroborated in our study, albeit the temperature dependence of the DR rate of C 6 D + 7 has been found to be considerably weaker (the exponential factor being −0.83 instead of −1.3). This could be due to a different reactivity of ground and rovibrationally excited states of the ion.…”

Model-data comparisons for Titan's nightside ionosphere

“…McClain et al (2004), and references therein, discuss many of the important dissociative recombination rate coefficients. Coates et al (2007a) described the discovery of negative ions in the lower ionosphere of Titan by Cassini.…”

The ionospheric source of the red and green lines of atomic oxygen in the Venus nightglow

“…Although in many experiments the vibrational distribution of O þ 2 ðvÞ is unknown, the DR coefficient has been found not to vary substantially from one measurement to another. In fact, the DR coefficient of O þ 2 is sometimes measured to verify the normalization of the apparatus (e.g., McLain et al, 2004). Its value at thermal energies is about 2 Â 10 À7 cm 3 s À1 to within about 10%, and the electron temperature dependence at low temperatures is about T À0:7 e .…”