Temperature dependence of ternary rate coefficients for the (CO)n−1++2CO⇌(CO)n++CO reaction, and the role of isomers for the growth of larger (CO)n+ clusters

Abstract

Ternary rate coefficients for the title reaction have been measured using a rf 22-pole ion trap in the temperature range 65–300 K. For the formation of dimers, the ternary rate coefficient, k3, follows the power law 2.3×10−28 cm6/s×(300 K/T)m, with m=1.6±0.2 over the full temperature range of the experiment. This result is in good agreement with previous measurements at higher temperatures and also with theoretical predictions. The formation of trimers is represented by k3=1.3×10−29 cm6/s×(300 K/T)m, with m=3.2±0.3. Including into the evaluation also previously published data measured at higher temperatures, leads to an even steeper temperature dependence. These findings are tentatively explained by the fact that (CO)2+ is a nonlinear molecule with low lying vibrational modes. The formation of tetramers, (CO)4+, is very slow and shows a positive or near zero temperature dependence. This behavior is explained by the presence of two isomers. In order to pass this bottleneck and to reach (CO)5+ and larger clusters, up to (CO)9+, a short but intense gas pulse has been injected into the trap. In contrast to previous high pressure studies where (CO)6+ appears to be especially stable, (CO)7+ turns out to be a favored cluster in the 80 K trap experiment. Also this result can be explained by the existence of two isomers, in which four CO molecules are loosely arranged around a strongly bound (CO)2+ or (CO)3+ core.