Metastable methane ions. Temperature dependence of the translational energy release

Abstract

The translational energy released in the unimolecular loss of H* from metastable methane ions has been examined and found to have a significant temperature dependence. Reaction mechanisms of the barrier traversal, tunnelling, and electronic predissociation types are all inconsistent with the observed temperature effect if centrifugal effects are neglected. The observed temperature dependence of the kinetic energy release correlates well with the variation in centrifugal barrier height with rotational energy derived from a simple form of Langevin collision theory. It is also in agreement with the more rigorous treatment of Klots which allows tunnelling through the centrifugal barrier.

The rotational predissociation of the CH4+ and CD4+ ions

“…The difference between our experiment and the measurements of Solka et al [10] can be ascribed to the ion source conditions. The temperature is 400-500 K in Solka's experiments [10] and around 1500 K in the monoplasmatron source.…”

“…The temperature is 400-500 K in Solka's experiments [10] and around 1500 K in the monoplasmatron source. The higher kinetic energy values observed in our experiments would not be detectible at lower temperature, due to the difference in the rotational distribution function of the ions.…”

“…Klots [9] proposed tunneling through a rotational barrier and showed the lifetimes to be equal for the loss of H or D. We intend to detail this behaviour in the frame of a theoretical model described in a following paper [19]. The rotational barrier can easily account for the temperature effect observed by Solka et al [10] on the kinetic energy release.…”

“…The aim of this work is to complete the results obtained with Nier-type ion sources [8,10] by the use of a monoplasmatron source characterized by a higher working temperature. The tandem mass spectrometer previously used to examine rotational predissociation of diatomic ions [11][12][13] was appropriate to investigate under high resolution, the same process occurring in the CH 4 + and CD 4 + molecular ions.…”

A study of the transition state for the unimolecular decomposition of a triatomic molecule

“…internal energy but low rotational energy, the latter depending principally on the temperature of the ion source [36]. In the second type of experiment dissociation of the ion is collision-induced and is normally caused by the introduction of an inert gas into the path of the accelerated ions [37,38].…”