Advances in low temperature gas-phase kinetics
Rate constants for elementary gas-phase reactions were first measured reliably following the development of pulsed photolysis and flow methods in the 1960's. These techniques have continued to be employed as kinetics experiments have been performed at lower and lower temperatures. Sub-ambient temperatures are reached either by cryogenic methods or by using expansion techniques. In this article, we review the possibilities and limitations of these cooling techniques and the results that have been obtained. These efforts have been driven both by the desire to understand the fundamental factors that control the rates of chemical reactions and also by the wish to provide rate constants that can be used in models of complex environments, such as planetary atmospheres and the interstellar medium. In this review, some emphasis is given to the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method which has now been used to determine rate constants for many elementary reactions, including those between neutral species, as well as ion–molecule reactions. This method has provided rate constants for a limited number of reactions to below 10 K. Major efforts are now being made to go to still lower temperatures and we describe some of the results obtained at these very low temperatures in the last section of this article.