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A study has been made of the kinetics of elementary reactions between neutral species at low (temperature greater or equal to 80 K) and ultralow (temperature less than or equal to 80 K) temperatures. 2 parallel, and interrelated sets of experiments have been carried out. Rate constants have been measured for reactions of the hydroxyl and cyanide radicals at temperatures down to 80 K. The experiments utilize a relatively conventional method, in which low temperatures are reached by cryogenically cooling cylindrical gas cells through which a gas mixture flows. Radicals are generated by pulsed laser photolysis (PLP) of a suitable precursor and the kinetic decays of these radical concentrations are observed by making laser induced fluorescence (LIF) measurements on the radicals at variable time delays after the photolysis pulse. The lowest temperatures in these experiments are achieved by cooling the reaction cells with liquid nitrogen.

The experiments apply the PLP-LIF technique in the ultracold environment provided when a gas mixture is expanded through a Laval nozzle in a Cinetique de Reaction en Ecoulement Supersonique Uniforme (CRESU) apparatus.

The results have provided a wealth of kinetic information for neutral neutral reactions in a temperature regime where there was no previous data. The experiments have revealed that far more gas phase reactions between natural species remain rapid at ultralow temperatures than was initially envisaged. Rate constants were measured for reaction between radicals and saturated molecules, radicals and unsaturated molecules, as well those for reactions between radicals. The results have attracted a great deal of attention, both from chemical theorists and from scientists attempting to model the chemistry which synthesizes a large number of molecules in dense interstellar clouds. There are strong indications that the data supplied by the experimental programme, which provides the first reliable information on neutral neutral reactions for these calculations, are likely to require considerable revision of the chemical models.
Rate constants for a number of gaseous radical-radical reactions will be measured at temperatures down to ca 10 K. Unstable radicals will be generated by pulsed laser photolysis and the rates of their subsequent removal observed by time-resolved laser-induced fuorescence. In Meudon these experiments will be performed in the CRESU apparatus which allows extremely low temperatures to be reached without condensation. The data obtained will be of exceptional theoretical and astrochemical interest.


B15 2TT Birmingham
United Kingdom

Participants (1)

Centre National de la Recherche Scientifique (CNRS)
4 Ter Route Des Gardes
92190 Meudon