Reaction Kinetics in Uniform Supersonic Flows at Very Low Temperatures

Abstract

There is a strong fundamental interest in the study of molecular collisions under conditions of very low temperatures or energies. For example, for reactive collisions, the effect of very small energy barriers or the influence of rotational or spin-orbit states on the reactivity can be highlighted. One of the strongest motivations however, comes from modern astrophysics and is related to the fascinating problem of the birth of stars and planets from the gravitational collapse of molecular cloud cores [1]. Molecular clouds are the densest part of the interstellar medium and typical conditions in a dense core are, a kinetic temperature of around 10K and a concentration of molecular hydrogen of 10⁴-10⁵ cm^-3. In the collapse of a rotating cloud core, a circumstellar disk is naturally generated in which planets, comets and various objects can be formed by accretion and this process is certainly not restricted to our solar system. If dust is present in various parts of the interstellar medium as grains of sub-micron size, its concentration in dense clouds makes them opaque to the harsh UV and visible radiation field of the neighbouring stars. With the striking developments of radioastronomy in the last few decades, it is, however, now possible to track the physical conditions prevailing inside these clouds mainly through the rotational spectra (most often in the sub-millimetre and millimetre range) emitted by polar molecules.