Post-recombination early Universe cooling by translation–internal inter-conversion: The role of minor constituents

Abstract

Little is known of the mechanism by which H and H2, the principal constituents of the post-re-combination early Universe, cooled sufficiently to permit cluster formation, nucleosynthesis, and, eventually, the formation of structured objects. Radiative decay primarily cools the internal modes of H2, as Δj = - 2 jumps accompany quadrupolar emission. This, however, would be a self-limiting mechanism. In this work, a translational energy cooling mechanism based on collision-induced, translation-to-internal mode conversion, is extended, following an earlier study [A. J. McCaffery and R. J. Marsh, J. Chem. Phys. 139, 234310 (2013)] of ensembles comprising H2 in a H atom bath gas. Here, the possible influence of minor species, such as HD, on this cooling mechanism is investigated. Results suggest that the influence of HD is small but not insignificant. Conversion is very rapid and an overall translation-to-internal energy conversion efficiency of some 5% could be expected. This finding may be of use in the further development of models of this complex phase of early Universe evolution. An unexpected finding in this study was that H2 + HD ensembles are capable of very rapid translation-to-internal conversion with efficiencies of >40% and relaxation rates that appear to be relatively slow. This may have potential as an energy storage mechanism.

Energy transfer in multi-collision environments; an experimental test of theory: LiH (10;2) in H2(0;0)

“…This and earlier studies [8][9][10][11][12] allow some conclusions to be drawn regarding the dominant processes of gas phase energy transfer when many collisions occur. A striking feature is the near-resonant V-V exchange process, found when a transition downwards in energy within the excited molecule's vibrational manifold is in near-energy coincidence with one upwards in energy from the ground state of the bath molecule.…”

“…A number of recent publications [8][9][10][11][12] have described the basis of the model and how it is used, and only a brief description is given here. The system of mechanics used for determining the outcome of individual diatomic-diatomic, or atom-diatomic collisions is the angular momentum (AM) model of collision -induced state change.…”

“…This program has been used in a number of studies of energy flow in gas ensembles containing a small fraction of highly excited diatomic molecules in a variety of atomic and diatomic bath gases. [8][9][10][11][12] The evolution of these ensembles was followed to equilibrium in a series of collision cycles in which vibrational and rotational populations, as well as modal temperatures of vibration, (Tv) rotation (Tr) and translation (Tt), are available after each collision cycle or group of collision cycles. The result has enhanced insight into the principal energy relaxation mechanisms in the multi-collision regime.…”

Quantum state-resolved, bulk gas energetics: Comparison of theory and experiment

“…The dominant mechanisms depend strongly on the fundamental properties of both excited species a) Email: A.J.McCaffery@sussex.ac.uk and bath gas; in particular, their energy level structures. A detailed description of the computational model and the collision physics on which it is based is given in a recent publications 7 and hence is outlined only very briefly below.…”

“…is given in a recent publication. 7 Here the ensemble computational method is used to investigate the relaxation of vibrationally excited CsH(v;j), present as the 1:10 ratio, minor component of a binary gas mixture in which the majority-or bath-species is H 2 . In addition, ALDS follow the equilibration of highly excited CsD(v;j) in D 2 and comparison between experiment and the predictions from the computational model in these two ensemble systems forms an important part of the evaluation process.…”