The chemistry of the early universe plays an important role in our understanding on the birth and evolution of galaxies and interstellar clusters. Molecular formation began at the end of the recombination era when the temperature was low enough that the newly formed atoms could survive for further evolution. After recombination, the matter density was still very low and three-body reactions were still very inefficient: however, it was there that the first molecular species were postulated to be formed through radiative association. In spite of the low fractional abundances which is expected for species like LiH, LiH+ and HeH+ these molecules have nevertheless been considered to be important in that domain, due to their large permanent dipole moment that make them possible candidates as coolant during the late stages of the gravitational collapse of the first cosmological objects. In fact, because of the high density of their rovibrational states, molecules can absorb thermal energy from the surrounding atomic gas via internal excitations and then release it through emission of photons, thereby efficiently cooling the clouds. In turn, these photons can increase the density of the cosmic background radiation inducing both spectral distortions and spatial anisotropies , representing a possible way to probe the features of the early universe chemistry. Moreover, at later stages, the molecular cooling mechanism is considered crucial for the formation of the first cosmological objects that are thought to be formed by collapse of the pristine molecular clouds. Among these objects, low-metallicity stars have formed, for which the only efficient cooling mechanism at low temperature can be provided by molecules like HD, HeH+, H3+, H2+, LiH+. Gas phase chemistry of their formation and destruction is therefore fundamental to have a good understanding of the early universe evolution. However, to evaluate the molecular abundance in the full redshift range from the Big-Bang to the formation of the first stars and galaxies, state-to-state rate coefficients in a large range of temperature (several order of magnitude) are required. Notwithstanding the simplicity of the reactive species, the broad range of the temperature required in the evolutionary cosmological models and the high complexity of chemical physical processes involving also non-adiabatic reactions, three-body recombination and collision induced dissociation processes, do not permit to obtain all the rate coefficients by 'exact'?quantum dynamical methods, so that benchmark quantum dynamical calculations are mandatory to properly assess dynamical approximations and/or models necessary to extend the numerical treatment to regimes where 'exact' quantum rates cannot be achieved. In the conference our recent effort [1-3] in this direction will be presented taking as example some prototypical key reaction of the early universe chemical network. References [1] F. Esposito and M. Capitelli; J. Phys. Chem. A 113 (2009) 15307. [2] D. De Fazio, M. de Castro, A. Aguado, V. Aquilanti and S. Cavalli; J.Chem. Phys.137 (2012) 244306. [3] Dario De Fazio; Phys. Chem. Chem. Phys. 16 (2014) 11662.
Early Universe Chemistry: a challenging problem for Reactive Scattering Methods
Dario De Fazio;
2015
Abstract
The chemistry of the early universe plays an important role in our understanding on the birth and evolution of galaxies and interstellar clusters. Molecular formation began at the end of the recombination era when the temperature was low enough that the newly formed atoms could survive for further evolution. After recombination, the matter density was still very low and three-body reactions were still very inefficient: however, it was there that the first molecular species were postulated to be formed through radiative association. In spite of the low fractional abundances which is expected for species like LiH, LiH+ and HeH+ these molecules have nevertheless been considered to be important in that domain, due to their large permanent dipole moment that make them possible candidates as coolant during the late stages of the gravitational collapse of the first cosmological objects. In fact, because of the high density of their rovibrational states, molecules can absorb thermal energy from the surrounding atomic gas via internal excitations and then release it through emission of photons, thereby efficiently cooling the clouds. In turn, these photons can increase the density of the cosmic background radiation inducing both spectral distortions and spatial anisotropies , representing a possible way to probe the features of the early universe chemistry. Moreover, at later stages, the molecular cooling mechanism is considered crucial for the formation of the first cosmological objects that are thought to be formed by collapse of the pristine molecular clouds. Among these objects, low-metallicity stars have formed, for which the only efficient cooling mechanism at low temperature can be provided by molecules like HD, HeH+, H3+, H2+, LiH+. Gas phase chemistry of their formation and destruction is therefore fundamental to have a good understanding of the early universe evolution. However, to evaluate the molecular abundance in the full redshift range from the Big-Bang to the formation of the first stars and galaxies, state-to-state rate coefficients in a large range of temperature (several order of magnitude) are required. Notwithstanding the simplicity of the reactive species, the broad range of the temperature required in the evolutionary cosmological models and the high complexity of chemical physical processes involving also non-adiabatic reactions, three-body recombination and collision induced dissociation processes, do not permit to obtain all the rate coefficients by 'exact'?quantum dynamical methods, so that benchmark quantum dynamical calculations are mandatory to properly assess dynamical approximations and/or models necessary to extend the numerical treatment to regimes where 'exact' quantum rates cannot be achieved. In the conference our recent effort [1-3] in this direction will be presented taking as example some prototypical key reaction of the early universe chemical network. References [1] F. Esposito and M. Capitelli; J. Phys. Chem. A 113 (2009) 15307. [2] D. De Fazio, M. de Castro, A. Aguado, V. Aquilanti and S. Cavalli; J.Chem. Phys.137 (2012) 244306. [3] Dario De Fazio; Phys. Chem. Chem. Phys. 16 (2014) 11662.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
