We show the importance of considering the interaction between photons and atomic and molecular species in the early Universe.
First we calculated spectroscopic properties of the HeH+ molecular ion. HeH+ is believed to be formed first in the early Universe. Due to its permanent dipole moment, a high efficiency of radiative cooling is expected. Transition probabilities between all rovibrational levels were also computed and used to predict the emission spectra of HeH+ in local thermodynamic equilibrium (LTE). Then we calculated the radiative cooling coefficients for the LTE case.
Secondly, we have calculated photodissociation cross sections for HeH+ . Compared to its formation process, the cross sections for the destruction process are not always treated precisely which is important, especially for UV irradiated environments. Photodissociation cross sections for the A 1 Σ+ ← X 1 Σ+ and X 1 Σ+ ← X 1 Σ+ transitions were obtained using a quantum method. We have calculated the respective cross sections as well as the case for LTE.
Lastly, we have considered the photodetachment cross section of H− involving a strong resonance around 11 eV. As for the radiation field, we have considered the case for a blackbody, quasar, and the average intergalactic radiation field in the early Universe.