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Abstract
In this dissertation, we show the importance of considering the interactionbetween photons and atomic and molecular species in theearly Universe as well as in current day photodissociation regions (PDRs).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. We used the time-independent radial nuclear Schr$ddot{{rm o}}$dinger equationand obtained 162 rovibrational levels for the X~$^{rm1}Sigma^{rm +}$ electronic ground state. This is 4 levels more than the result previouslycalculated.Transition probabilities between all rovibrational levels were also computed andused to predict the emission spectra of HeH$^{+}$ in local thermodynamicequilibrium (LTE).Then we calculated the radiative cooling coefficients for the LTE case. We found that the value of the radiative cooling coefficients of HeH$^{+}$ isabout ten orders of magnitude larger than those of H$_{2}$.However, the abundance of HeH$^{+}$ is low compared to that of H$_{2}$in the early Universe. Therefore, the absolute cooling efficiency of HeH$^{+}$ is a minor effectoverall and depends highly on the primordial cloud circumstances.Secondly, we have calculated photodissociation cross sections for HeH$^{+}$.Compared to its formation process, the cross sections for thedestruction process are not always treated precisely which is important,especially for UV irradiated environments.Photodissociation cross sections for the A~$^{mathrm{1}}Sigma^{mathrm{+}}leftarrowmathrm{X~}^{mathrm{1}}Sigma^{mathrm{+}}$ andX~$^{mathrm{1}}Sigma^{mathrm{+}}leftarrowmathrm{X~}^{mathrm{1}}Sigma^{mathrm{+}}$ transitions were obtained using aquantum method. We have calculated the respective cross sections as well as thecase for LTE.Those data will be included in future releases of the plasmacode Cloudy.Thirdly, photodissociation cross sections of C$_{2}$ have been calculatedfrom its ground electronic state X~$^{mathrm{1}}Sigma^{mathrm{+}}$to the electronic excited statesA~$^{mathrm{1}}Pi_{mathrm{u}}$, 2~$^{mathrm{1}}Pi_{mathrm{u}}$, and3~$^{mathrm{1}}Pi_{mathrm{u}}$.C$_{2}$ is important as a temperature diagnostic in diffuse interstellar cloudsand PDRs. More accurate values have been obtained from the current calculation andimportant resonance features have also been found.Next, we have considered the photodetachmentcross 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. We conclude that the photodetachment rate is enhanced by $> 30~%$ ifthe resonance is included. A reduction of H$^{-}$ results in the loss of H$_{2}$. Therefore the reduction of the H$_{2}$ abundance affects significantlythe formation of Population III (Pop III) stars and may influence the era ofthe reionization of the Universe. We believe that this result may have a significant impact on large scalecosmological simulations because they adopt the H$^{-}$ cross section without the resonance contribution.Lastly, we have considered the spectra of primordial objects at high redshift.We have used the publicly-available plasma code Cloudy in order to simulatethe first generation star, Pop III star.We considered the case for the single Pop III star as well as for the case ofa large star cluster in a dwarf galaxy. Strong features due to H I lines, He II lines as well as few H$_{2}$ linesare predicted.The simulated primordial emission features, may be observable with future farinfrared or submillimeter detectors.