## Photoprocess effects in the early Universe

##### Abstract

In this dissertation, we show the importance of considering the interaction
between photons and atomic and molecular species in the
early 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 equation
and obtained 162 rovibrational levels for the X~$^{rm
1}Sigma^{rm +}$ electronic ground state.
This is 4 levels more than the result previously
calculated.
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.
We found that the value of the radiative cooling coefficients of HeH$^{+}$ is
about 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 effect
overall 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 the
destruction process are not always treated precisely which is important,
especially for UV irradiated environments.
Photodissociation cross sections for the
A~$^{mathrm{1}}Sigma^{mathrm{+}}leftarrow
mathrm{X~}^{mathrm{1}}Sigma^{mathrm{+}}$ and
X~$^{mathrm{1}}Sigma^{mathrm{+}}leftarrow
mathrm{X~}^{mathrm{1}}Sigma^{mathrm{+}}$ transitions were obtained using a
quantum method.
We have calculated the respective cross sections as well as the
case for LTE.
Those data will be included in future releases of the plasma
code Cloudy.
Thirdly, photodissociation cross sections of C$_{2}$ have been calculated
from its ground electronic state X~$^{mathrm{1}}Sigma^{mathrm{+}}$
to the electronic excited states
A~$^{mathrm{1}}Pi_{mathrm{u}}$, 2~$^{mathrm{1}}Pi_{mathrm{u}}$, and
3~$^{mathrm{1}}Pi_{mathrm{u}}$.
C$_{2}$ is important as a temperature diagnostic in diffuse interstellar clouds
and PDRs.
More accurate values have been obtained from the current calculation and
important resonance features have also been found.
Next, 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.
We conclude that the photodetachment rate is enhanced by $> 30~%$ if
the resonance is included.
A reduction of H$^{-}$ results in the loss of H$_{2}$.
Therefore the reduction of the H$_{2}$ abundance affects significantly
the formation of Population III (Pop III) stars and may influence the era of
the reionization of the Universe.
We believe that this result may have a significant impact on large scale
cosmological 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 simulate
the first generation star, Pop III star.
We considered the case for the single Pop III star as well as for the case of
a large star cluster in a dwarf galaxy.
Strong features due to H I lines, He II lines as well as few H$_{2}$ lines
are predicted.
The simulated primordial emission features, may be observable with future far
infrared or submillimeter detectors.