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\begin{document}

\title{Methyl halides}



\author[1]{sinhanidhi00}%
\affil[1]{Affiliation not available}%


\vspace{-1em}



  
  \date{May 14, 2024}


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\section*{lide~Positron - methyl halides scattering
dynamics}

{\label{106359}}

Nidhi~ Sinha, Paresh Modak, Suvam Singh and Bobby Antony~~~

\emph{Atomic and Molecular Physics Lab,~ Department of Applied Physics,
Indian Institute of Technology (Indian school of~ Mines),
Dhanbad-826004, Jharkhand, India~~}

\subsection*{Abstract}

{\label{986558}}

The well known Spherical Complex Optical Potential (SCOP) and Complex
Spherical Potential-ionisation contribution (CSP-ic) method with proper
modification is applied to the CH\textsubscript{3}X (X=Cl, Br and I)
molecules to compute the positron impact scattering cross sections,
which rather is a first theoretical attempt to report the same. Besides,
the various inelastic cross sections for the present targets is new in
literature. In general, a reasonable accordance has been found between
the present total cross sections and the experimental measurements.
However, the observed inconsistency between the theoretical values and
experimental cross sections in the low energy region persist in this
work. Furthermore, based on the present data set, we have attempted to
propose a semi-empirical formula for~\(Q_{tot}\), depending on
the target parameters which would be useful to determine the total cross
sections in the high energy region.~\(\sigma\)

\par\null

\subsection*{Introduction}

{\label{131771}}

Recent decades have witnessed augmented focus on positron scattering
from various targets. This rest on the vast contribution of such
phenomenon in different fields. Guided by these applications, our group
have extended the research focus on positron-target collisions. The
present work is an addition to this endeavor.~ We have chosen the less
attended methyl halide group as targets in this article. These molecules
yield majority of the inorganic free halogen radicals to the
stratosphere resulting in ozone depletion. Also CH\textsubscript{3}F
find vital applications in biochemistry and mass spectroscopy.
Furthermore, methyl bromide and methyl iodide are potential candidates
as soil fumigant. The cross sectional data for these targets are used in
analysis of various phenomena. The cross sections for positron-molecule
interactions help us give a better insight of the particle track
simulations. Modelling software such as GEANT, PENELOPE and LEPTS use
these cross sections as their basic input to estimate radiation induce
damage.~

\(sigma\ b\)

The only available previous data is the experimental measurements of~
Kimura~\emph{et al.}~\cite{Kimura_2001}; however, no such theoretical
effort has been made. The present computed cross sections are thus
the~first theoretical report for the present targets. Furthermore, the
various inelastic cross sections are reported for the first time.~

\subsection*{Result and discussion}

{\label{441149}}

\subsubsection*{Total cross section}

{\label{576737}}\selectlanguage{english}
\begin{figure}[h!]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/ch3cl/ch3cl}
\caption{{Positron impact total cross section for methyl chloride
{\label{780644}}{\label{780644}}%
}}
\end{center}
\end{figure}

The total cross sections for the present set of targets is shown in fig.
1, 2 and 3 with the only available experimental measurements of
Kimura~\emph{et al.}~\cite{Kimura_2001}. ~The present cross sections for
the three molecules show a similar variation with increasing energy.
These molecules have~\emph{IP}s around 10 eV, beyond which an increase
in cross section is observed which is due to the addition of the
contributions from various inelastic channels in the scattering process.
However, at sufficiently high energy the interaction time between the
target molecule and the projectile is adequately reduced and hence the
cross sections which is well~substantiated in the graph.

The present results overestimate the cross sections of Kimura~\emph{et
al.}~\cite{Kimura_2001} in the intermediate energies, though
underestimates them above 40 eV. The MSCOP (multi-scattering centre
spherical complex optical potential) formalism adopted to compute the
present results may be a possible reason for the higher cross sections
in the intermediate region. However, for CH\textsubscript{3}Br and
CH\textsubscript{3}I, present data lie close to the error limits of the
experimental cross sections of~Kimura~\emph{et al.}~\cite{Kimura_2001}.
Nonetheless. the present results show a similar nature as that of the
experimental measurements.

Furthermore, the paucity of data for the present set of molecules demand
for more theoretical as well as experimental endeavors to be attempted
to validate the present results.~

\par\null\par\null\selectlanguage{english}
\begin{figure}[h!]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/ch3br/ch3br}
\caption{{Positron impact total cross sections for methyl bromide
{\label{411841}}{\label{411841}}%
}}
\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[h!]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/ch3i/ch3i}
\caption{{Positron impact total cross section for methyl iodide
{\label{966495}}{\label{966495}}%
}}
\end{center}
\end{figure}\selectlanguage{english}
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\caption{{This is a caption
{\label{122594}}%
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\end{figure}\selectlanguage{english}
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\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/direct-ion/direct-ion}
\caption{{This is a caption
{\label{587286}}%
}}
\end{center}
\end{figure}

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