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\subsection{Part c \textit{following Anjali Tripathi}}   \href{In \href{http://ay201b.wordpress.com/2011/04/12/course-notes/#excitation_processes}{In  class we derived}{http://ay201b.wordpress.com/2011/04/12/course-notes/#excitation_processes} derived}  the timescale for neutral-neutral (nn) and neutral-ion (ni) interaction between particles in the ISM. For $n=1~\rm{cm^{-3}}$ and $T=10^4\rm{K}$