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\subsection{Part c \textit{following Anjali Tripathi}}   \href{http://ay201b.wordpress.com/2011/04/12/course-notes/#excitation_processes}{In class we derived} the timescale for neutral-neutral (nn) and neutral-ion (ni) interaction between particles in the ISM. For $n=1~\rm{cm^{-3}}$ and $T=10^4\rm{K}$ $T=10^4\rm{K}$, $\tau_{\rm{nn}}\sim50~\rm{yr}$ and $\tau_{\rm{ni}}\sim10~\rm{yr}$. This illustrates the importance of the electromagnetic potential in influencing interaction rates! Ions, protons, and electrons are always more likely to interact than neutral particles (assuming the ionization fraction is $\gtrsim10^{-3}$). This physics determines the role of processes including thermalization and collisional excitation in different ISM regimes.