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\subsubsection{Collisional Pumping}  \label{subsub:coll_pump}  Collisionally pumped (Type I) masers occur in higher-density regions, where the molecules responsible for the masers are excited from collisions primarily with H$_2$. In order to maintain the inversion through collisions: (1) the spontaneous decay from higher energy states into the upper maser level must be faster than into the lower maser level, or (2) the spontaneous decay of the upper maser level must be slower than the lower maser level \citep{Goldreich_1974}. However, the transition will thermalize beyond the critical number densities ($n_{\mathrm{H}}$ Table \ref{subsub:rad_pump}), setting an upper limit on the energy transfer to the masing transition. The H$_2$O and SiO transitions in Table \ref{subsub:rad_pump} are examples of masers that are typically collisionally excited \citep{Elitzur_1992}. Note that both of these transitions have particularly high critical number densities.  \subsubsection{Radiative Pumping}  \label{subsub:rad_pump}  Radiatively pumped (Type II) masers rely primarily on absorption of IR photons, and correlations are observed between the IR luminosity and maser luminosity \citep{darling2002_paperIII}. Successful use of this pump generally relies on having more transitions in the masing molecule into the upper maser state than the lower one when IR photons are absorbed \citep{lo2005}. Inverted populations levels in the 18 cm line quadruplet of OH result from radiative pumping \citep[][see \S XXX ADD NUM XXX for more on the OH transitions]{Elitzur_1992, lo2005}.  \begin{table}   \begin{tabular}{ c c c c c c }