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OH megamasers (OHMs) emit primarily in the main lines at 1667 and 1665 MHz,and differ from their Galactic counterparts due to their immense luminosities,large linewidths and 1667/1665 MHz flux ratios, which are always greater thanone. We find that these maser properties result from strong 53 micron radiativepumping combined with line overlap effects caused by turbulent linewidths ofabout 20 km/s; pumping calculations that do not include line overlap areunreliable. A minimum dust temperature of about 45 K is needed for inversion,and maximum maser efficiency occurs for dust temperatures in the range 80 - 140K. We find that warmer dust can support inversion at lower IR luminosities, inagreement with observations. Our results are in good agreement with a clumpymodel of OHMs, with clouds sizes about 1 pc and OH column densities about 5e16cm^2, that is able to explain both the diffuse and compact emission observedfor OHMs. We suggest that all OH main line masers may be pumped by far-IRradiation, with the major differences between OHMs and Galactic OH maserscaused by differences in linewidth produced by line overlap. Small Galacticmaser linewidths tend to produce stronger 1665 MHz emission. The large OHMlinewidths lead to inverted ground state transitions having approximately thesame excitation temperature, producing 1667/1665 MHz flux ratios greater thanone and weak satellite line emission. Finally, the small observed ratio ofpumping radiation to dense molecular gas, as traced by HCN and HCO$^+$, is apossible reason for the lack of OH megamaser emission in NGC 6240.

The Effect of 53 micron IR Radiation on 18 cm OH Megamaser Emission