We model the OH megamaser emission from the luminous infrared galaxy IIIZw35 as arising from a narrow rotating starburst ring of radius 22 pc enclosing a mass of 7 \times 10 ^ { 6 } M _ { \odot } .
We show how both the compact and apparently diffuse maser emission from this ring can arise from a single phase of unsaturated maser clouds amplifying background radio continuum .
The masing clouds are estimated to have a diameter of < 0.7 pc and internal velocity dispersion of \sim 20km s ^ { -1 } .
We find that the clouds are neither self-gravitating nor pressure confined but are freely expanding .
Their dispersal lifetimes may set the vertical thickness of the ring .
For an estimated internal density of 3 \times 10 ^ { 3 } cm ^ { -3 } , cloud masses are of order 24M _ { \odot } .
The observed spectral features and velocity gradients indicate that the clouds must be outflowing and escaping the nucleus .
The cloud mass outflow rate is estimated to be 0.8M _ { \odot } yr ^ { -1 } , while the star formation rate is \sim 19M _ { \odot } yr ^ { -1 } .
Associated ionised gas , possibly generated from dissipated clouds , provides free-free absorption along the source axis , explaining the observed East-West asymmetries .
We show that the clumpiness of a maser medium can have a dramatic effect on what is observed even in a relatively low gain OH megamaser .
Specifically , in IIIZw35 our clumpy maser model naturally explains the large line to continuum ratios , the large 1667MHz:1665MHz line ratios and the wide velocity dispersions seen in the compact maser spots .
Other astrophysical masers showing both compact and apparently diffuse emission might be explained by similar clumpy structures .