Excitation temperature of the 44 GHz, 95 GHz, and 36 GHz lines (lower panels), and the 84 GHz and 6₂−6₁-E masers (upper panels) for a temperature of 200 K and a specific column density of 10¹⁶ cm^-2 km^-1 s (left) and of 10¹⁷ cm^-2 km^-1 s (right) as functions of volume density. The dashed horizontal line separates maser from non-maser behaviour. Vertical lines mark the transitions from maser to non-maser behaviour.