A scheme that is analogous to the operation of a heat engine is proposed for exciting maser action in the far infrared and submillimeter parts of the spectrum. A beam of hot molecules interacts through distant collisions with a cold gas of another species at very low pressure. Certain energy levels in the hot gas are cooled faster than others owing to coincidental resonances in the spectra of the two gases. In the partially cooled nonequilibrium state, population inversions are possible in the hot species.To demonstrate feasibility, the interaction between pure rotational states of HCl and HF is discussed in detail. Barring unforeseen experimental difficulty (e.g., chemical activity of HF) the Schawlow‐Townes condition for maser action can be satisfied by theJ=3 → 2 transition in HF at 123.1 cm−1, or 81.25 &mgr;.