Collisional relaxation times in a three-level maser

We derive the Bloch equations for a three-level system, introducing phenomenological relaxation rates. Microscopic expressions are developed for these rates in the case of a gas of molecular absorbes for which relaxation is due to collisions with other molecules. Assuming that the colliding species move on classical paths and interact through long-range dipole terms, we can derive simple formulae for all the rates. A notable prediction is that certain off-diagonal rates are non-zero, though smaller than the diagonal terms. The physical significance of the results is briefly discussed.