Relaxation rates in molecular beam maser experiments

A theoretical explanation is given of recent measurements by Kukolich, Wang and Oates on the relaxation of pure inversion states and superposition states in a molecular-beam ammonia maser spectrometer. The Feynman-Vernon-Hellwarth representation is used, combined with an impact theory of collision damping, to derive the rate equations. It is shown, in agreement with experiment, that the longitudinal-component decay rate (analogous to a T₁ process in the Bloch equiations in magnetic resonance) is larger than the transverse-component decay rate (the analog of a T₂ process), in contrast to the usual magnetic-resonance case. The difference between the two rates depends on the efficiency of inelastic collisions in mixing the two maser levels, and is therefore more likely to occur with polar scattering gases having appropriate energy level spacings and populations.