Keele Research Repository

The construction of an ammonia molecular beam maser employing an open resonator is described. The operation of this maser as an amplifier enables the effects of mode degeneracy in the resonator to be examined in a controlled manner. Its operation above oscillation threshold can give rise to biharmonic effects with beat frequencies of a few hundred Hz. A transient behaviour of the maser makes possible the measurement of low field Zeeman splittings within the maser linewidth.