  The mechanism of the shift, broadening and quenching of the ammonia inversion
frequency with gas pressure has been a problem of lively interest for over
seventy years. A simple quantum model of the ammonia molecule perturbed by
collisions with ideal gas molecules displays the essential features of the
experimental data for NH3 and for ND3. The model does not display the behaviour
expected from theories of quantum localisation such as quantum state diffusion
and decoherence. On the other hand, models of perturbed classical oscillators
do display similar behaviour to our model. The quenching of the ammonia
inversion transition cannot therefore be interpreted as spatial localisation of
the wavefunction.
