Classical ghosts in quantal microwave ionisation

We provide a brief review of the response of an excited hydrogen atom to a microwave field, and a detailed discussion of the behaviour of a one‐dimensional model in a high frequency field. In particular we show that although classical dynamics is incorrect for the parameters of experimental interest an examination of the classical behaviour is helpful in understanding the quantal behaviour. We show further that, for these parameters, the quantal probabilities fluctuate dramatically with the variation of most system parameters and attribute this non‐classical behaviour to the dominant influence of relatively few, about ten, quasi‐resonant states.