The authors compare the classical dynamics of the one-dimensional model of the hydrogen atom when an applied microwave field is represented by a smoothly varying function and when it is represented by a series of impulses, the latter being a computationally more convenient system. They show that there are fundamental differences between the dynamics of the two models: for impulsive fields there are almost no rotationally invariant phase curves, and the bifurcation sequence of stable periodic orbits is completely different in the two models. Further, in the weak-field case and when the applied field frequency is less than the unperturbed frequency most orbits of the impulsive model are chaotic in contradistinction to smooth fields for which most orbits are stable. From these sample calculations the authors conclude that the impulsive model of microwave ionisation should not be used either as an approximation to experiment or to compare the classical and quantal dynamics of the ionisation process.