  A simple theory of the Rydberg atoms ionisation by electromagnetic pulses and
microwave field is presented. The analysis is based on the scale transformation
which reduces the number of parameters and reveals the functional dependencies
of the processes. It is shown that the observed ionisation of Rydberg atoms by
subpicosecond electromagnetic pulses scale classically. The threshold electric
field required to ionise a Rydberg state may be simply evaluated in the
photonic basis approach for the quantum dynamics or from the multiphoton
ionisation theory.
