It is shown that certain unitary gauge operations that are normally of negligible importance in the theory of laser-atom interactions become significant for pulses containing only a few cycles, and it is discussed how to make gauge invariant predictions in general, and in particular in the velocity and length gauges, and in the Henneberger frame.

• Received 4 February 2002

DOI:https://doi.org/10.1103/PhysRevA.65.053417