The success of this windowing procedure can be interpreted as follows. In the case that the basis set was complete and covered all of space extending to infinity, then some part of the H 2 wave packet would quickly be ionized by the strong laser field, never to return. In practice, the finite extent of the basis set means that there are spurious peaks corresponding to excitations into unbound states, the rate of which is dominated by the first-order transition integrals and the most significant spurious features coincide with excited states having nonzero oscillator strength. By windowing the spectrum, we only include µ(t) data from later times in the Fourier transform, by which time some of the high-frequency, artificiallytrapped electron wave has been absorbed by the CAP.