  The Keldysh theory of photoionization in a solid dielectric is generalized to
the case of arbitrarily short driving pulses of arbitrary pulse shape. We
derive a closed-form solution for the nonadiabatic ionization rate in a
transparent solid with a periodic dispersion relation, which reveals ultrafast
ionization dynamics within the field cycle and recovers the key results of the
Keldysh theory in the appropriate limiting regimes.
