Numerically determined ionization rates for the field ionization of atomic hydrogen in strong and short laser pulses are presented. The laser pulse intensity reaches the so-called “barrier-suppression ionization” regime where field ionization occurs within a few half laser cycles. Comparison of our numerical results with analytical theories frequently used shows poor agreement. An empirical formula for the “barrier-suppression ionization” rate is presented. This rate reproduces very well the course of the numerically determined ground-state populations for laser pulses with different length, shape, amplitude, and frequency.

• Received 24 February 1998

DOI:https://doi.org/10.1103/PhysRevA.59.569