Suppression of photoionization by a static field.

The dc field Stark effect is studied theoretically for atoms in high intensity laser fields. We prove that the first-order perturbation corrections for the energy and photoionization rate vanish when the dc field strength serves as a perturbational strength parameter. Our calculations show that by applying a dc field in the same direction as the polarization direction of the ac field, the photoinduced ionization rate is almost entirely suppressed. This suppression is attributed to changes in the phase shift of the continuum atomic wave functions which can be controlled by the dc field.