The ionization of Rydberg states by sub-picosecond, unipolar electromagnetic field pulses shows a strong orientation dependence. A small static electric field permits us to study Rydberg Stark states with oriented permanent electric dipole moments. The probability for ionizing a Stark state depends on its energy and the orientation of its electric dipole moment with repsect to the direction of the pulsed electric field. This orientation dependence is most prominent for pulse durations comparable to the classical orbit period.
Classical simulations reproduce many of the observed features.