The ionization of Rydberg states by subpicosecond, 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 respect 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. PACS number(s): 32.80.RmRecently developed sources of high-power, picosecond electromagnetic field pulses [1] have been used to study Rydberg atoms [2]. These pulses are nearly unipolar (i.e., "half" of an optical cycle), with peak powers up to 1 MW and coherent bandwidths of several THz, centered at 0.5 THz

Rydberg atoms are ionized by nearly unipolar, subpicosecond electromagnetic pulses. Deviations from a perfectly unidirectional pulse are found to alter substantially the ionization probability as a func tion of peak field. Quantitative agreement between classical theory and experiment is achieved if the pulse imperfections are significantly attenuated.In recent experiments [1,2], Rydberg atoms have been exposed to ultrashort, electric-field pulses. The electric field in these pulses is nearly unipolar, and has a temporal shape which resembles one-half of a cycle of THz-band radiation [3]. In the experiments [1,2], the 0.5-psec dura tion of these "half-cycle pulses" (HCPs), 1"HCP' is shorter than or comparable to the classical Kepler period of the Rydberg states, 1"K =21Tn 3. The dynamics of the ioniza tion process in these experiments is distinctly different from that in either a long field pulse ('Tpulse> 'TK) [4] or a short laser pulse (1"laser < 1"K ) [5].In a long electric-field pulse, essentially no energy is transferred to the Rydberg electron. Instead, ionization occurs due to the modification of the electronic binding potential [4]. In a HCP, the electron is unable to respond to the rapid changes in the binding potential, and the electron must gain energy from the field in order to es cape the Coulomb attraction of the nucleus. Classically speaking, the electron receives an energy "kick" or im pulse from the rapidly changing field [1],-00where F(t) is the HCP field and v(t) is the velocity of the electron. Therefore the ionization probability is strongly dependent on the velocity or momentum distribution of the initial-state wave function as well as the temporal shape of the electric-field pulse. The time dependence of the electric field is extremely important in the ionization of Rydberg atoms where 1"pulse «1"K· In a Rydberg atom, the electron probability distribution is peaked far from the ion core, and the prob ability for finding the electron near the nucleus during the pulse is 1"pulse l 'TK. Therefore, in order to have an ion ization probability greater than 1"pulse l 1"K' energy transfer between the electron and the time-dependent field must occur far from the ion core where the electron is essen tially free. Equation (1) clearly shows that a free electron can gain energy from a unipolar field pulse. However, this electron cannot gain energy from a pulse whose time-integrated electric field is zero. Indeed, Rydberg atoms may be ionized with 100% efficiency by a HCP, but not by a short laser pulse [1,5].The results of the ionization experiments [1] are in qualitative agreement with classical simulations [1,6].The agreement is quantitative if the experimental field values are rescaled by a multiplicative factor of 2.5. A more recent field calibration suggests that the field discrepancy is actually only a factor of 1.6. A description of the differences between the two calibration techniques is given later in the paper. A possible source for the per sistent disagreement between experiment and theory...