Effects of imperfect unipolarity on the ionization of Rydberg atoms by subpicosecond half-cycle pulses.

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],