Precision Tests of Laser-Tunneling Ionization Models

Abstract

The energy and number distributions of photoelectrons observed outside the ionizing field can give insight into the ionization process in a strong laser field. The physics of this process can be considered to occur in a two-step process known as the “simpleman’s model.”¹ First, the electron escapes the atom by tunneling, and second, it interacts with the external field. If the time it takes the electron to tunnel out of the atom is much shorter than the laser period, then ionization occurs in the adiabatic limit (the ionization potential Ip is greater than the photon energy ω), and static field calculations of the ionization rate are valid.¹ In the experiments described below, the Keldysh adiabaticity parameter γ = (I _p /2U _p )^1/2 ≪ 1,² where Ip is the ionization potential of the atom and Up = F ²/2ω² is the ponderomotive energy, indicating that ionization occurs in the tunneling regime. (Here the electric field F is proportional to the square root of the laser intensity.) Once liberated, the electron will gain a drift momentum which is determined by the phase of the laser field at the time of ionization. If the polarization is linear, then the drift momentum will be zero unless the external electric field is not at its peak value. The drift momentum will be nonzero and directed perpendicular to the polarization vector if the laser is elliptically polarized. If the laser is circularly polarized, the drift due to initial conditions will give the electron a directed kinetic energy of about 2Up when ponderomotive effects are included. We have used a 2-ps, 1-μm laser with intensities up to 1 × 10¹⁸ W/cm² and two electron spectrometers to test the static tunneling models.