Precise measurements of electron initial conditions for tunneling ionization in an intense, elliptically polarized laser field

摘要

The energy and angular distributions of electrons liberated during the ionization of neon by a high-intensity, elliptically polarized laser field have been measured. These distributions, measured outside the ionizing laser field, give insight into the ionization process occurring inside the laser focus. The purpose of the experiments presented in this dissertation is to test the validity of a physical model based on tunneling ionization of atoms by an intense laser field.;The relevant physics can be described by a two-step semiclassical model in which, first, the electron escapes the atom by tunneling, and, second, interacts with the external field classically. An electron liberated in a laser field gains a directed component of momentum, or a drift, which is determined by the phase of the laser field at the time of ionization. Depending on the ellipticity of the laser's polarization, the distribution of drift momenta may be directed along the polarization direction, as is the case with linear polarization, perpendicular to the major axis of polarization (elliptical polarization), or equal in all directions in the plane perpendicular to the laser propagation direction (circular polarization). If the laser pulse is longer than the electron's interaction time with the field, the average oscillation energy of the electron in the field is also converted to directed kinetic energy. The drift momentum distribution determines the shape of the angular distribution of electrons outside the laser focus.;A 1-mum, 1-J, 2-ps laser, with a peak intensity of 1018 W/cm2, has been used to ionize neon to high charge states. The energy and angular distributions of electrons ejected from the focus were measured with a magnetic spectrometer as a function of laser ellipticity, and electrons with energies up to 85 keV and angular distribution asymmetries as large as 8 : 1 were observed. The experimental observations show that tunneling ionization is occurring and are in excellent agreement with predictions of the Ammosov-Delone-Krainov (ADK) tunneling model. In addition, the measured distributions agree with the semiclassical model's assumption that the electrons are liberated into the field with zero initial kinetic energy.