Theory of Strong-Field Atomic Ionization for Elliptical or Circular Polarization.

摘要

Interaction between intense laser fields (with intensities on the order of 1 PW/cm2) and gas-phase atoms or molecules has led to many new physical phenomena, such as multiphoton ionization, above-threshold ionization, nonsequential double ionization, high harmonic generation, attosecond pulse generation, coherent X-ray generation, etc. These phenomena have been of interest from the perspectives of both fundamental physics and potential applications.;The first step of all these strong-field phenomena is atomic ionization. Therefore understanding atomic ionization is the basis of understanding all strong-field phenomena. Extensive work has been done on atomic ionization, but mainly with linearly polarized laser fields. Little attention has been paid to elliptical or circular polarization, for two reasons. First, many of the above-mentioned strong-field processes can only be generated or maximized with linear polarization. Second, elliptical or circular polarization is very difficult to deal with, especially theoretically.;In this dissertation, we will be focusing on atomic ionization with elliptical or circular polarization. The first question: why would one bother to study elliptical or circular polarization? The answer is that elliptical or circular polarization is able to reveal important ionization information that is not accessible with linear polarization. For example, at what time during a pulse was an electron emitted? Surprisingly, this obvious question cannot be answered with linear polarization, but it can be answered with elliptical or circular polarization.;Atomic ionization with elliptical or circular polarization will be studied systematically, mainly using a classical ensemble method. Our study covers a range of ionization channels, including single ionization, double ionization, and triple ionization. We will explain how to decode new ionization information from experimentally measurable ion momentum distributions. We will also resolve a couple of interesting puzzles with elliptical or circular polarization. For example, is recollision, which has been accepted as playing a central role in initiating many strong-field phenomena with linear polarization, possible under elliptical or circular polarization? For another example, can the two electrons emitted sequentially in a double ionization event be viewed as independent?