This project focuses on understanding theoretically the interaction of intense coherent radiation with atoms and molecules and on investigating the use of ultrafast (attosecond scale) laser or electron pulses to pump or probe electronic processes in atoms and molecules. Such processes are difficult to treat theoretically because electronic interactions with both intense laser fields and with atomic and molecular potentials must in general be described non-perturbatively; also, interactions with short laser or electron pulses must be described time-dependently. In support of these investigations we have developed a number of new theoretical approaches. Investigations include using attosecond scale electron pulses as both temporal and spatial probes of electronic motion in both atoms and molecules; investigating the origin of laser-intensity-induced enhancements of ionized electron yields for electron energies corresponding to the so-called "ATI plateau"; developing novel closed form analytic formulas for high order harmonic generation amplitudes and rates for negative ions, extending those results to neutral atoms, and applying those results to seeking ways to enhance high order harmonic yields; and investigating two-electron effects in high order harmonic generation by short-pulse lasers.
Graduate students and postdoctoral researchers involved with this project are given a broad education in theoretical atomic physics, first hand experience in all aspects of scientific communication, and in teaching undergraduates at a large AAU Land Grant university. Project results are not only published in leading physics journals and presented at national and international meetings, but are also periodically distilled and integrated with related work by others in review articles written by the PI and collaborators. All graduate students and postdoctoral researchers involved with this project in the past have been sought after by a variety of other employers, including technology companies, medical researchers, and other leading AMO theory groups. The basic research supported by this project contributes broadly to our understanding of means to control matter on an atomic scale. Our work on increasing the intensities of high order harmonics may one day lead to sources of coherent x-rays, thus providing a new means for visualizing living biological structures as well as nanoscale materials structures. Our investigations involving attosecond pulses of electrons may lead to ways of resolving electron motion both temporally and spatially. A workshop on attosecond science at the Kavli Institute for Theoretical Physics co-organized by the P.I. in 2006 has helped to enhance the infrastructure for research in this emerging new discipline. The P.I.'s co-editing of a special focus issue of New Journal of Physics on attosecond science in 2008 is evidence of his continuing service to the research community.
Award ID #0901673; A. F. Starace, P.I. The work of my group and our collaborators on this project includes a significant number of predictions for intense laser interactions with atomic systems using fully quantum, nonperturbative formulations, benchmark calculations for possible applications of attosecond electron pulses to image electronic motion in atoms and molecules, and an initial analysis of static field ionization of a model molecular system that we regard as providing the groundwork for investigations of strong field processes involving molecules. A primary goal of this project is to understand strong field atomic and molecular interactions as well as ultrafast (attosecond) processes at a fundamental quantum level. Our approach is to select an analytically solvable model problem, then to solve it completely and understand the implications of the solution, and finally to apply the knowledge gained for the model system to real atoms and molecules and the experiments involving them. Since much of our work is analytical, we are able to scan the experimental parameter space (i.e., intensity, frequency, polarization, target system, etc.) to discern general features of the physics of laser-matter interactions in a way that completely numerical approaches have difficulty doing, owing to demands on computational resources. The model system that we employ to investigate strong field interactions with atoms is denoted the time-dependent effective range (TDER) model. It describes an electron that is weakly bound in an arbitrary short-range potential and interacting with an intense laser field. The theory combines effective range theory (commonly used in nuclear physics and in cold atom physics) with Floquet theory (for the interaction of the electron with a monochromatic laser field) to describe exactly the non-perturbative interactions of the electron with both the atomic potential and the laser field. In the current project period, we published a complete description of the TDER model and used it to derive analytically factorized formulas for high-order harmonic (HHG) yields and above-threshold ionization (ATI) or detachment (ATD) rates. These factorized formulas explain completely and analytically the detailed high energy structures of HHG and ATI/ATD plateaus. Moreover, knowing analytically the meaning of the factors for our model system permits immediate generalization to real atomic systems. We thus predicted that the well-known giant dipole resonance in the high energy photoionization spectrum of Xe should dominate the structure of the HHG spectrum of Xe, which prediction has now been confirmed experimentally. Similarly, we were able to explain the physical reason for experimental observations of strong enhancements of particular high-order harmonics in plasmas of transition metal ions as originating from a potential barrier effect known from photoionization of transition metals. We have also generalized our theoretical approach to treat short laser pulses (instead of only monochromatic fields) and applied this generalization to obtain analytic factorized formulas for both HHG and ATI that incorporate the parameters of the short laser pulse. In addition, we have carried out benchmark calculations showing that ultrafast electron diffraction using attosecond electron pulses can "image" electron motion in atoms and molecules. These latter results have been highlighted in Physical Review Focus: http://focus.aps.org/story/v26/st25 We have also developed a simple theoretical model for a diatomic molecule that we expect will provide us with the analytical insight necessary to describe strong field processes involving real diatomic molecules. The current project has had a number of broader impacts. First, the P.I. has strived to give the students and postdoctoral researchers involved with this project as broad an education as possible and to ensure that they have the requisite communication skills to succeed. Second, all project results are published in leading scientific journals. Nearly all are also presented at one or more national or international scientific conferences. Third, the P.I. has played an active role in furthering the development of attosecond physics. Together with A.D. Bandrauk (U. Sherbrooke), and several other colleagues, he proposed, organized, and chaired a 3-week international workshop on Attosecond Science – Exploring and Controlling Matter on Its Natural Time Scale at the Kavli Institute for Theoretical Physics (KITP) in Beijing, China (9-27 May 2011). Also, the P.I. is currently a member of the American Physical Society (APS)’s Publications Oversight Committee (2010-2013), which provides recommendations and advice to APS officers concerning "operating philosophy" and "general editorial policy" of the APS’s scientific journals, which are the world’s leading physics journals. Fourth, among benefits to society, all of the graduate students and postdoctoral research associates involved with this and with previous NSF-supported projects have been sought after by other employers. Also, the basic research supported by this project contributes broadly to our understanding of means to control matter on an atomic scale, which may lead in the future to important practical applications.
