RUI: Image-Based Strong-Field Adaptive Control of Molecular Dynamics

Augustana University Association, Sioux Falls SD

Investigators

Abstract

Non-technical description: Traditional chemical synthesis methods resemble cooking in the sense that the ingredients, mixing methods, temperature, and pressure all may be varied to produce a desired result. Despite control over all of these macroscopic variables there are some chemical processes that remain elusive. Since the invention of the laser in the 1960s, the field of coherent control has sought to use the laser to manipulate chemical dynamics by applying energy of suitable color and duration directly to individual molecules. In this sense, the laser can be thought of as a new type of reagent that drives a chemical reaction. While easily stated, this task has proved challenging. Molecules are complicated and dynamic, making it difficult to determine the correct laser characteristics to drive a particular process. A proven method for approaching this problem is to use experimental feedback to guide an adaptive search of the possible laser pulses. In a physics version of natural selection, laser pulses that provide a better outcome are given an increased chance to survive and have their characteristics contribute to the tailored pulse that ultimately produces the desired outcome. Such a method, however, is only as good as the feedback that drives it. The goal of these studies is to develop enhanced image-based feedback techniques that enable this adaptive approach to coherent control of chemical dynamics. Technical description: The PI has recently developed the ability to use three-dimensional momentum imaging of the laser-molecule reaction products to define the control objective described in the non-technical description above. Using three-dimensional imaging to target a specific final state has led to better understanding of the mechanisms that undergird the laser-based control, especially in situations in which obtaining precise optical spectroscopic feedback is impractical. Increased mechanistic understanding can subsequently lead to better search parameterization, enhancing the adaptive control process. Current efforts are focused on applying image-based adaptive control to study and influence photoisomerization processes in small molecules such as ethylene. By studying photoionization, it is possible to probe how electronic excitation is rapidly converted to nuclear motion, an essential step in many ultrafast chemical processes. Ethylene is particularly interesting as a benchmark molecule for examining the role of conical intersections in these electronic to nuclear energy conversions. These studies will be advanced by using two-pulse experiments, which allow the separation of the ionization step from the subsequent evolution of the molecular ion, which the group hopes to control. Strong-field tunneling ionization of polyatomic molecules often involves multiple molecular orbitals, and studies of these relationships help link the image-based feedback to more specific target states, again with the objective of refining adaptive control methods. A goal of this project will be extending this work to explore the control of laser driven electron rescattering via feedback derived from angle resolved photoelectron distributions. Electron rescattering is an essential part of many ultrasfast laser-based processes, such as high-harmonic generation and the production of attosecond pulses, and therefore control of this sort has a number of potential applications. Finally, as an undergraduate institution, this project helps identify and develop talented students by immersing them in forefront research.