Shaped ultrafast laser pulses can powerfully influence molecular dynamics, thus allowing access to outcomes not typically available by other means. The complexities of the field-molecule interaction generally make an a priori determination of the required field characteristics impossible, and so adaptive feedback algorithms are often employed to identify the optimal pulse. This technique efficiently selects pulses that enhance the desired pathway. Exactly how this is accomplished, however, is often obscure. This research examines ways to extract mechanistic information from closed-loop control by approaching this problem from the perspective of the feedback signal. By incorporating images into the feedback loop or using vibrational-state specific feedback we can query the search algorithm in very specific ways. Correlation of small changes in the feedback target with changes in the optimal pulse traits can lead to mechanistic insight. Furthermore, the mechanisms underlying the control can be subsequently probed with the power of velocity map imaging (VMI) or cold-target recoil-ion momentum spectroscopy (COLTRIMS). Experiments will be conducted at Kansas State University with much of the pre- and post-experiment work carried out at Augustana College. The primary aim of these experiments is to uncover the fundamentals of the molecular dynamics in these interactions. To this end, we will construct, test, and incorporate into the feedback loop a Doppler-free kinetic energy release spectrometer that is capable of resolving specific vibrational states of CO2+ through its dissociation into C+ + O+. Using this high resolution feedback, we will seek to manipulate the vibrational population and thereby gain a window into the dynamics leading to population of the transient CO2+. Using VMI as feedback allows simultaneous access to angular and kinetic energy release (KER) information for a given ion species. Shaped pulses will be used to control the isomerization of the acetylene di-cation into CH2 + + C+ and the ethylene cation into CH3 + + CH. Both of these processes can be probed with VMI and/or COLTRIMS. Understanding how isomerization is controlled in these benchmark hydrocarbons can provide a foundation for improved control in larger molecules. A concurrent secondary direction will be the implementation of new feedback techniques for adaptive control, such as the rapid inversion of VMI spectra to obtain unambiguous KER data.
Broader Impacts: This work integrates undergraduates at all levels, from experiment design to manuscript preparation. Experience has shown that this activity both encourages students to continue their science training and provides a strong background for graduate work. The group has a good record of promoting the participation of women in physics. Scientifically, improved understanding of the molecular dynamics involved in closed-loop coherent control will benefit several applications, including detection of trace amounts of materials with undesirable environmental or national security traits and the use of shaped pulses for the creation of molecular qubits for quantum computing. Improved understanding of how isomerization dynamics can be manipulated could further enable the development of molecular switches and laser-controlled chemical synthesis. Dissemination of results will occur through peer-reviewed publications, conference presentations, and seminars and symposiums that often feature students.
Three-dimensional view helps laser in building new molecules In many ways, traditional chemical synthesis is similar to cooking. To alter the final product, you can change the ingredients or their ratio, change the method of mixing ingredients, or change the temperature or pressure of the environment of the ingredients. Like an accomplished chef, chemists have become very skilled at the manipulation of these parameters to produce many of the products that make our lives better. But there are some chemical reactions that resist these methods, and, as a result, researchers are continually looking for new techniques to apply. In particular, laser-based chemistry has been a goal for scientists since the invention of the laser in the 1960s. Applying a laser pulse of the correct color and duration to a molecule could, in principle, inject just the right amount of energy to modify a specific chemical bond and change the molecule into a more desirable configuration. In this sense, the laser can be thought of as a new type of reagent that drives a chemical reaction. In practice, even a single molecule is a complicated system and finding the correct laser pulse characteristics to influence molecules is difficult. In addition, sophisticated laser pulse shaping devices can produce a nearly infinite number of pulse shapes, making a systematic search for the correct laser-molecule solution daunting. A proven method for approaching this problem is to use experimental feedback to guide an adaptive search of the possible laser pulses. As in 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. In the work supported by this award, researchers from Augustana College (SD) along with collaborators from Kansas State University (KSU) in the United States and from the Max Planck Institute for Quantum Optics (MPQ) and the Ludwig Maximilian University (LMU) in Munich, Germany, have reported an improved feedback technique. By imaging the dissociating molecule in three dimensions, a laser pulse can be optimized to drive the molecule to a very specific final state. This image-based technique can complement feedback methods that depend on optical spectroscopy. Furthermore, the researchers were able to use the dissociation images to guide theoretical work that revealed how the laser pulse was able to control the molecule, in this case driving acetylene ions from the normal HCCH configuration to the unusual HHCC configuration. Building on the initial work done at MPQ, Augustana College students Chris Rallis (’11), Bethany Jochim (’11) and Phillip Andrews developed a method for converting the image into feedback quickly enough to be useful in the experiment. They then developed a system of computer control linking the entire experiment as well as refining image-analysis techniques to evaluate the experimental data. Once this was accomplished, the Augustana group traveled to the J.R. Macdonald Laboratory (JRML) – a state-of-the-art ultrafast laser facility located at KSU, to conduct the experiment as part of the long-standing Augustana-KSU collaboration. "The experiment shows that improved feedback, provided by multi-dimensional imaging, enhances both our abilities to control chemical reactions and the physical insight that can be gained", said Matthias Kling, research group leader at MPQ and assistant professor at KSU at the time the studies were conducted. "The new methodology provides new possibilities for the control of more complex systems including larger molecules, clusters, and nanoparticles. Multi-dimensional data provide stricter limitations for theoretical modeling and will help to improve our models", explains Regina de Vivie-Riedle, professor at LMU and leader of the group that performed the theory. The Augustana-KSU collaboration has been very successful in giving undergraduates from Augustana an opportunity to work in a world-class lab like JRML. Several of the Augustana undergraduates participating in the research have gone on to be excellent students in the K-State graduate program, including Fulbright Scholar Nora (Johnson) Kling, graduating with a Ph.D. in December, and current Ph.D. student Bethany Jochim, recipient of a prestigious Department of Energy fellowship. Both graduate students are in the group of Itzik Ben-Itzhak, JRML director, who says: "This project serves as a model for research university?liberal arts college collaboration. Both institutions benefit greatly from the partnership." The results should point the way toward new possibilities for control of more complex systems including larger molecules, clusters, and nanoparticles. The next group of Augustana undergraduate students is already working on future experiments.
