A variety of chemical processes have been controlled in recent years using closed loop schemes involving shaped laser pulses. For all but the most basic systems, however, it has proved challenging to deconstruct the optimal laser pulse to recover information about the molecular dynamics involved in the laser-molecule interaction. On the other hand, momentum imaging techniques can produce rich, correlated, multi-dimensional data that has led to significant advances in our understanding of many photon- and ion-molecule interactions. The program of research described in this proposal would use momentum imaging to probe the molecular fragmentation dynamics arising from optimized laser pulses. By combining numerical analysis capable of reducing the optimal pulse shape to its essential features with a posteriori reconstruction of the dynamics afforded by momentum imaging, we hope to deduce the control mechanism(s). Specifically, the PI proposes to identify interesting "optimal" pulses for selective molecular fragmentation using closed-loop control and then use momentum imaging to investigate the control mechanisms produced by these pulses. Information gleaned from the imaging measurements will also be used to explore the interesting topology of the multidimensional surface that defines the optimal control parameters.
Traditional chemical synthesis is like cooking; a chef or chemist can change ingredients, the amount and timing of how those ingredients are mixed, apply or remove heat, and perhaps even change the pressure or volume available for the process. A broad goal of the field of coherent control is to learn how to manipulate the electrons of individual atoms and molecules, so that one day the synthetic chemist may have a tool that a chef does not. There are other potential uses of this basic research into atomic and molecular dynamics, including trace detection of harmful substances and quantum computing. Ultrafast lasers have durations that are short compared to many molecular motions and can be as strong as the forces that bind electrons to nuclei. Modern laser pulse shaping devices can produce complicated pulses that can influence molecules in complex ways. When seeking to manipulate a molecule in a specific way, however, these pulse shapers can produce far too many potential pulse shapes to try one after the other. This problem was solved (following a suggestion by Princeton professor Herschel Rabitz and co-workers) by using a closed-loop approach, in which a pulse is tested and feedback from these tests are used to develop new trial pulses, and so forth until an optimal pulse shape is found for the particular problem. This scheme is illustrated in the first figure. In the last decade many researchers have used this approach to find shaped ultrafast laser pulses that control atoms and molecules in a number of ways. A common problem is that the resulting pulse is so complex that it is often unclear how it manages to control the atomic or molecular process. The intellectual merit the work associated with this award centers on the role of the feedback signal in this search process. For simple molecular systems, we applied a number of detection techniques that allow different kinds of feedback signals to be used in the search. The search for an optimal laser pulse is often likened to finding a path up a mountain to a peak. This analogy can frame our work as well: If the feedback signal is not defined well enough, how do you know if you are ascending a mountain that has only a single route to the top rather than a plateau with numerous paths that all reach the same elevation? In one case, we found that more specific feedback produced numerically simpler laser pulse shapes. In another experiment, we used a detailed feedback condition to greatly excite molecules but not break them apart. These basic experiments help us understand the processes that the laser pulse shapes use to drive the control of the molecular system. In other words, we gain some insight into why a particular path is the best route to the top. By examining enough of these paths, one day we may be able to determine some common characteristics of the best routes. This can help us determine a generic plan for scaling an arbitrary mountain (or control a molecule in some new way) in the future. The most significant broader impact of this award is the undergraduate research experience it provides to regional students from South Dakota and neighboring states. Ten Augustana College students, including those in the second image, participated in the work funded by this award. Many have gone on to graduate school in science-related disciplines at institutions including Cornell, Kansas State University, and the Universities of Colorado and South Dakota, and the rest plan to follow these students following graduation. These students will become the highly skilled workers needed for tomorrow’s labor force.
