In this project funded by the Chemical Structure, Dynamics and Mechanisms Program of the Chemistry Division, Professor Suits of Wayne State University will conduct investigations of the dissociation of polyatomic molecules using laser techniques that are sensitive to the electronic and spin angular momentum of the products. The findings can then be used to unravel the detailed dynamics of these dissociation events. A key aspect of the proposed work is development of an approach for efficient detection of electron spin polarization in hydrogen atom photo-fragments, a principal product of the dissociation of a great many polyatomic molecules. A second aspect of the proposed studies is a strategy to control the novel "roaming radical" reaction mechanism identified in formaldehyde and acetaldehyde decomposition.
Broader impacts of the proposed work include development of a new high-resolution image acquisition system that will be freely distributed to other groups. In addition, recruitment and training of a diverse group of undergraduate students, graduate students and post-docs in methods in ion imaging, laser spectroscopy and data analysis is a core aspect of the group activity.
An important question in the decomposition of photoexcited molecules is the sequence of electronic states that are encountered in the course of dissociation, and the ways in which they influence the final outcome. This question has direct bearing in many diverse areas from solar energy capture to vision to atmospheric chemistry: almost anywhere that molecules interact with light.
Our main achievement during the previous grant period was the development of a new method to probe spin-polarized hydrogen atoms produced in photodissociation, and to record its velocity distribution with high resolution and high sensitivity. Hydrogen atoms are produced in the dissociation of countless molecules, and in the past one would study these events by detecting the number hydrogen atoms going in a particular direction with a particular speed. With our new method, at the same time we can detect the magnitude and direction of the electron spin in these hydrogen atoms and its angular and speed distribution. Each electron is like a small magnet, and with this method we can detect this for each scattered atom. This essentially doubles the amount of information we obtain in each such photodissociation process. The importance of this is that it can reveal the entire history of the dissociation event, revealing a great deal more about what happened just as the parent molecule began to fall apart. These are the crucial moments when the chemical fate is established. In addition, it can reveal "matter-wave interference", a purely quantum phenomenon that allows these atoms to follow two pathways at once. The spin polarization we detect reveals the interference between the H atom waves propagating along these two paths as highlighted in the accompanying image. The second major direction was theoretical in nature, and involved trying to understand what happens to a highly excited molecule as it responds to an energetic impulse. The system we chose was ethylene cation, C2H4+, which is the simplest organic system that has a π-type molecular orbital. This then is a prototype for countless organic systems. This computer-based "experiment" allowed us to examine what happens on the ultrafast timescale following an initial photoexcitation. We could see the internal motions in the molecule directly and how the complex motion of the atoms interacted with the electron motions as the system began to relax. These studies will help shed light on new ultrafast experiments that are now becoming available owing to rapid advances in laser technology. In addition to these direct scientific pursuits, there are numerous ways in which our research efforts exert broader scientific, intellectual, social and cultural impacts beyond the immediate domain of the research field. Foremost among these is in the research and training component in a diverse international community. Our current group includes a broad array of faiths and cultures from all reaches of the earth. Countries currently represented range from South Korea to Ghana, Latvia to Michigan, France to Sri Lanka, India to England. Students and post-docs learn demanding physics and chemistry material at the forefront of the field, technical skills such as laser and molecular beam and optical techniques, design and drafting for construction of sophisticated new instruments, and software development for data acquisition and analysis. And they learn this from each other, and in doing so they come to appreciate the value and rewards of this diversity in the research effort. Opportunities for women in science are also strongly promoted in the group with a typical composition of 25-40% female. The bulk of the spin-polarized hydrogen experiment, in fact, was the full responsibility of a female graduate student beginning in her second year. She has led the effort from initial apparatus construction to debugging the subtle issues in Rydberg spectroscopy through now obtaining robust and routine spin polarization spectra, analyzing data and writing papers. This is training that builds expertise, experimental acumen, and self-confidence. Along with our ongoing research efforts and the broader impacts that ensue by virtue of sophisticated training and experience in a diverse community, we continue to develop new tools for ion image acquisition and slice imaging techniques and share these freely with the broader community.
