Search for: All records

The formation of two-electron chemical bonds requires the alignment of spins. Hence, it is well established for gas-phase reactions that changing a molecule’s electronic spin state can dramatically alter its reactivity. For reactions occurring at surfaces, which are of great interest during, among other processes, heterogeneous catalysis, there is an absence of definitive state-to-state experiments capable of observing spin conservation and therefore the role of electronic spin in surface chemistry remains controversial. Here we use an incoming/outgoing correlation ion imaging technique to perform scattering experiments for O(³P) and O(¹D) atoms colliding with a graphite surface, in which the initial spin-state distribution is controlled and the final spin states determined. We demonstrate that O(¹D) is more reactive with graphite than O(³P). We also identify electronically nonadiabatic pathways whereby incident O(¹D) is quenched to O(³P), which departs from the surface. With the help of molecular dynamics simulations carried out on high-dimensional machine-learning-assisted first-principles potential energy surfaces, we obtain a mechanistic understanding for this system: spin-forbidden transitions do occur, but with low probabilities.

 

Surface reaction rate constants were measured accurately so that a meaningful comparison with theory can now be made. 

Despite the critical role of teachers in the educational process, few advanced learning technologies have been developed to support teacher-instruction or professional development. This lack of support is particularly acute for middle school math teachers, where only 37% felt well prepared to scaffold instruction to address the needs of diverse students in a national sample. To address this gap, the Advancing Middle School Teachers’ Understanding of Proportional Reasoning project is researching techniques to apply pedagogical virtual agents and dialog-based tutoring to enhance teachers' content knowledge and pedagogical content knowledge. This paper describes the design of a conversational, agent-based intelligent tutoring system to support teachers' professional development. Pedagogical strategies are presented that leverage a virtual human facilitator to tutor pedagogical content knowledge (how to teach proportions to students), as opposed to content knowledge (understanding proportions). The roles for different virtual facilitator capabilities are presented, including embedding actions into virtual agent dialog, open-response versus choice-based tutoring, ungraded pop-up sub-activities (e.g. whiteboard, calculator, note-taking). Usability feedback for a small cohort of instructors pursuing graduate studies was collected. In this feedback, teachers rated the system ease of use and perceived usefulness moderately well, but also reported confusion about what to expect from the system in terms of flow between lessons and support by the facilitator. 

Adsorption involves molecules colliding at the surface of a solid and losing their incidence energy by traversing a dynamical pathway to equilibrium. The interactions responsible for energy loss generally include both chemical bond formation (chemisorption) and nonbonding interactions (physisorption). In this work, we present experiments that revealed a quantitative energy landscape and the microscopic pathways underlying a molecule’s equilibration with a surface in a prototypical system: CO adsorption on Au(111). Although the minimum energy state was physisorbed, initial capture of the gas-phase molecule, dosed with an energetic molecular beam, was into a metastable chemisorption state. Subsequent thermal decay of the chemisorbed state led molecules to the physisorption minimum. We found, through detailed balance, that thermal adsorption into both binding states was important at all temperatures.