Dynamics of Molecules in Extreme Rotational States Made with an Optical Centrifuge

University Of Maryland, College Park, College Park MD

Investigators

Abstract

Most molecules are stable at room temperature, but become more reactive at higher temperatures. Increasing the temperature makes molecules fly through space faster (in other words their "translational kinetic energy" increases), and this in turn can lead to greater chemical reactivity because the collisions between molecules occur with greater energy. As temperature increases, molecules also vibrate more (their bonds stretch and bend with more energy); higher vibrational energies also can increase chemical reactivity. Many studies in the past have asked how translational and vibrational energies affect chemical reactions. However, there is another kind of molecular motion that is not so well-studied: molecules also rotate, and if they do so at very high rates, their bonds can stretch or even break. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Amy S. Mullin of the University of Maryland uses sophisticated laser techniques to prepare and study molecules with large amounts of rotational energy. Professor Mullin and her students create molecules in high-energy rotational states using what they call an optical centrifuge. This technique uses intense pulses of light to trap molecules and cause them to spin with very high energy. This approach enables them to study how fast molecular spinning affects how closely atoms are held together in a chemical bond and how energy is shared by collisions with other molecules. The research has broader impacts of potential societal benefits from an increased understanding of molecules in high-energy environments such as found in nature (i.e., in astrophysics and planetary atmospheres) and in high temperature industrial and geological applications. The project is also training students in the design, construction and use of advanced experimental instruments and computer-based modeling. In addition, the research promotes dialogue with the general public through interactive live-streaming videos. This project is investigating the behavior of CO2, N2O, CO and H2CO molecules in extreme rotational states (e.g., J > 200). The optical centrifuge functions by spatially and temporally combining two oppositely chirped pulses of light, each with opposite circular polarization, from a Ti:sapphire laser system with two stage amplification. The optical centrifuge spins the molecules unidirectionally into high rotational states with oriented angular momenta. The creation and collision dynamics of the centrifuged molecules are measured using time-resolved IR polarization spectroscopy. Doppler line profiles for individual rotational states give information about the translational energy distribution and spatial orientation of the centrifuged molecules as they undergo collisions. Relative capture and acceleration efficiencies for different species in the optical centrifuge identify the molecular properties of efficient centrifugation. These spectroscopic studies of high-energy states are testing whether low-energy molecular models are valid for high rotational energies. The experimental observations are interpreted using to current models for structure and collision dynamics of low energy molecules to identify the bonding and collision forces of rotationally excited molecules. Research results may impact knowledge about molecular behavior in environments that have non-local thermodynamic equilibrium such as in astrophysics, planetary and stellar atmospheres, plasmas, combustion, and high temperature industrial and geological applications. The research is training research students in sophisticated laser-based research at the forefront of controlling and interrogating molecular motion. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.