Coherent Spectroscopy and Coherent Control of Molecules and Materials

Massachusetts Institute Of Technology, Cambridge MA

Investigators

Abstract

In this project, funded by the Chemical Structure Dynamics and Mechanism-A (CSDM-A) program of the Chemistry Division, Professor Keith Nelson of the Massachusetts Institute of Technology is developing novel techniques that use short pulses of light in the terahertz (THz) frequency range of the spectrum. The terahertz range is just above the lower-frequency microwave (or GHz) range that is used in common devices like cellphones. Magnetic resonance measurements (similar to those in magnetic resonance imaging, or MRI) can reveal key information about molecular structure and bonding, but they have hardly ever been made at THz frequencies because of technological challenges with the sources of THz light. Professor Nelson overcomes these challenges using short THz pulses generated in a simple, tabletop-sized, experimental setup to make THz magnetic resonance measurements in about one minute. Measurements of molecular and biomolecular complexes with iron or similar elements are being conducted to understand molecular structures and how they change during chemical processes. The project has broader impacts in making facile and inexpensive THz measurement available to scientific communities in chemistry, biology, and materials science with applications spanning energy harvesting to chemical catalysis to biomedical measurement. In addition to providing training in cutting-edge spectroscopy techniques for graduate students and post-doctoral scientists, the Nelson research group also runs a unique outreach program (the Lambda Project) in which high school students come to MIT for hands-on experiments to learn about modern optics and modern materials. The project builds on recent demonstrations of THz electron paramagnetic resonance (EPR) in both the linear regime in which free-induction decays (FIDs) are measured and in the nonlinear regime in which 2-dimensional (2D) electron paramagnetic resonance (EPR) is carried out. THz EPR spectra of high-spin transition metal complexes with zero-field splittings that arise from spin-orbit coupling are measured. The splittings are highly sensitive to the ligands and their configuration around the central transition metal ion. The EPR measurement can be preceded by an optical ("pump") pulse that initiates photochemical changes, with associated changes in the spin states monitored on picosecond time scales. 2D THz EPR measurements, demonstrated on the collective spin states of magnetic materials, are being extended to multinuclear organometallic complexes in order to measure spin-spin interactions between the transition metal ions, revealing detailed information about the ground and excited-state electronic structures of the complexes. In addition to these measurements in which THz magnetic fields coherently drive electron spins, the combined THz electric and magnetic field components are being used to drive both electric and magnetic dipole moments of chiral molecules in experiments that seek new methods for enantiometric separation. Further impacts in chemistry and biology may be realized if improved enantiomeric separation can be achieved.