Ultrafast Spectroscopy With Frequency Combs The Reber lab uses fiber-laser frequency combs to do ultrafast chemistry. Ultrafast spectroscopy is used for real-time tracking of dynamics in quantum mechanical systems on the femtosecond (10-15 s) timescale. We build and develop our own fiber-laser frequency combs to do this research. NIST has a great introduction to frequency combs: https://www.nist.gov/topics/physics/optical-frequency-combs Project 1: Cavity-Enhanced Ultrafast Transient Absorption Spectroscopy We employ a newly-developed technique called Cavity-Enhanced Transient Absorption Spectroscopy to increase the sensitivity of ultrafast absorption spectroscopy. We use ultrafast fiber-laser frequency combs and couple them to external enhancement cavities to both increase the power and effective absorption pathlength, thus giving signal enhancements of several orders of magnitude over traditional transient absorption spectroscopy. This technique allows us to study samples in molecular beams on the femtosecond timescale with transient absorption spectroscopy. Before the development of this technique, ultrafast transient absorption was limited to concentrated condensed phase samples and study of gas phase samples generally required an indirect probe, such as measuring the energy of an ejected electron, or was limited to molecules and states that fluoresce. Now, not only can we now do the same experiment on condense phase molecules as gas phase molecules via transient absorption spectroscopy, we are able to directly probe the molecular states undergoing the dynamics we wish to study. We are currently building up the fiber lasers and cavity-enhanced transient absorption spectrometer with unique frequency tuning capabilities. We will combine this with a molecular beam source to create a relatively cold source of molecules and the ability to make clusters, radicals, and other designer molecules. Here is a simplified diagram of the experimental layout: Current Transient Absorption Projects: Towards a Fundamental Understanding of Singlet Fission One research direction will study the fundamentals of light-harvesting molecules, with a focus on the process of singlet fission, in which one absorbed photon becomes two excited electrons. Single fission is already starting to be used to increase the efficiency of solar cells, even though it is not well understood. We aim to look at model systems and understand the fundamental mechanisms of singlet fission. We are synthesizing molecules with covalently-bound chromophores that are known to undergo singlet fission. We will put these molecules in our molecular beam to study singlet fission in the gas phase, for the first time. This will enable direct comparison of the process of singlet fission with high-quality computational studies. Ultrafast Dynamics of Hydrocarbon Radicals Hydrocarbon combustion mechanisms are dominated by reactions involving radicals, yet these highly-reactive and short-lived species have been challenging experimentalists for decades. While, linear spectroscopy has been a powerful method for the detection and characterization of these key reactive intermediates, many radicals are seen to have broad and unstructured electronic absorptions, as a result of ultrafast dynamics, and only minimal information can be obtained from these spectra. Cavity-enhanced transient absorption spectroscopy has sufficient sensitivity for observing femtosecond electronic excited state dynamics of radicals with absorption, for the first time. Allyl radical is our first combustion radical to study because of the integral importance in hydrocarbon combustion and use as model theoretical systems. Project 2: Two-Dimensional Spectroscopy with Frequency Combs We are again leveraging the unique properties of frequency combs to open new regimes of study with 2D spectroscopy. The first 2D cavity-enhanced spectroscopy on condensed-phase systems is in progress! Project 3: Development of Frequency Comb Technology We are building and testing a frequency comb based upon electro-optic modulation of laser light. Some test spectroscopy experiments are in progress, so stay tuned for results! Funding: Current Funding: National Science Foundation, AMO-E Physics and CSDM-A Chemistry Department of Energy, Gas Phase Chemical Physics Program University of Georgia Seed Grant Past Funding: University of Georgia Start-up funds University of Georgia Innovation Gateway IP Development Award UGA Junior Faculty Seed Grant American Chemical Society Petroleum Research Fund, DNI Grant
