Probing Ultrafast Dynamics of Bacterial Reaction Centers Using Two-Dimensional Electronic Spectroscopy

In the initial steps of photosynthesis, solar energy is converted to stable charge separated states with high efficiency. Understanding the relationship between structure and function in the photosynthetic reaction centers where these conversion steps take place could guide the development of more efficient artificial light harvesting systems. Reaction centers are complicated pigment-protein complexes with multiple spectrally overlapped absorption bands, making interpretation of spectroscopic data challenging. The sub-picosecond time scales involved in the energy transfer and charge separation processes present another challenge. Two-dimensional electronic spectroscopy (2DES) has proven to be a powerful tool for disentangling features in spectrally congested systems like reaction centers by resolving the optical response with respect to excitation and detection frequencies. 2DES also obtains the excitation frequency dependence without sacrificing time resolution, which is necessary to resolve energy transfer processes in reaction centers occurring on time scales faster than 100fs. We perform 2DES on bacterial reaction centers (BRCs) from the purple bacterium Rhodobacter capsulatus, using a degenerate optical parametric amplifier producing 12fs pulses with bandwidth spanning the broad near-IR absorption bands of the BRC. The 2D spectra are analyzed using several global analysis methods to extract the underlying energy transfer and charge separation kinetics, and we compare the results to published transient absorption studies on BRCs. Commonly used 2DES global analysis techniques proved inadequate for resolving specific branched and parallel reaction mechanisms. We developed an improved 2D kinetic fitting approach which employs a common set of basis spectra for all excitation frequencies, and uses information from the linear absorption spectrum and BRC structure to model the excitation frequency dependence of the 2D spectrum. Using the improved fitting method, we show that the entire time-dependent 2D spectrum is well-represented by a sequential reaction scheme with a single charge-separation pathway. We tested several proposed alternative reaction schemes involving branched charge separation pathways, and did not find compelling evidence from our data that favors a particular branched model. Based on this analysis, we conclude that our data supports the simpler, single pathway charge separation model.PHDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/138758/1/aniedrin_1.pd

Development of a thin section device for space exploration: Rock cutting mechanism

We have developed a rock cutting mechanism for in situ planetary exploration based on abrasive diamond impregnated wire. Performance characteristics of the rock cutter, including cutting rate on several rock types, cutting surface lifetime, and cut rock surface finish are presented. The rock cutter was developed as part of a broader effort to develop an in situ automated rock thin section (IS-ARTS) instrument. The objective of IS-ARTS was to develop an instrument capable of producing petrographic rock thin sections on a planetary science spacecraft. The rock cutting mechanism may also be useful to other planetary science missions with in situ instruments in which sub-sampling and rock surface preparation are necessary. © 2012 COSPAR. Published by Elsevier Ltd. All rights reserved

Ultrafast Charge-Transfer Dynamics at the Boron Subphthalocyanine Chloride/C₆₀ Heterojunction: Comparison between Experiment and Theory

Photoinduced charge-transfer (CT) processes play a key role in many systems, particularly those relevant to organic photovoltaics and photosynthesis. Advancing the understanding of CT processes calls for comparing their rates measured via state-of-the-art time-resolved interface-specific spectroscopic techniques with theoretical predictions based on first-principles molecular models. We measure charge-transfer rates across a boron subphthalocyanine chloride (SubPc)/C₆₀ heterojunction, commonly used in organic photovoltaics, via heterodyne-detected time-resolved second-harmonic generation. We compare these results to theoretical predictions based on a Fermi’s golden rule approach, with input parameters obtained using first-principles calculations for two different equilibrium geometries of a molecular donor–acceptor in a dielectric continuum model. The calculated rates (∼2 ps^–1) overestimate the measured rates (∼0.1 ps^–1), which is consistent with the expectation that the calculated rates represent an upper bound over the experimental ones. The comparison provides valuable understanding of how the structure of the electron donor–acceptor interface affects the CT kinetics in organic photovoltaic systems