Coherent Two-Quantum Two-Dimensional Electronic Spectroscopy Using Incoherent Light.

Two-quantum two-dimensional electronic spectroscopy (2Q 2D ES) may provide a measure of electron-correlation energies in molecules. Attempts to obtain this profound but elusive signal have relied on experimental implementations using femtosecond laser pulses, which induce an overwhelming background signal of nonresonant response. Here we explore theoretically the signatures of electron correlation in coherent 2Q 2D ES measurements that use spectrally incoherent light, I(4) 2Q 2D ES. One can use such fields to suppress nonresonant response, and therefore this method may better isolate the desired signature of electron correlation. Using an appropriate treatment of the multilevel Bloch electronic system, we find that I(4) 2Q 2D ES presents an opportunity to measure electron-correlation energies in molecules.

Lineshape analysis of coherent multidimensional optical spectroscopy using incoherent light.

Coherent two-dimensional electronic spectroscopy using incoherent (noisy) light, I((4)) 2D ES, holds intriguing challenges and opportunities. One challenge is to determine how I((4)) 2D ES compares to femtosecond 2D ES. Here, we merge the sophisticated energy-gap Hamiltonian formalism that is often used to model femtosecond 2D ES with the factorized time-correlation formalism that is needed to describe I((4)) 2D ES. The analysis reveals that in certain cases the energy-gap Hamiltonian is insufficient to model the spectroscopic technique correctly. The results using a modified energy-gap Hamiltonian show that I((4)) 2D ES can reveal detailed lineshape information, but, contrary to prior reports, does not reveal dynamics during the waiting time.

Coherent multidimensional optical spectra measured using incoherent light.

Four-wave mixing measurements can reveal spectral and dynamics information that is hidden in linear spectra by the interactions among light-absorbing molecules and with their environment. Coherent multidimensional optical spectroscopy is an important variant of four-wave mixing because it resolves a map of interactions and correlations between absorption bands. Previous coherent multidimensional optical spectroscopy measurements have used femtosecond pulses with great success, and it may seem that femtosecond pulses are necessary for such measurements. Here we present coherent two-dimensional electronic spectra measured using incoherent light. The spectra of model molecular systems using broadband spectrally incoherent light are similar but not identical to those expected from measurements using femtosecond pulses. Specifically, the spectra show particular sensitivity to long-lived intermediates such as photoisomers. The results will motivate the design of similar experiments in spectral ranges where femtosecond pulses are difficult to produce.

Two-dimensional electronic spectroscopy using incoherent light: theoretical analysis.

Electronic energy transfer in photosynthesis occurs over a range of time scales and under a variety of intermolecular coupling conditions. Recent work has shown that electronic coupling between chromophores can lead to coherent oscillations in two-dimensional electronic spectroscopy measurements of pigment-protein complexes measured with femtosecond laser pulses. A persistent issue in the field is to reconcile the results of measurements performed using femtosecond laser pulses with physiological illumination conditions. Noisy-light spectroscopy can begin to address this question. In this work we present the theoretical analysis of incoherent two-dimensional electronic spectroscopy, I((4)) 2D ES. Simulations reveal diagonal peaks, cross peaks, and coherent oscillations similar to those observed in femtosecond two-dimensional electronic spectroscopy experiments. The results also expose fundamental differences between the femtosecond-pulse and noisy-light techniques; the differences lead to new challenges and new opportunities.

Strategies for Fostering Synergy between Neuroscience Programs and Chemistry Departments.

The successful model of the Neuroscience Program at Concordia College is used as a source of illustrative examples in a presentation of strategies to foster synergy between neuroscience programs and chemistry departments. Chemistry is an increasing voice in the dialog of modern neuroscience. To be well-prepared to engage in this dialog, students must have strong chemistry training and be comfortable applying it to situations in neuroscience. The strategies presented here are designed to stimulate thought and discussion in the undergraduate neuroscience education community. Hopefully this will lead to greater interaction between chemistry and neuroscience at the undergraduate level in other institutions.

Establishment of a neurochemistry track under the new ACS guidelines.

The new guidelines put forth by the ACS for approved chemistry degrees provide departments with greater flexibility in designing their ACS majors. Under these guidelines, students receive foundational and in-depth chemistry training while allowing individual departments to use their creativity in developing a curriculum that best meets the needs of their students and plays to the strength of the department. The chemistry department at Concordia College has developed an ACS Neurochemistry track and shares how the program arose, some of the practical matters in developing it, and how it can be made to work well within a liberal arts college.

Halogen bonding in iodo-perfluoroalkane/pyridine mixtures.

Mole fraction and temperature studies of halogen bonding between 1-iodo-perfluorobutane, 1-iodo-perfluorohexane, or 2-iodo-perfluoropropane and pyridine were performed using noisy light-based coherent anti-Stokes Raman scattering (I((2)) CARS) spectroscopy. The ring breathing mode of pyridine both is highly sensitive to halogen bonding and provides a strong I((2)) CARS signal. As the lone pair electrons from the pyridinyl nitrogen interact with the sigma-hole on the iodine from the iodo-perfluoroalkane, the ring breathing mode of pyridine blue-shifts proportionately with the strength of the interaction. The measured blue shift for halogen bonding of pyridine and all three iodo-perfluoroalkanes is comparable to that for hydrogen bonding between pyridine and water. 2-Iodo-perfluoropropane displays thermodynamic behavior that is different from that of the 1-iodo-perfluoroalkanes, which suggests a fundamental difference at the molecular level. A potential explanation of this difference is offered and discussed.

Ion-pair interaction in pyridinium carboxylate solutions.

Ion-pair interactions between pyridinium cations and various carboxylate anions are explored using noisy light based coherent anti-Stokes Raman scattering (I(2)CARS). Binary mixtures of pyridine and various carboxylic acids (including halo-acetic acids, straight-chain carboxylic acids, and pivalic acid) are prepared. A Brønsted type acid-base reaction occurs in these mixtures to create pyridinium and carboxylate ions. Both pyridine, itself, and pyridinium have strong I(2)CARS signals originating from their ring breathing modes. The vibrational frequency of the ring breathing mode for pyridine is blue-shifted by hydrogen bonding, and that same mode for pyridinium is red-shifted by ion-pair interaction. Frequency shift data for the ring breathing mode of pyridine and pyridinium are presented. These data are discussed in terms of a simplistic model for the electronic behavior of these compounds.

Effects of hydrogen bonding on the ring stretching modes of pyridine.

The effects of hydrogen bonding on the ring stretching modes (both ring breathing and triangle) of pyridine are experimentally investigated using noisy light based coherent Raman scattering spectroscopy. Three systems, pyridine/formamide, pyridine/water, and pyridine/acetic acid, provide varying degrees of strength for the diluent-pyridine hydrogen bond complex. Formamide forms a relatively weaker hydrogen bond, while acetic acid essentially fully transfers a proton to pyridine. Both dilution studies and temperature studies are performed on the three systems. Together, these provide a broad context in which a very simple model for the electronic behavior of pyridine is formulated. This model is based on a molecular orbital picture and electrostatic arguments, and it well explains the observed experimental results. Additionally, a new mechanism for the line broadening of the ring breathing mode for the pyridine-water hydrogen bonded complex is proposed.

Factorized time correlation diagram analysis of paired causal systems excited by twin stochastic driving functions.

This work examines the properties and mathematical structure of factorized time correlation (FTC) diagram analysis in a general context. The goal is to extract general principles and analytic behavior that are not tied to any particular phenomenon in physics. It is hoped that this will provide a basis for expanded use of FTC diagram analysis beyond its current employment in the study of noisy light-based nonlinear optical spectroscopy. Furthermore, the concept of indirect correlation in a two-channel system driven by twin stationary circular Gaussian stochastic inputs is defined and discussed both analytically and through FTC diagram analysis.