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Real-time mapping of electronic structure with single-shot two-dimensional electronic spectroscopy.

ABSTRACT: Electronic structure and dynamics determine material properties and behavior. Important time scales for electronic dynamics range from attoseconds to milliseconds. Two-dimensional optical spectroscopy has proven an incisive tool to probe fast spatiotemporal electronic dynamics in complex multichromophoric systems. However, acquiring these spectra requires long point-by-point acquisitions that preclude observations on the millisecond and microsecond time scales. Here we demonstrate that imaging temporally encoded information within a homogeneous sample allows mapping of the evolution of the electronic Hamiltonian with femtosecond temporal resolution in a single-laser-shot, providing real-time maps of electronic coupling. This method, which we call GRadient-Assisted Photon Echo spectroscopy (GRAPE), eliminates phase errors deleterious to Fourier spectroscopies while reducing the acquisition time by orders of magnitude using only conventional optical components. In analogy to MRI in which magnetic field gradients are used to create spatial correlation maps, GRAPE spectroscopy takes advantage of a similar type of spatial encoding to construct electronic correlation maps. Unlike magnetic resonance, however, this spatial encoding of the nonlinear polarization along the excitation frequency axis of the two-dimensional spectrum results in no loss in signal while simultaneously reducing overall noise. Correlating the energy transfer events and electronic coupling occurring in tens of femtoseconds with slow dynamics on the subsecond time scale is fundamentally important in photobiology, solar energy research, nonlinear spectroscopy, and optoelectronic device characterization.

SUBMITTER: Harel E

PROVIDER: S-EPMC2944741 | biostudies-literature | 2010 Sep

REPOSITORIES: biostudies-literature

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Real-time mapping of electronic structure with single-shot two-dimensional electronic spectroscopy.

Harel Elad E Fidler Andrew F AF Engel Gregory S GS

Proceedings of the National Academy of Sciences of the United States of America 20100901 38

Electronic structure and dynamics determine material properties and behavior. Important time scales for electronic dynamics range from attoseconds to milliseconds. Two-dimensional optical spectroscopy has proven an incisive tool to probe fast spatiotemporal electronic dynamics in complex multichromophoric systems. However, acquiring these spectra requires long point-by-point acquisitions that preclude observations on the millisecond and microsecond time scales. Here we demonstrate that imaging t ...[more]

PMID: 20810917

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Project description:The theory of electronic structure of many-electron systems, such as molecules, is extraordinarily complicated. A consideration of how electron density is distributed on average in the average field of the other electrons in the system, that is, mean field theory, is very instructive. However, quantitatively describing chemical bonds, reactions, and spectroscopy requires consideration of the way that electrons avoid each other while moving; this is called electron correlation (or in physics, the many-body problem for fermions). Although great progress has been made in theory, there is a need for incisive experimental tests for large molecular systems in the condensed phase. In this Account, we report a two-dimensional (2D) optical coherent spectroscopy that correlates the double-excited electronic states to constituent single-excited states. The technique, termed 2D double-quantum coherence spectroscopy (2D-DQCS), uses multiple, time-ordered ultrashort coherent optical pulses to create double- and single-quantum coherences over time intervals between the pulses. The resulting 2D electronic spectrum is a map of the energy correlation between the first excited state and two-photon allowed double-quantum states. The underlying principle of the experiment is that when the energy of the double-quantum state, viewed in simple models as a double HOMO-to-LUMO (highest occupied to lowest unoccupied molecular orbital) excitation, equals twice that of a single excitation, then no signal is radiated. However, electron-electron interactions, a combination of exchange interactions and electron correlation, in real systems generates a signal that reveals precisely how the energy of the double-quantum resonance differs from twice the single-quantum resonance. The energy shift measured in this experiment reveals how the second excitation is perturbed by both the presence of the first excitation and the way that the other electrons in the system have responded to the presence of that first excitation. We compare a series of organic dye molecules and find that the energy offset for adding a second electronic excitation to the system relative to the first excitation is on the order of tens of millielectronvolts; it also depends quite sensitively on molecular geometry. These results demonstrate the effectiveness of 2D-DQCS for elucidating quantitative information about electron-electron interactions, many-electron wave functions, and electron correlation in electronic excited states and excitons. Our work helps illuminate the implications of electron correlation on chemical systems. In a broad sense, we are trying to help address the fundamental question "How do we go beyond the orbital representation of electrons in the chemical sciences?"

Dataset Information

Real-time mapping of electronic structure with single-shot two-dimensional electronic spectroscopy.

Publications

Real-time mapping of electronic structure with single-shot two-dimensional electronic spectroscopy.

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