Two-Dimensional Optical Spectroscopy

Two-dimensional optical spectroscopies are recognised as incisive spectroscopic tools to explore structure and dynamics of complex molecular systems. 2D electronic spectroscopy (2DES) allows the investigation of energy relaxation and transfer in molecules, nanomaterials and molecular aggregates. Two-dimensional vibrational spectroscopy (2DIR) is capable of probing the ground state structure and vibrational coupling of many chemical and biological systems.

2D optical spectroscopies couple the benefits of high time and energy resolution to correlate the respective absorption and emission frequencies of a system, for a given waiting time (t2). The two-dimensional correlation map can separate the inhomogeneous and homogeneous components of the spectral line shapes, and identify any intra/inter-molecular couplings via cross-peaks.

Take for example a linear absorption spectrum with two peaks. Do the two peaks originate from two different species with different associated optical excitation frequencies, or a coupled dimer system, with energies split by the coupling constant, J. Ultrafast spectroscopies, such as transient absorption cannot distinguish between these two scenarios. However, 2D spectroscopies can provide a solution. A cut along the diagonal of the 2D correlation spectrum returns the populations (at e1e1 and e2e2) for a given waiting time, t2. Off-diagonal peaks represent coupling between the two states. For example, the green peak at e2e1 demonstrates that energy was initially absorbed at e2, and subsequently emitted at e1.  Further insights can be learned from the time evolving amplitudes and line shapes of the peaks, such as action of coherent transfer pathways, or the role of the environment inducing inhomogeneous broadening.

Illustrative linear absorption spectrum, and two possible energy structures that can explain the two peaks at e1 and e2 in the spectrum. Corresponding 2D frequency-frequency correlation spectrum, for the coupled species case.

We have recently constructed a 2DIR spectrometer in our laboratory using a Germanium acousto-optic modulator pulse shaper in a partially co-linear geometry.  This experimental configuration has several significant advantages over the original boxcar geometry; the signal is self-heterodyned by the probe pulse, and thus the phase between the probe and signal pulses is known, obviating any need to phase spectra. Further, the pulse shaper can be used to rotate the frame of the pump-pair, which shifts the observed oscillatory signal to lower frequencies, loosening the sampling criteria, allowing more rapid data collection.

Schematic diagram of the set-up for a 2DIR experiment in which the pump pulses are controlled by a pulse shaper. The top part of the figure shows the temporal sequence of pulses used in our experiments.

In the above figure, the insert at the top shows the pulse ordering for 2D optical experiments, illustrating the three inter-pulse time delays and associated k-vectors of light.  The middle portion shows the partially co-linear implementation of 2D optical spectroscopies, in which the t1 delay and relative phase are controlled by a pulse shaper. The t2 delay (the waiting time) is incremented by a mechanical delay stage. The emitted signal from the sample is self-heterodyned by the probe pulse and frequency dispersed onto an array detector and digitised. For a given t2 delay, time-frequency 2D surfaces are collected, and then Fourier-transformed along the t1 dimension to obtain the frequency-frequency correlation plot.

With our current 2DIR spectrometer, we have explored the role of vibrational coherence transfer between the coupled anti-symmetric and symmetric carbonyl stretch vibrations in a model rhodium di-carbonyl compound (RDC). By shaping the excitation pulse with the pulse shaper, we have isolated the vibrational population transfer from the coherence transfer pathway.

We are currently constructing a 2DES spectrometer which will capable of capturing correlated dynamics of electronic states in the visible, and then extended to the ultraviolet region. Our first 2DES experiments will be to identify the main energy transfer pathways in biohybrid nanomaterial quantum dot–bacterial reaction centres, which are aimed to optimise the solar energy capture of reaction centres from purple bacteria. This is a collaborative project with Dr. Mike Jones in Biochemistry and Prof. David Fermin in Chemistry.

2D electronic-vibrational spectroscopy (2DEV) is a multidimensional optical spectroscopy technique that combines the advantages of 2DES and 2DIR, to measure the interplay and coupling between electronic states and vibrational manifolds. 2DEV was recently developed by Tom Oliver (while working in Prof Graham Fleming’s laboratory at UC Berkeley) to follow the coupled electronic-nuclear motion of biomolecules and nanomaterials. The technique has also been utilised to connect directly the excitonic and site bases of pigment-protein light harvesting complexes.

2DEV spectroscopy was recently used to explore the role of conical intersections in the model carbonyl containing carotenoid, β-apo-8′-carotenal, in acetonitrile and cyclohexane solutions.  Correlated and anti-correlated electronic and vibrational transition frequencies were observed in 2DEV spectra for different vibrations on the excited state. For the C=C anti-symmetric stretching mode in acetonitrile, the correlation between the electronic and vibrational degrees of freedom, as determined by the center line slope(s), persists for longer than the measured S2 lifetime, indicating that the transfer of molecules to the lower lying S1 state is impulsive and involves a conical intersection in the vertical Franck−Condon region, and likely linked to this nuclear co-ordinate.

2DEV spectra for beta-apo-8'-carotenal in acetonitrile. 2DEV spectra correlate the initial electronic excitation energy with the subsequent vibrational emission. The centre line-slopes are displayed in dashed white lines, and remain correlated beyond the S2 lifetime for the probed C=C anti-symmetric stretch vibration.

2DEV spectroscopy, incorporating broadband ultraviolet and mid-infrared laser pulses, will be developed to explore delocalisation, charge-transfer and electron/hole transport in model DNA systems.

Schematic DNA double helix. Overlaid arrows illustrate two different charge transportation mechanisms: super-exchange and incoherent hopping.

The arrows in the above figure illustrate two potential mechanisms responsible for charge-transportation in DNA. Super-exchange (pink arrow) involves through-bond tunnelling, wherein electrons (illustrated here) can migrate between non-adjacent nucleobases in a single step. The black arrows indicate an incoherent sequential hopping mechanism.


Representative publications

Correlating the motion of electrons and nuclei with two-dimensional electronic-vibrational spectroscopy, T.A.A. Oliver, N.H.C. Lewis and G.R. Fleming, Proc. Natl. Acad. Sci. U.S.A. 111, 10061–10066 (2014).

Vibrational and electronic dynamics of nitrogen–vacancy centres in diamond revealed by two-dimensional ultrafast spectroscopy, V.M. Huxter, T.A.A Oliver, D. Budker and G.R. Fleming, Nat. Phys. 9, 744–749 (2013).

Following coupled electronic-nuclear motion through conical intersections in the ultrafast relaxation of beta-apo-8′-carotenal, T.A.A. Oliver and G.R. Fleming, J. Phys. Chem. B 119, 11428–11441 (2015).

Waveguide-enhanced 2D-IR spectroscopy in the gas phase, G.M. Greetham, I.P. Clark, D. Weidmann, M.N.R. Ashfold, A.J. Orr-Ewing and M. Towrie. Opt. Lett. 38, 3596–3599 (2013).

Measuring correlated electronic and vibrational spectral dynamics using line shapes in two-dimensional electronic-vibrational spectroscopy, N.H.C. Lewis, H. Dong, T.A.A. Oliver and G.R. Fleming, J. Chem. Phys. 142, 174202 (2015).

A method for the direct measurement of electronic site populations in a molecular aggregate using two-dimensional electronic-vibrational spectroscopy, N.H.C. Lewis, H. Dong, T.A.A. Oliver and G.R. Fleming, J. Chem. Phys. 143, 124203 (2015).