Two-dimensional electronic spectroscopy (2DES)

Two-dimensional electronic spectroscopy (2DES) is an ultrafast laser spectroscopy technique that can probe the electronic, energetic, and spatial landscapes of a sample. 2DES is the optical analogue of Nuclear Magnetic Resonance, a spectroscopic technique that has revolutionized structural biology, enabling the determination of complex molecular structures with high spatial resolution. 2DES is the “ultimate” time-resolved nonlinear optical experiment, since it provides the maximum amount of information about the third-order nonlinear response of a system, and any other third-order nonlinear spectroscopy (such as pump-probe) is contained in the 2D spectrum. 2DES displays its full power in systems with multiple interacting components. 2DES provides two dimensional spectra that shows correlations between excitation and detection frequencies, with simultaneously high spectral and temporal resolution. 2DES can dissect congested spectra and reveal the molecular connections between transitions, thus providing a means to elucidate the overall functionality of a sample.

Model 2D spectrum for a three-component system. The elongation of the peaks along the diagonal displays the instantaneous distribution of frequencies in the transition, whereas the minor axes of the elliptically shaped diagonal peaks are measures of the homogeneous linewidths

This technique requires the interaction of the sample with a sequence of 3 laser pulses. Two of them must be phase-locked replicas, with variable delay, and can be easily generated with NIREOS’ GEMINI-2D interferometer by placing it in the pump beam before the sample, thus turning a pump-probe setup into a state-of-the-art 2DES spectrometer.

Pulse sequence in a 2DES experiment. In a 2-colour experiment pulses 1-2 (interferometrically stable) can have a different colour with respect to pulses 3/LO (also interferometrically stable). NIREOS’ GEMINI-2D is employed to generate pulses 1 and 2 and control their coherence time τ.

Advantages of 2des

With respect to other nonlinear spectroscopic techniques, 2DES has the following advantages:

(i)      It is possible to separate, and thus distinguish, contributions to the nonlinear signal that are spectrally overlapped in one-dimensional experiments. Analysis of cross-peaks reveals whether the different transitions seen in the sample absorption spectrum arise from the same or different molecular species and can quantify couplings and correlations between different excited states.

(ii)     2DES removes inhomogeneous broadening and thus measures the homogeneous linewidth of optical transitions, enabling the individual levels to be singled out in strongly congested spectra.

(iii)    2DES can follow the parallel pathways by which the coupled electronic dynamics evolve after photoexcitation in real time. This makes the 2D technique a particularly powerful tool for tracking excitation energy transfer processes from origin to terminus through multiple channels simultaneously.

(iv)    2DES overcomes the Fourier limit, obtaining simultaneously high temporal resolution (to follow dynamics occurring on the femtosecond timescale) and spectral resolution (to resolve excitation and emission energies over significant bandwidths with high spectral accuracy).

(v)     It is possible to retrieve structural information on the relative spatial arrangement of the chromophores, by either probing the intermolecular electronic couplings through the analysis of the cross-peaks or manipulating the pulse polarizations. 2DES can connect the molecular structure (determined by X-ray crystallography) with the electronic energy levels and, in favorable cases, it can provide insight into the molecular structure when unknown.

Working principle

Oscillating transient signal as a function of the relative delay T1 between the two pump pulses at a selected probe wavelengh and at a fixed population time T2.

A Forurier Transform ad a funcion of T1 for each probe wavelength allows one to retrieve the 2D maps ad a function of detection and exitation wavelengths.

Bidimensional maps of 2D Electronic Spectroscopy measurements obtained on a Light Harvesting (LH1) complexo f a sample of Rhodospirillum Rubrum for three diferente populations time T2 (15 fs, 45 fs, 4000 fs).