Vibrations play a critical role in many photochemical and photophysical processes in which excitations reside on the electronically excited state. However, difficulty in assigning signals from spectroscopic measurements uniquely to a specific electronic state, ground or otherwise, has exposed limitations to their physical interpretation. Here, we demonstrate the selective excitation of vibrational coherences on the ground electronic state through impulsive Raman scattering, whose weak fifth-order signal is resonantly enhanced by coupling to strong electronic transitions. The six-wave mixing signals measured using this technique are free of lower-order cascades and represent correlations between zero-quantum vibrational coherences in the ground state and single-quantum coherences between the ground and electronic states. We believe that this technique has the potential to shed much-needed insight onto some of the mysteries regarding the origin of long-lived coherences observed in photosynthetic and other coupled chromophore systems.

We demonstrate use of spatial interference for the complete electric field reconstruction of two-dimensional (2D) coherent spectroscopic signals generated through four-wave mixing (4WM) in a single laser shot. Until now, the amplitude and phase characterization of 4WM signals has relied primarily on Fourier transform spectral interferometry (FTSI), which limits the measurement’s sensitivity and resolution. We show that spatial spectral interferometry (SSI) is a generalized approach to 4WM signal detection that eliminates these inherent limitations of FTSI without introducing additional experimental complexity. SSI is used to measure the 2D photon echo spectra of two systems with dramatically different line widths, the coupled D line transitions in rubidium vapor and the energy-transfer dynamics in the light-harvesting protein LH2.

Accurate mapping of the electronic and vibrational structure of a molecular system is a basic goal of chemistry as it underpins reactivity and function. Experimentally, the challenge is to uncover the intramolecular interactions and ensuing dynamics that define this structure. Multidimensional coherent spectroscopy can map such interactions analogous to the way in which nuclear magnetic resonance provides access to the nuclear spin structure. Here we present two-dimensional coherent spectra measured using few-cycle continuum light. Critically, our approach instantaneously maps the energy landscape of a complex molecular system in a single laser pulse across 350 nm of bandwidth, thereby making it suitable for rapid molecular fingerprinting. We envision few-cycle supercontinuum spectroscopy based on the nonlinear optical response as a powerful tool to examine molecules in the condensed phase at the extremes of time, space, and energy.Keywords: molecular spectroscopy; nonlinear spectroscopy; single-shot; supercontinuum; two-dimensional spectroscopy;