Condensed-phase molecular systems are difficult to study using optical spectroscopy because the low dimensionality of the measurement naturally averages over many of the quantum-mechanical degrees of freedom of the system. Multi-dimensional spectroscopy is an approach to increase the dimensionality and hence the information content of spectroscopic methods to reveal properties otherwise hidden in ensemble measurements. While third-order methods such as 2D electronic or 2D infrared spectroscopy are powerful, for many systems the optical response remains an ambiguous metric of the underlying physical structure and dynamics. For instance, in studies of energy transfer in photosynthetic pigment-protein complexes like Fenna-Matthews-Olson (FMO), there has been considerable debate on the physical origin of oscillatory signals. On one hand, these signals may represent localized vibrational wave packet motion on the ground electronic state, or, they may represent delocalized wave like motion on the excited electronic state. Measurements using 2D electronic spectroscopy cannot, in general, distinguish these two processes since the spectral signatures are effectively identical. Changing the structure of the system also presents pitfalls because local changes oftentimes affect the spectral response in unexpected or unpredictable ways, especially in cases in which the disorder may dominate. Other problems in which signal ambiguity become problematic involve those in which electronic and vibrational degrees of freedom strongly couple, creating so-called vibronic states. Or cases in which carriers and phonons couple, creating polaronic states. In an attempt to tackle these difficult, but important, classes of problems, we have developed a fifth-order spectroscopic method which we call GRadient-Assistead Multi-dimensional Electronic Raman Spectroscopy or GAMERS, for short. GAMERS uses two light sources: one source for selectively creating ground-state coherences by means of impulsive Raman scattering by a below-resonance excitation, and a second source that is resonant with an electronic excitation in the system for creating both ground- and excited-state coherences among specific vibronic (or polaronic) states of the molecular system. The GAMER spectrum has four dimensions of spectral information: one corresponding to low-frequency vibrational modes on the ground-electronic state, one corresponding to low-frequency vibronic states on the electronically-excited state (or on the ground-state), and two corresponding to ground-to-electronic transitions. Using near-impulsive light sources, GAMERS achieves over 1013 cm-4 of spectral volume (i.e. volume of the hypercube), which translates to a capacity of over 108 distinct peaks. While in any real system the hypercube space is extremely sparse, this means that peaks that are normally overlapped in lower-dimensionality experiments, may readily spread across multiple spectral dimensions. Our group has developed the GAMERS method from the ground up, including a theoretical description that illustrates the new information content available. An exciting prospect for GAMERS is that it may allow the ground- and excited-state potential surfaces to be experimentally determined owing to its sensitivity to anharmonicity along different normal mode coordinates. In fact, GAMERS is sensitive both to diagonal and off-diagonal anharmonicity, and affords the spectral resolution to extract mode-by-mode displacements. The displacements are measures of electron-vibrational-phonon couplings. GAMERS may also directly distinguish dynamics that occur on the ground state from those that occur on the electronically excited state, which should be of relevance to coherent energy transfer or to other photochemical processes such as those involving conical intersections.
Team Members: Austin Spencer, William Hutson, Shawn Zhao