The coupling of specific nuclear and electronic degrees of freedom of a molecular system during non-radiative electronic transitions plays a central role in photochemistry and photobiology. This breakdown of the Born-Oppenheimer approximation during
processes such as internal conversion determines the mechanism and product distribution of photochemical reactions and is responsible for the high efficiency of photobiological
processes. In order to explore this phenomena in
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-carotene, a molecule that plays a primary role as an auxiliary light-harvesting pigment in photosynthesis, a spectroscopic method was employed that allows for the individual vibrational modes to be monitored selectively within the dynamics of an internal conversion
process. This spectroscopic technique employs an initial pump laser to excite the molecule into an excited electronic state and resolves the subsequent relaxation
process by interrogating the system with a time-delayed, coherent anti-Stokes Raman
process (CARS), which acts as a mode-selective filter for observing the population flow within specific vibrational modes with a time resolution in the femtosecond regime.