Population dynamics in vibrational modes during non-Born-Oppenheimer processes: CARS spectroscopy used as a mode-selective filter

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 beta-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.