The application of femtosecond time-resolved CARS-spectroscopy for the characterization of vibrational modes upon photoexcitation

Abstract: This contribution reports on the application of a CARS process as a powerful probe for the investigation of vibrational dynamics of polyatomic molecules (e.g. β-carotene, stilbene-3). In the case of e.g. stilbene-3, the vibrational modes active in the structural rearrangements upon photo-excitation could be characterized. In order to investigate the coupling of vibrational and electronic degrees of freedom during the dynamics of radiationless electronic transitions such as internal conversion (IC), a time-resolved spectroscopic method must be employed that is sensitive to the different vibrational modes of the polyatomic system. This is accomplished by integrating coherent anti-Stokes Raman scattering (CARS) as a probe mechanism in a femtosecond time-resolved pump-probe scheme. This configuration will be referred to in the following as a pump CARS scheme and employs four femtosecond laser pulses. We will show that within this experimental arrangement it is possible to identify the electronic states participating in an IC and gain information about vibrational modes of the transient electronic states. The principle of these experiments is illustrated by exploring the IC process in β-carotene, an active chromophore in photosynthesis and the photo-induced structural rearrangements of the C=C bonds of stilbene-3.