| Conical intersections between electronic states are important for many fast reactions in chemistry and biochemistry. Pump-probe experiments have shown that nuclear wavepackets can be transferred between electronic states in less than 100 fs at a conical intersection. Femtosecond pump-probe techniques are widely used to study nuclear wavepacket motion, excited state reaction dynamics, and solvation processes. This thesis begins with studies of the manifestation of optical frequency electronic coherence decay caused by molecular motion in the isotropic pump-probe signal recorded with spectrally selective pulses. Building on this work, it is shown how the anisotropic pump-probe signal recorded with linearly polarized pump and probe pulses reflects electronic motion at a Jahn-Teller conical intersection. A diagrammatic treatment is used to show that the anisotropy of vibrational quantum beats can be used to determine vibrational symmetry but also reflects electronic dephasing and population transfer processes. Numerical calculations explore how these results are manifested with finite pulses. Experiments exciting the Q (0-0) transition of silicon 2,3-naphthalocyanine bis(trihexylsilyloxide) in benzonitrile revealed the anisotropy of five vibrational quantum beats, and were used to deduce the Jahn-Teller reorganization energy of the three asymmetric modes. The asymmetric vibrational quantum beats lead to a zero-adjustable parameter prediction of the initial anisotropy decay. Further, the vibrational quantum beat anisotropies in the experiment require that both electronic dephasing and population transfer are complete within ∼200fs. We predict that the timescale of electronic reorganization at a typical, reactive conical intersection could be 100x faster. In summary, this thesis presents the first use of vibrational polarization signatures to follow femtosecond dynamics on an excited electronic state and the first time domain measurement of electronic motion through a conical intersection. |
