Sebastian Wachsmann-Hoglu, Ph.D.
CEDARS-SINAI MEDICAL CENTER
8700 Beverly Blvd, D-6061
Los Angeles, CA 90048
 
The main objectives of this seminar are: (1) to understand the concept of resonance Raman scattering; (2) to learn how molecular vibrations participate in energy relaxation after ultrafast electron transfer, (3) to identify ways of using  this method for biomedical applications. 
 
Resonance Raman Spectroscopy has exceptional sensitivity and selectivity due to enhanced Raman scattering of vibrational modes coupled to molecular electronic transitions. By resolving the resonance Raman spectra in time, it is possible to determine the role of vibrational modes in processes such as photochemical reactions, and/or transfer of excitation between molecules and their environment. Betain-30 is a solvatochromic molecule that, after photoexcitation, undergoes ultrafast back-electron transfer in condensed phase. After a short pump pulse initiates the chemical reaction, a probe pulse explores the vibrational modes involved in this process. High-frequency Franck-Condon modes were identified as accepting modes of energy, and non-equilibrium vibrational population was observed for approximately 10ps after the reaction ceased. This was followed by slower vibrational relaxation and energy flow into additional intra and intermolecular (solvent) modes. Molecular temperatures in excess of 600K were determined immediately after intramolecular vibrational equilibrium has been achieved. A similar pump-probe experiment using Coherent Anti-Stokes Raman Spectroscopy revealed strong vibronic coupling of two adjacent electronic states in diphenylhexatriene. 
 
While resonance Raman spectroscopy is a powerful method to study photochemical processes, its use is mainly limited to non-fluorescent molecules. To make it useful for biomedical applications, where it may have a great potential due to its high selectivity, novel methods need to be developed to reduce the fluorescence background. One possible approach is to temporally separate the instantaneous Raman signal from the delayed fluorescence. A time-resolved approach would, in addition, have the ability to measure fluorescence lifetimes with the same instrument.

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