Real-time structural observation of ultrafast chemical reaction dynamics with femtosecond stimulated Raman Public Deposited

http://ir.library.oregonstate.edu/concern/technical_reports/fn107061s

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  • I have used the unique capabilities of femtosecond stimulated Raman spectroscopy (FSRS) to observe the structural changes associated with ultrafast chemical reaction dynamics. The unique capabilities of the FSRS setup are exploited to obtain vibrational structural information with simultaneous high instrumental time (<100 fs) and energy (<10 cm⁻¹) resolution and deduce the actual nuclear rearrangements that occur during a chemical reaction in real time. The femtosecond time-resolution of FSRS is established by measuring the vibrational spectra of short-lived (<200 fs) excited electronic states in polyenes. In diphenyloctetraene, I observed resonantly enhanced vibrational bands that are broadened by and disappear with the 100-fs lifetime of the optically excited 1B[subscript u] excited electronic state. Measurement of vibrational structural features throughout the first picosecond after photoexcitation of [beta]-carotene establish that no additional intermediate electronic states beyond the 1B[subscript u] and 2A[subscript g] states are involved in the electronic relaxation dynamics of this important sensitizer. I have used FSRS to directly observe anharmonic coupling in the time-domain in deuterio-chloroform. Anharmonic coupling between non-stationary low-frequency bending motion and the high-frequency C-D stretch results in vibrational side bands adjacent to the C-D stretch fundamental. Modeling of these sidebands and their dynamics demonstrates the ability of FSRS to detect vibrational structural changes that occur much faster than the vibrational dephasing time. The first direct observation of the atomic rearrangements occurring during a photochemical process in real time is achieved in FSRS studies of the primary event in vision, the cis-trans isomerization of the retinal chromophore in rhodopsin. The dynamic evolution of dispersive hydrogen-out-of-plane (HOOP) bands is used in combination with spectral modeling and density functional theory calculations to determine the structures of retinal throughout the reaction. The reactant-like shape of the primary photorhodopsin intermediate suggests that a large fraction of the isomerization occurs on the ground state surface, while rapid excited state decay is mediated by fast HOOP motion revealing the origins of rhodopsins unique reactivity. These experiments demonstrate the unique ability of FSRS to observe chemical change in real time and usher in a new era of direct structural studies of chemical reaction dynamics on the femtosecond time scale.
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  • description.provenance : Submitted by Laura Wilson (laura.wilson@oregonstate.edu) on 2012-10-23T20:29:49Z No. of bitstreams: 1 Real time structural observation.pdf: 7854904 bytes, checksum: 74d9de517715af9bb5b522affece4f04 (MD5)
  • description.provenance : Made available in DSpace on 2012-10-23T20:31:56Z (GMT). No. of bitstreams: 1 Real time structural observation.pdf: 7854904 bytes, checksum: 74d9de517715af9bb5b522affece4f04 (MD5) Previous issue date: 2006
  • description.provenance : Approved for entry into archive by Laura Wilson(laura.wilson@oregonstate.edu) on 2012-10-23T20:31:56Z (GMT) No. of bitstreams: 1 Real time structural observation.pdf: 7854904 bytes, checksum: 74d9de517715af9bb5b522affece4f04 (MD5)

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