faculty

Professor of Chemistry
Ph.D., University of California-Berkeley

Physical Chemistry, Ultrafast Laser Spectroscopy and Chemical Reaction Dynamics

Phone: (734) 763-6074
E-mail: rsension@umich.edu

Research Group

The majority of interesting reactions from a biological, environmental, or engineering perspective occur in fluid condensed phase environments. In such an environment a chemical reaction is controlled by intermolecular interactions with the surrounding solvent bath as well as by the intramolecular Hamiltonian. Interaction with the solvent often results in a situation where reactions are controlled by the competition between intramolecular and intermolecular energy relaxation on time scales ranging from femtoseconds to picoseconds.

In the laboratory, state-of-the-art ultrafast lasers and spectroscopic techniques are employed in the observation and control of photoinitiated reactions. These investigations are made possible by the continuing development of tunable femtosecond light sources from the far UV to the near IR, and the development of techniques to precisely control the phase and amplitude of coherent light pulses The principle objectives of the research program in our group are:

(1) To develop a detailed understanding of the fundamental processes which govern chemical reaction dynamics in fluid environments. Studies of small molecules permit connections between theoretical calculations and experimental measurements. We are using ultrafast spectroscopy and theoretical modeling to investigate isomerization dynamics in small polyene molecules.

(2) To use "designer" light pulses to control chemical reactions in condensed phases. Bond-selective control of chemical reactions has been a long standing goal of modern chemical physics. Early attempts using selective laser excitation were thwarted by fast intramolecular energy redistribution. Now ultrafast laser pulses, optical pulse shaping, and feedback algorithms have been successfully combined to control bond dissociation reactions in simple isolated molecules. We are using coherent sculpted light pulses to control unimolecular bond dissociation, and isomerization reactions in solution.

(3) To use short pulses to establish synchronization and study enzyme mechanism in complicated biological systems. Our current investigations are concentrating on the bond-cleavage mechanism in B12 dependent enzymes. this critical system.

AWARDS

• LS&A Excellence in Education Award-2005

REPRESENTATIVE PUBLICATIONS

1. Harris, D. A., Orozco, M. B., and Sension, R. J., 2006, "Solvent dependent conformational relaxation of cis-1,3,5 hexatriene" Journal of Physical Chemistry A, v110 (30) 9325-9333.
   
2. Florean, A. C., Carroll, E.C. Spears, K.G., Sension, R.J. and Bucksbaum P.H., 2006, "Optical control of excited state vibrational coherences of a molecule in solution: The influence of the excitation pulse spectrum and phase in LD690." Journal of Physical Chemistry B, available in ASAP.
   
3. Carroll, E. C., Pearson, B. J., Florean, A. C., Bucksbaum, P. H., and Sension, R. J., 2006, "Spectral phase effects on nonlinear resonant photochemistry of 1,3-cyclohexadiene in solution" The Journal of Chemical Physics, v124 (11) 114506 (10 pages).
   
4. Shiang, J. J., Cole, A. G., Sension, R. J., Hang, K., Weng, Y., Trommel, J. S., Marzilli, L. G. and Lian, T., 2006, "Ultrafast Excited State Dynamics in Vitamin B12 and Related Cob(III)alamins" Journal of the American Chemical Society, v128 (3) 801-808.
   
5. Sension, R. J., Harris, D. A., and Cole, A. G., 2005 "Time-Resolved Spectroscopic Studies of B12 Coenzymes: A Comparison of the Influence of Solvent on the Primary Photolysis Mechanism and Geminate Recombination of Methyl-, Ethyl-, n-Propyl-, and 5'-Deoxyadenosylcobalamin" Journal of   Physical Chemistry B, v109 (46) 21954-21962.
   
6. Sension, R. J., Harris, D. A., Stickrath, A. B., Cole, A. G., Fox, C. C., Marsh, E. N. G., 2005 "Time Resolved Measurements of the Photolysis and Recombination of Adenosyl-cobalamin Bound to Glutamate Mutase" Journal of   Physical Chemistry B, v109 (38) 18146-18152.