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Departmental Colloquium

Uncovering Molecular Relaxation Processes in Condensed Phases with Nonlinear Optical Spectroscopy  
Guest Speaker
Andy Moran  
Guest Affiliation
University of North Carolina at Chapel Hill  
Thursday, October 18, 2012 4:00 pm - 5:00 pm  

Photoinduced electrocyclic ring opening of cylcoalkene molecules are among the most elementary processes in organic chemistry.  One prototypical light-activated reaction transforms cyclohexadiene into hexatriene.  It is known that a sequence of extremely fast non-radiative transitions precedes bond breaking in cyclohexadiene.  However, these excited state dynamics have never been directly monitored in solution.  We explore such photoinduced relaxation processes in a closely related derivative of cyclohexadiene, α-terpinene, using femtosecond four-wave mixing spectroscopies carried out in the deep UV spectral range.  Of particular interest are the primary molecular geometry changes induced by light absorption.  The importance of these nuclear motions for the ring opening process will be discussed.

Intriguing fundamental physics surround photoinduced relaxation processes in DNA.  Non-radiative transitions deactivate the excited electronic states of the DNA bases in less than 1 picosecond.  Such ultrafast electronic relaxation holds implications for biological photoprotection because all slower excited state chemical reactions are necessarily suppressed.  Following ground state recovery, the nucleobases are left in “hot” quantum states, wherein a subset of vibrational modes possesses a highly non-equilibrium distribution of excitation quanta (i.e., >4 eV in excess energy).  We use laser spectroscopies to follow these dynamics in thymine model systems at temperatures ranging from 100K-300K.  Our data suggest a competition between internal conversion and vibrational cooling processes in this family of molecules.