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Dr. Andrew Petit

Research Interests

The Petit research group uses theoretical and computational chemistry to model the spectroscopy and excited state dynamics of experimentally relevant systems.  In doing so, we will develop strong collaborations with experimental research groups to not only aid in the interpretations of experimental data but also make predictions about what will be observed in future experiments. 

A common theme in much of the research performed in our group is what happens when the Born-Oppenheimer approximation, which underlies most of chemistry, breaks.  We normally think of the electrons (because they are so much lighter than the nuclei) as being able to keep up with whatever motions the atoms in a molecule are doing; electrons get excited when the molecule absorbs light, not when it flops around.  However, this is just an approximation and there are many important situations where the dynamics of the nuclei significantly affects the electrons.  For example, in electron transfer, the motions of the molecule cause an electron to jump from the donor to the accepter.  In photochemistry, a molecule can become electronically excited by absorbing a photon but because of molecular motions, suddenly find itself back in the electronic ground state without emitting light.  We are interested in determining both the locations of the molecular geometries where two electronic states are coupled (i.e., conical intersections) as well as modeling the non-adiabatic dynamics that occurs when the Born-Oppenheimer approximation breaks. 

Some of the types of questions that we are interested in include:

Living organisms protect themselves from dangerous ultraviolet (UV) radiation from the sun by using molecules that quickly and with high efficiency transform UV photons into heat.  For example, the human eye is protected from UV light by kynurenine molecules in the lens.  The mechanism through which these molecules, after absorbing a UV photon and becoming electronically excited, undergo ultrafast relaxation back to the ground state is not currently known.  Research in this area will not only give insights into the photochemistry of biologically relevant molecules but also potentially aid in the development of new sunscreens and dyes.

When living organisms are exposed to radiation, low-energy electrons produced in solution are known to attach to DNA and ultimately induce strand breaks.  Similarly, electron-capture dissociation mass spectrometry works based on the attachment of low-energy electrons to molecules and subsequent fragmentation.  Although qualitative explanations for these processes have been proposed, quantitative theoretical approaches for modeling the attachment of low-energy electrons to molecules and the subsequent dynamics that this causes have been lacking, particularly when multiple coupled electronic states are involved.  We intend to develop semi-classical methods for describing these processes, beginning with modeling recent time-resolved photoelectron experiments that directly probe the dynamics after the electron attachment.

Very few experimental techniques exist that can directly probe the interface between an electrode and the electrolyte in an electrochemical cell or battery.  One such approach, sum frequency generation (SFG) spectroscopy, effectively measures the vibrational spectra of all molecules at the interface.  However, interpretation of experimental SFG spectra can be very challenging and as such, the theoretical modeling of SFG spectra offers great potential for understanding what is seen in experiment.  We plan to combine separate theoretical approaches for modeling the interface between an electrified electrode and solution and for calculating SFG spectra.  This will allow us to not only model SFG spectra at electrified interfaces but also, in some cases, make predictions before the experiments have been performed.