Today, our most fundamental challenge in chemistry is to develop predictive theories of reactivity rigorously grounded in the laws of physics. Of course, this can only be done with approximations, perhaps the most important of which was introduced by Born and Oppenheimer. The Born-Oppenheimer approximation (BOA) is the basis for important chemical concepts like: the transition state, the reaction coordinate and more generally the electronically adiabatic potential energy surface. For reactions at metal surfaces, we naturally doubt the BOA as such physical systems exhibit electronic continua. This talk will present recent experimental results designed to test the validity of the BOA and reveal aspects of the mechanisms that lie behind BOA failure when molecules interact with metal surfaces. Experiments on H-atom translational inelasticity suggest electronic friction is a useful concept – here BOA failure is treated as a drag force correction to motion on what is effectively an electronically adiabatic potential energy surface. For state-to-state vibrational relaxation studies, electron transfer involving hops between neutral and anionic potential energy surfaces appears to be essential to describing the dynamics. How BOA failure might affect our current methods of calculating reaction rates – e.g. transition state theory – remains an important unanswered question.  This points out how little effort has been expended testing theories of reaction rates in surface chemistry a problem we are beginning to address.<br/>

Breakdown of the Born-Oppenheimer approximation in surface chemistry

2019 Spring National Meeting

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