| Abstract
| - The one-particle electron transition current density (TCD) forvibronic transitions between pairs of stationarystates in molecules is defined. Expressions for TCD are developedusing the complete adiabatic (CA) formalismin which the electronic wave function carries an explicit dependence onthe nuclear momenta, as well as theusual dependence on nuclear positions. In the case of vibronictransitions, the principal non-Born−Oppenheimer (non-BO), nuclear-momentum-dependent contribution to TCD isaccompanied by a less importantBO, nuclear-position-dependent contribution. For vibrationaltransitions within a single electronic state, theBO contribution vanishes, leaving only non-BO, nuclear-momentum-drivenTCD. In the limit of pure electronictransitions, or vibrational transitions within a single electronicstate, it is shown that electron TCD satisfiesthe continuity equation for the conservation of electron transitionprobability density (TPD) for any pair ofstationary states. TCD is a vector field having a uniquerepresentation at each point in the Cartesian spaceof a molecule. It is shown that TCD is a dynamic representation ofthe changes in TPD associated withelectrons in molecules under the influence of a transition-inducingperturbation and that it provides directvisual information concerning the participation of all spatial regionsof the molecule in quantum transitions.The use of TCD provides an opportunity to view uniquely electronicmotion associated with quantummechanical transitions in molecules.
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