Change as Electronic Spectroscopy Processes

A theoretical description of chemical reactions comports three important aspects i) equilibrium limit ii) rate processes iii) mechanism(s). In the R-BO framework, a chemical reaction is described as the change of electronic state from the chemical species defining reactants into the electronic state of the species 

It is apparent from the above considerations that the external energy E plays a role in this approach. With respect to the eigenstates of H , the system is open. Energy conservation principle brings the description into the realm of thermodynamics. The inclusion of the kinetic energy related to the inertial frame used to unfold H is then required by energy conservation. 

We assume that each Flc(r,Q)-state can be associated either to an asymptotic a-configuration or to a full molecular species with its stationary a-configuration. 

Note that by giving to the system an external energy E, excited vibration-rotation and electronic states can be activated (populated too). These states can radiate energy into the field by photon emission or can be populated by photon absorption. The electromagnetic field is considered here as a source or as a sink of energy ensuring energy conservation. The total Hamiltonian must include the 

We focus attention on both the theory of rates and the electronic mechanism for generic chemical processes. An exact quantum mechanical transition state theory was early developed by Miller [18] and rate expressions were derived from quantum scattering theory [19]. That approach and subsequent developments are based upon the reactive BO potential energy surfaces where the familiar case concerns a reaction accompanied by a smooth change in the overall electronic energy surface [20], The R-BO approach does not have such adiabatic changes of electronic states and it is of interest to see the way quantum scattering theory handles the problem in this new context. 

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