Franck-Condon transitions and chemical reactions

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Abstract
By using certain excited electronic states (potentials) of diatomic molecules in the initial and final species channel as distortion potentials in distorted wave theory of scattering, we perform a collision theoretical study of the Franck-Condon transition model for chemical reactions proposed by Herschbach et al. A study with a collinear collision model indicates that to an approximation bimolecular reactions in general may be viewed as two simultaneous Franck-Condon transitions between the ground and excited electronic states of the product and reactant diatomic molecules, if excited electronic states are allowed to participate in the reaction processes. In the one-dimensional model A … X … Y of the reaction geometry for the reaction A + XY → AX + Y the product translational energy distribution P ya , when XY is in the ground vibrational state, is found to be approximately equal to the formula, where u xy is the repulsive excited electronic state of XY evaluated at the equilibrium distance r e of XY in the ground electronic state and gb is related to its derivative at r e. The exponential factor is the same as the formula obtained by Herschbach for the product translational energy distribution function. We discuss the role and probable importance of excited electronic states in the ordinary chemical reactions.

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