Radiationless Relaxation in Isolated Polyatomics

We finally comment on the vibrational selection rules for vibronically induced transitions. The intensity of a vibrational band which occurs through vibronic coupling in normal coordinate is proportional to both the Herzberg-Teller and Born-Oppenheimer theories. When mode k is nontotally symmetric, the symmetry selection rule in that mode will be Av = 1, 3,. .. if the equilibrium position is undisplaced along in the electronic transition. If mode k is also harmonic, with similar frequencies in both electronic states, the more restrictive selection rule Av— applies. In such a case, the band intensity becomes proportional to [1 , times a 

Since the molecule is isolated, these radiationless processes must be energy-conserving. Since they are energy-conserving, it might seem a priori that they should be reversible. For example, Ti benzene formed by intersystem crossing 

The general problem of isolated-molecule nonradiative relaxation may be stated as follows. A Born-Oppenheimer molecular state in electronic state manifold A is prepared in a molecule by photon absorption from some lower 

In analogy to what was done in Section 3.1, we multiply this equation by (2)1, and use the facts that 

It is important to differentiate here between the energies E, , (which are eigenvalues of the BO Hamiltonian f q) + U q, Q)) and the energies (which 

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A previous review provides a description of the theory of electronic relaxation in polyatomic molecules with particular emphasis on the vibronic state dependence of radiationless transition rates. A sequal review considers the general question of collisional effects on electronic relaxation, while the present one covers only the special phenomenon of collision-induced intersystem crossing. It departs from the other collisional effects review in presenting only a qualitative description of the theory the full theoretical details can be obtained from the previous review and the original papers.As a review of the basic concepts of radiationless transitions theory is necessary as a prelude to a discussion of collision-induced intersystem crossing, considerable overlap exists between this section and Section II of the previous collision effects review. However, since many concepts from radiationless transition theory, such as the nature and criteria for irreversible decay, the role of the preparation of the initial state, the occurrence of intramolecular vibrational relaxation, etc. pervade the other papers on laser chemistry in these volumes, it is useful to recall the primary results of the theory of electronic relaxation in isolated molecules and its relevance to the material in the present volume as well as to this review. 

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