Schematic mechanism of indirect or accidental predissociation

This chapter concludes with a brief outline of the information crucial to selection and testing of a mechanistic model for an observed predissociation. 

If experimental data concerning the variation of T with v, J, and /z axe available, then a detailed interpretation of the predissociation may be attempted making use of computer programs for calculating vibrational factors. These calculations may involve either semiclassical formulas [Eq. (7.6.3)] or numerical integration of vibrational wavefunctions. 

There axe several pitfalls to be avoided in formulating a predissociation mechanism. A linear variation of the linewidth with J(J +1) can be caused by either gyroscopic interaction or penetration through a centrifugal barrier. Another 

When the linewidth exhibits no oscillations, this suggests the occurrence of an inner crossing, but two cases exist where an outer crossing is shown to display no linewidth oscillation. The first example concerns the OD molecule. Below the energy of the curve crossing, the bound free vibrational overlap comes only from the tail of the discrete wavefunction (tunnelling). The nonradiative decay rate is very slow, but it increases smoothly with J [predissociation of the OD A2E+(v = 0-2) levels by the 4E state (Bergeman, et ai, 1981)]. 

These ambiguities can be eliminated if levels belonging to successive values of v or J are examined. Isotope effects are also useful for confirming the nature of a predissociation. In the absence of detailed information, one must be cautious about inferring the origin of observed predissociation effects. 

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