The Electronic Interaction Strength

It is insufficient, having obtained an approximate shape for the dissociative curve from the study of the relative variation of T (or r), for the interpretation of the predissociation to be complete. An equally important but often quite difficult step consists of accounting for the absolute value of T. The mere presence of a crossing by a repulsive curve is not sufficient to cause predissociation of bound levels. A value for the electronic interaction He may be deduced from an experimental value for r j if the calculated value of (Xv,j Xe,j) is introduced into Eq. (7.5.16). This semiexperimental value for He may be compared to a calculated or estimated value. The few cases that have been completely interpreted are listed in Tables 7.3 and 7.4. 

The coupling operator, H, for predissociation in Eq. (7.5.1) has exactly the same origin as for perturbations (see Table 3.2). It can be an electronic, spin- 

The selection rules and v, J-dependence of predissociation effects depend on the identity of the operator responsible for the predissociation. From knowledge of the selection rules, qualitative information can immediately be obtained from the variation of the total interaction with v or J. For example, if lines from low-7 levels are missing in emission, the predissociation is certainly not due to a gyroscopic (Afi 0) interaction, which would be zero for J = 0, but must arise from a homogeneous (Afl = 0) interaction. 

When a predissociation is weak, its interpretation is often difficult small first-order effects can be masked by second-order effects. If only a few lines are missing or weakened, it is necessary to consider the possibility of an accidental predissociation, or, in other words, a three-state interaction involving a local perturbation by a weakly predissociated level (See Section 7.13). Predissociation of normally long-lived (metastable) states detected in emission may originate from very small interactions such as spin-spin or hyperfine interaction, as is the case for the I2 B3II0+ state (Broyer, et al., 1976). 

Some general conclusions may be drawn from Tables 7.3 and 7.4. 

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