The potential energy curves of CH

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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Figure 7.24 The potential energy curves of CH. Although the A2A potential is not crossed by that of the X2n state, the A2A state is weakly predissociated by the continuum of X2n. [Reprinted courtesy of Brzozowski, et al., (1976) and the Astrophysical Journal, published by the University of Chicago Press 1976 The American Astronomical Society.]...

In fig. 38 we have illustrated the conclusions just reached by the potential energy curves of CH obtained from the band spectroscopic data by means of the method of Morse (see section 21). The fully drawn curves represent states actually known, the dotted curves states to be expected according to the theory but not yet found. These are the states and expected to be repulsive and hence not observable, the Allstate for which the correlation was doubtful and which, also on the viewpoint of MuUiken represented in the figure, should be... 

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